Open Access

Comparisons of protein profiles of beech bark disease resistant and susceptible American beech (Fagus grandifolia)

Proteome Science201311:2

DOI: 10.1186/1477-5956-11-2

Received: 9 May 2012

Accepted: 23 December 2012

Published: 14 January 2013

Abstract

Background

Beech bark disease is an insect-fungus complex that damages and often kills American beech trees and has major ecological and economic impacts on forests of the northeastern United States and southeastern Canadian forests. The disease begins when exotic beech scale insects feed on the bark of trees, and is followed by infection of damaged bark tissues by one of the Neonectria species of fungi. Proteomic analysis was conducted of beech bark proteins from diseased trees and healthy trees in areas heavily infested with beech bark disease. All of the diseased trees had signs of Neonectria infection such as cankers or fruiting bodies. In previous tests reported elsewhere, all of the diseased trees were demonstrated to be susceptible to the scale insect and all of the healthy trees were demonstrated to be resistant to the scale insect. Sixteen trees were sampled from eight geographically isolated stands, the sample consisting of 10 healthy (scale-resistant) and 6 diseased/infested (scale-susceptible) trees.

Results

Proteins were extracted from each tree and analysed in triplicate by isoelectric focusing followed by denaturing gel electrophoresis. Gels were stained and protein spots identified and intensity quantified, then a statistical model was fit to identify significant differences between trees. A subset of BBD differential proteins were analysed by mass spectrometry and matched to known protein sequences for identification. Identified proteins had homology to stress, insect, and pathogen related proteins in other plant systems. Protein spots significantly different in diseased and healthy trees having no stand or disease-by-stand interaction effects were identified.

Conclusions

Further study of these proteins should help to understand processes critical to resistance to beech bark disease and to develop biomarkers for use in tree breeding programs and for the selection of resistant trees prior to or in early stages of BBD development in stands. Early identification of resistant trees (prior to the full disease development in an area) will allow forest management through the removal of susceptible trees and their root-sprouts prior to the onset of disease, allowing management and mitigation of costs, economic impact, and impacts on ecological systems and services.

Keywords

Beech bark disease Beech scale Disease resistance Insect resistance Fagus Cryptococcus Neonectria

Background

Beech bark disease (BBD) is an insect-fungus complex that has been killing American beech (Fagus grandifolia Ehrh.) trees since the accidental introduction of the beech scale insect (Cryptococcus fagisuga Lind.) to Canada around 1890 [1, 2]. The first phase of BBD is beech scale insect infestation resulting in the production of small fissures in the bark [1]. The fungal component, either Neonectria ditissima Samuels & Rossman or Neonectria faginata Castlebury then infects these fissures causing extensive tissue damage. Mortality in the first wave of the disease can be as high as 50 % [3], with consequent loss to stand health, merchantable timber, and many wildlife and ecosystem services.

An estimated 1% of American beech trees remain disease free in forests long-affected by beech bark disease [4]. Insect challenge experiments have demonstrated that resistance is to the beech scale portion of the disease complex [4, 5]. Although it has been reported that scale infestation without Neonectria infection may play a role in mortality events [6], there has been no documentation of Neonectria infection leading to widespread stand or landscape level mortality in the absence of prior scale infestation. In aftermath forests, where scale populations have declined, presumably due to loss or reduced quality of habitat [7] and environmental factors [8], the population dynamics of scale and Neonectria are no longer directly correlated [7, 9]. However, in these cases it is suggested that the lower density of scale is still sufficiently high that infection sites (scale-feeding wounds) are not limiting and once the fungus has established in the tree it is no longer influenced by fluctuations in scale density [7]. Therefore, the focus of recent breeding and tree improvement efforts has been on resistance to the scale insect [5, 10]. Current management approaches are based on the objective of increasing the proportion of disease resistant beech by removing susceptible trees along with any resulting root and stump sprouts, retaining the disease-free trees prior to the height of BBD development [11, 12] and supplementing with genetically scale-resistant seedling plantings once such materials are available (management plans of US Forest Service Allegheny National Forest [13] and Michigan Dept. of Natural Resources [14]). However, it is impossible to identify the most resistant beech trees until the scale infestation is heavy, at which point economic losses have already occurred and management operations are more complicated and expensive. Identification of a biomarker for Cryptococcus resistance would provide land managers the opportunity to begin management operations before the economic and ecological losses have occurred, and to spread BBD management activities over several budget cycles. A biomarker for resistance may also be utilized to expedite the breeding and selection process.

Bark protein differences are likely to be a good source for biomarker candidates. The scale insect feeds in the tissue layers vernacularly known as bark (cork, cork cambium, phloem, and cambium) [1]. Wargo et al. [15] found significant differences in bark amino acid concentrations and total amino nitrogen in different seasons and scale infestation levels. These results suggest bark protein content may be important in the insect-tree interaction, and that bark protein profiles may differ between scale-resistant and susceptible beech trees. Therefore differentially expressed bark proteins may be reliable biomarkers of resistance to beech scale in American beech. One method to identify differentially expressed bark proteins is to examine the proteome of a number of trees using two dimensional electrophoresis (2-DE) gels. A proteomics approach allows the examination and quantification of large numbers of proteins anonymously and simultaneously. Typically, 2-DE analysis is limited to two-sample comparison with simple experimental structure (e.g. one treatment). Utilization of analysis of variance (ANOVA) for statistical analysis allows the testing of three or more treatment levels for several technical and biological factors in one model, and supports unbalanced experimental and sampling designs. This more sophisticated analysis allows the identification of more complex protein quantity patterns and the interactions of factors in protein quantity. In this study we employ the use of 2-DE gel based proteomics and ANOVA to identify proteins in the bark of American beech that are different between healthy and BBD diseased trees, while also considering if the BBD effect is present alone or with a stand effect or interaction between stand and BBD effects. Although the healthy trees in this study are known to be resistant to the scale insect, the diseased trees are susceptible to both scale infestation (Table 1), and had symptoms of an active fungal infection at the time of tissue collection. Proteins that are expressed in response to the scale insect cannot be distinguished from proteins expressed in response to the fungal pathogen in diseased trees so in our analysis we refer to the more general BBD response which includes responses to both. This approach allows selection of proteins for further study that are most likely to be broadly linked to BBD response rather than different in protein quantity due to the relatedness of trees within stands.
Table 1

Tree information including stand, field disease score, controlled test insect challenge result, and spot quantities

Tree name

Stand code a

Disease Condition b

Scale Interaction c

Total protein spots

Number of matched spots

Number of unique spots

1504

LM

H

R (8)

382

295

87

1506

LM

D

S (3)

197

184

13

CL01d

CL

H

R (10)

258

241

17

CM01

CM

H

R (1)

280

270

10

CM02Dd

CM

D

S(11)

329

329

0

DT02d

DT

H

R (2)

228

215

13

DT02D

DT

D

not tested

286

266

20

DTOAd

DT

H

R (2)

522

421

101

JC01d

JC

H

R (14)

367

338

29

JC02D

JC

D

S (4)

487

408

79

OD02d

OD

H

R (10)

284

272

12

OD08Dd

OD

D

S (12)

239

225

14

RS01d

RS

H

R (9)

288

265

23

RS02D

RS

D

S (7)

229

223

6

SL01d

SL

H

R (4)

249

240

15

SL03d

SL

H

R (8)

260

243

17

aLM=Ludington, MI, USA; CL= Charlie Lake, NB, Can; CM= Curry Mountain, NB, Can; DT= Roachville, NB, Can; JC=Highfield, NB, Can; OD= Odell Park, Fredericton, NB, Can; RS= Regent St., Fredericton, NB, Can; SL=Spednick Lake, NB, Can.

bH=Healthy (lacking scale infestation and signs of fungal infection), D= Diseased (beech bark disease present both scale infestation and fungal infection observed (on ortet) in the field).

cTest result of artificial inoculation of grafted ramets with scale insect eggs. Result (number of ramets tested, i.e. replicates). R= Resistant, 0–5 avg. live scale, S= Susceptible, 30–100 avg. live scale and expected to sustain scale population and subsequent fungal infection.

dCorrespondence to Ramirez et al. [16] CL01=E1, CM02d=H1, DT02=B3, DT0A=B1, JC01=C1, OD02=A2, OD08d=A3, RS01=I1, SL01=D1, and SL03=D3.

Results

Individual tree analysis and spot matching

The location, field disease score, and the artificial infestation (scale resistant/scale susceptible) results for the ten healthy trees and six diseased trees studied are summarized in Table 1. Artificial infestation of grafted ramets of the healthy trees demonstrated that all of these genotypes are resistant to the scale insect, the details of these experiments are reported elsewhere [16]. Protein was extracted and 2-DE was conducted for three technical replicates per tree. Figure 1 shows a randomly selected experimental gel to illustrate the typical resolution and spot density we achieved in the experiment. PDQuest was used to create a master gel for each tree and the number of protein spots per tree ranged from 197 to 522 with an average of 305.3 and standard error of 23.
https://static-content.springer.com/image/art%3A10.1186%2F1477-5956-11-2/MediaObjects/12953_2012_Article_405_Fig1_HTML.jpg
Figure 1

Raw gel image of one randomly selected gel. Gel image from tree DT02, shown to illustrate the number of spots and quality of separation typical of the experimental gels. The gel has been cropped to remove gel edge and markers, but include both pI and electrophoresis fronts.

An experiment-wide master gel was constructed using the 'compare experiments' function of PDQuest where each individual tree master was considered an 'experiment'. The experiment wide master gel (Figure 2) included all spots on individual tree masters that were added to the experiment wide master because they were present in two or more trees. Most, but not all, of these spots were present in more than two trees, and some spots were present in all trees. The number of matched and unique spots for each tree is listed in Table 1. The total number of spots added to the experiment wide master (matched in two or more trees) was 531. Matched spot per tree ranged from 184 to 421 and average 277.2 +/− 16.7. Unique spots per tree (spots in the tree master not matched and added to the experiment wide master) range from 0 (base tree CM02d has all spots on the master gel by definition) to 101 with an average of 28.5 +/− 7.75. The grand total of spots identified is 987 which is the sum of the 531 matched spots plus 456 spots unique to only one tree. Spots unique to a single tree were excluded from further analysis. While unique spots in aggregate are 46% of total spots, a maximum of 22.7% of the spots in any one tree are unique indicating that the spot matching of trees to the experiment wide master was efficient.
https://static-content.springer.com/image/art%3A10.1186%2F1477-5956-11-2/MediaObjects/12953_2012_Article_405_Fig2_HTML.jpg
Figure 2

Global master gel showing the Gaussian spots from all experimental gels. Spots identified as having a significant BBD effect are marked and numbered, with a blue circle for spots that are higher in healthy trees (compared to BBD diseased trees), and a green box for spots that are lower in healthy trees . The image is a composite of the full 48 gels so weaker spots may not be visible in the image (but are present and can be seen at higher magnification and contrast).

To consider whether these unique spots could be artifacts due to low spot intensity, four trees were selected at random and the spot intensity distribution examined. Graphing showed that the distribution of unique spots was slightly biased toward lighter spots. However, comparison of summary statistics (mean, min, Q1, mean, Q3, max) illustrates that spots of similar intensity are both matched and unmatched, and the faintest spot is matched in three of the four trees (data not shown). This indicates that the spots unique to each tree are not artifacts of poor matching related to the intensity of the spots or variances in protein quantification. Spot quantities for all matched spots in all trees were exported for analysis (missing spots were estimated) in order to use more robust statistical procedures than PDQuest allows.

Constitutive proteins and technical effects

General linear models were fitted to assess whether technical effects (gel and batch) were significant or could be ignored, and to identify constitutive proteins (spot quantity equal in all individual trees, tested by fitting a tree effect). Technical effects were significant for only six spots and these spots were dropped from further analysis, allowing technical effects to be dropped from later models. The biological effects of interest are stand and disease state, and constitutive spots will obviously not vary for these effects. Therefore the 103 constitutive spots (tree effect not significant) were removed from the dataset to reduce its size for more efficient analysis (leaving 422 spots that had a significantly different mean spot quantity in at least one tree and no significant technical effects).

Stand and disease condition effects

General linear models were fitted to test for disease condition effect (BBD), stand (STAND) and the disease condition by stand interaction (INT) for the retained spots. The count of spots significant for different combinations of effects is shown in Table 2. The largest class of proteins contains those that are quantitatively different in several trees (demonstrated in the first model, but not significant for any tested effects (209 spots)). Protein spots with a significant STAND effect but no significance for BBD (61 spots of which 28 have a significant interaction and 33 do not) are interesting as potential markers for geographic variation in beech. These spots and an additional 32 spots with only the interaction effect found significant were not studied further in this experiment.
Table 2

Significant protein counts

 

Stand x BBD Interaction

 

Significant

Not Significant

STAND effect

Significant

Not Sig.

Significant

Not Sig.

BBD Effect Significant

47

9

14

50

BBD Effect Not Significant

28

32

33

209

Count of proteins with significant effects for stand, beech bark disease state, and stand x disease state interaction. The most likely biomarker candidates are the protein spots with a significant BBD effect in the absence of stand effects and interactions. To see the correspondence to Figure 3 may require summing across cells, for example the 56 spots where the interaction and BBD effect are significant is the sum of the two stand levels, 47 plus nine.

Table 3 shows the protein spots with significant BBD effect, including the p-value and q-value for the BBD effect, the mean spot quantity and standard error for the diseased and healthy trees, and the ratio and direction of differences of spot quantity of healthy to diseased to trees (for all p-values underlying Table 3, see Additional file 1). One-hundred and twenty spots have a significant BBD effect, and of these 50 have no other significant effects and are the most logical candidates for biomarker development (marked with an asterisk in Table 3).
Table 3

Significant effects, means, and expression ratio for protein spots with significant beech bark disease (BBD) effect

Spot

 

BBD q-value a

STAND b

INT c

mean_H d

sd_H d

mean_D d

sd_D d

Ratio of means e

Direction in healthy

11

 

0.0470

NS

SIG

26,174

65,030

72,775

87,910

2.7804

down

13

 

0.0328

SIG

SIG

94,894

219,401

59,736

143,489

1.5886

up

19

 

0.0000f

SIG

SIG

8,004

5,915

62,232

97,523

7.7749

down

103

*

0.0199

NS

NS

55,653

177,669

296,935

581,023

5.3354

down

108

 

0.0004

SIG

SIG

258,261

376,114

111,723

168,407

2.3116

up

109

*

0.0023

NS

NS

617,595

986,852

153,958

248,739

4.0114

up

110

 

0.0001

SIG

SIG

39,356

97,166

220,462

387,308

5.6017

down

210

 

0.0004

NS

SIG

757,282

605,616

285,096

446,196

2.6562

up

303

 

0.0002

SIG

SIG

173,237

616,608

633,878

976,099

3.6590

down

306

 

0.0022

NS

SIG

8,004

5,915

149,834

274,377

18.7195

down

307

 

0.0275

SIG

NS

759,264

688,274

302,438

516,028

2.5105

up

408

 

0.0000f

SIG

SIG

258,525

539,218

856,239

1,833,144

3.3120

down

410

 

0.0041

SIG

NS

26,580

63,389

118,326

141,769

4.4517

down

411

 

0.0295

SIG

NS

165,226

431,575

708,030

1,274,849

4.2852

down

412

 

0.0054

SIG

SIG

447,495

1,004,238

310,856

722,114

1.4396

up

505

*

0.0470

NS

NS

925,741

886,816

477,691

544,289

1.9379

up

601

*

0.0318

NS

NS

333,862

424,610

725,308

843,336

2.1725

down

602

 

0.0000f

SIG

SIG

365,242

873,890

1,394,639

1,721,661

3.8184

down

603

 

0.0076

SIG

NS

1,533,077

1,657,207

1,053,773

1,360,574

1.4548

up

606

 

0.0216

SIG

SIG

43,523

126,690

8,417

8,283

5.1708

up

610

 

0.0001

SIG

SIG

146,957

447,440

8,417

8,283

17.4593

up

704

 

0.0000f

SIG

SIG

68,064

212,603

899,841

2,069,461

13.2205

down

804

*

0.0013

NS

NS

8,004

5,915

148,985

250,618

18.6134

down

1004

*

0.0023

NS

NS

8,004

5,915

175,554

294,631

21.9328

down

1005

 

0.0012

SIG

SIG

83,946

197,401

354,800

440,759

4.2265

down

1109

 

0.0000f

SIG

SIG

83,319

265,430

503,759

1,143,792

6.0462

up

1303

 

0.0012

SIG

SIG

364,487

1,135,686

155,251

282,766

2.3477

up

1309

*

0.0043

NS

NS

297,936

428,232

70,605

95,878

4.2197

up

1401

*

0.0075

NS

NS

1,381,518

1,428,751

573,957

882,187

2.4070

up

1402

*

0.0168

NS

NS

1,221,781

1,194,937

2,454,219

2,386,797

2.0087

down

1405

 

0.0219

SIG

NS

536,821

589,544

164,445

364,867

3.2644

up

1704

 

0.0076

NS

SIG

107,476

344,505

651,980

1,109,723

6.0663

down

2101

*

0.0238

NS

NS

1,068,400

1,220,098

824,152

883,156

1.2964

up

2107

 

0.0158

SIG

SIG

65,274

230,953

293,355

519,761

4.4942

down

2312

*

0.0284

NS

NS

350,710

704,649

8,417

8,283

41.6664

up

2506

*

0.0469

NS

NS

1,897,806

1,096,881

2,497,681

1,221,943

1.3161

down

2507

*

0.0012

NS

NS

417,660

599,615

1,048,242

1,050,906

2.5098

down

2611

 

0.0498

SIG

SIG

1,535,001

1,783,491

2,205,570

2,796,718

1.4369

down

2614

 

0.0000f

SIG

SIG

8,004

5,915

285,838

432,421

35.7112

down

2615

 

0.0008

SIG

SIG

62,288

208,601

431,929

1,018,078

6.9344

down

2801

 

0.0000f

SIG

SIG

8,004

5,915

869,550

1,285,546

108.6371

down

2803

 

0.0010

SIG

SIG

712,668

1,115,376

580,029

825,345

1.2287

up

2806

 

0.0001

SIG

SIG

443,822

1,084,784

129,571

442,770

3.4253

up

3201

*

0.0016

NS

NS

1,336,983

1,205,431

794,583

908,778

1.6826

up

3305

 

0.0000f

SIG

SIG

89,683

248,936

303,148

448,565

3.3802

down

3306

 

0.0078

SIG

SIG

116,998

272,340

59,537

124,166

1.9651

up

3404

 

0.0082

SIG

NS

4,068,995

2,823,334

3,452,364

2,764,490

1.1786

up

3406

 

0.0000f

SIG

SIG

44,870

125,570

462,745

547,706

10.3130

down

3412

 

0.0177

NS

SIG

322,035

683,636

130,942

334,190

2.4594

up

3502

 

0.0188

SIG

NS

442,850

650,679

786,812

1,011,948

1.7767

down

3607

*

0.0105

NS

NS

2,247,225

2,722,559

1,430,733

2,093,163

1.5707

up

3702

*

0.0001

NS

NS

633,076

1,243,273

3,792,473

3,730,678

5.9905

down

3708

*

0.0096

NS

NS

633,632

796,621

407,271

620,398

1.5558

up

3802

 

0.0015

SIG

SIG

8,004

5,915

111,820

262,030

13.9703

down

3905

*

0.0039

NS

NS

8,004

5,915

137,656

242,371

17.1980

down

4106

 

0.0066

SIG

SIG

105,844

212,452

397,149

845,289

3.7522

down

4201

 

0.0010

NS

SIG

8,004

5,915

254,207

454,577

31.7593

down

4305

 

0.0000f

SIG

SIG

46,506

128,075

470,932

768,247

10.1263

down

4706

*

0.0481

NS

NS

2,739,148

3,047,552

917,692

1,062,566

2.9848

up

4802

 

0.0000f

SIG

SIG

48,748

143,131

408,963

769,834

8.3894

down

4906

*

0.0302

NS

NS

873,770

1,402,616

628,704

1,038,074

1.3898

up

5301

 

0.0202

SIG

SIG

450,470

363,776

399,574

418,863

1.1274

up

5303

*

0.0062

NS

NS

3,456,984

1,373,851

5,360,468

2,202,726

1.5506

down

5505

*

0.0009

NS

NS

604,226

742,627

2,522,891

2,358,561

4.1754

down

5601

*

0.0063

NS

NS

456,028

681,368

1,497,166

1,370,793

3.2831

down

5605

 

0.0000f

SIG

SIG

124,350

177,741

8,004

5,915

15.5356

up

5606

 

0.0000f

SIG

SIG

110,019

153,695

8,004

5,915

13.7452

up

5703

*

0.0297

NS

NS

3,051,574

5,434,263

478,287

1,621,642

6.3802

up

5802

*

0.0011

NS

NS

1,190,828

915,352

508,682

528,700

2.3410

up

5908

 

0.0010

SIG

SIG

161,910

387,868

8,004

5,915

20.2282

up

5909

 

0.0012

SIG

SIG

8,417

8,283

143,259

352,729

17.0200

down

6208

 

0.0297

SIG

SIG

186,317

276,277

80,227

222,514

2.3224

up

6303

 

0.0328

NS

SIG

253,951

331,973

397,930

349,663

1.5670

down

6304

 

0.0018

NS

SIG

2,888,828

3,401,904

820,317

1,445,597

3.5216

up

6306

*

0.0099

NS

NS

1,800,296

1,365,419

2,730,178

2,859,788

1.5165

down

6501

*

0.0318

NS

NS

1,468,936

1,183,570

604,825

1,255,327

2.4287

up

6707

*

0.0193

NS

NS

11,970,218

12,515,695

6,247,307

6,508,633

1.9161

up

6709

*

0.0238

NS

NS

8,974,280

5,564,143

5,289,975

6,857,853

1.6965

up

6710

*

0.0238

NS

NS

1,400,824

2,287,877

404,306

569,323

3.4648

up

6807

 

0.0070

SIG

SIG

118,758

291,241

8,004

5,915

14.8370

up

6808

 

0.0009

SIG

SIG

1,174,714

1,224,320

239,503

580,829

4.9048

up

6809

 

0.0002

SIG

SIG

376,863

587,605

95,365

213,349

3.9518

up

7009

 

0.0004

SIG

SIG

175,933

391,166

265,335

410,728

1.5082

down

7106

 

0.0000f

SIG

SIG

962,980

1,282,010

39,820

101,833

24.1835

up

7107

*

0.0009

NS

NS

387,387

501,608

50,505

130,300

7.6703

up

7110

*

0.0245

NS

NS

84,648

139,984

256,705

307,196

3.0326

down

7111

 

0.0000f

SIG

SIG

658,929

1,249,222

148,370

341,871

4.4411

up

7112

 

0.0022

NS

SIG

73,598

133,128

8,004

5,915

9.1949

up

7203

 

0.0261

SIG

NS

1,311,783

732,419

927,380

799,253

1.4145

up

7207

 

0.0186

SIG

NS

60,957

126,879

21,092

40,907

2.8901

up

7303

 

0.0052

SIG

SIG

246,921

602,207

615,797

1,105,070

2.4939

down

7408

*

0.0126

NS

NS

3,049,802

2,337,843

1,336,825

1,182,165

2.2814

up

7510

 

0.0098

SIG

SIG

305,493

558,423

397,578

1,073,627

1.3014

down

7602

*

0.0412

NS

NS

7,296,723

7,488,294

3,609,984

5,505,222

2.0213

up

7609

 

0.0016

SIG

SIG

381,570

946,188

8,004

5,915

47.6713

up

7803

*

0.0238

NS

NS

716,697

474,399

341,082

391,607

2.1012

up

7808

*

0.0075

NS

NS

704,023

626,350

305,591

487,526

2.3038

up

7809

*

0.0267

NS

NS

582,174

1,475,329

8,004

5,915

72.7338

up

7810

*

0.0297

NS

NS

442,246

1,119,936

8,004

5,915

55.2519

up

8002

*

0.0082

NS

NS

175,231

300,741

22,986

74,010

7.6235

up

8104

*

0.0188

NS

NS

1,096,503

874,012

394,837

541,448

2.7771

up

8202

 

0.0230

SIG

NS

29,407

58,674

56,069

97,123

1.9066

down

8303

*

0.0311

NS

NS

1,850,424

1,058,481

1,293,017

1,164,851

1.4311

up

8309

*

0.0008

NS

NS

340,383

449,968

902,563

716,532

2.6516

down

8311

 

0.0318

SIG

NS

85,387

133,171

26,149

58,170

3.2654

up

8312

*

0.0498

NS

NS

306,017

504,921

549,086

736,070

1.7943

down

8503

*

0.0193

NS

NS

587,330

907,526

123,323

558,974

4.7626

up

8506

*

0.0010

NS

NS

1,709,206

2,026,022

334,292

701,018

5.1129

up

8508

*

0.0158

NS

NS

1,395,005

1,198,636

633,833

820,628

2.2009

up

8512

*

0.0012

SIG

NS

8,417

8,283

1,173,776

1,657,190

139.4515

down

8604

 

0.0267

SIG

SIG

2,262,933

3,169,913

1,147,023

2,200,353

1.9729

up

8607

 

0.0020

SIG

NS

1,865,790

3,095,124

369,996

861,448

5.0427

up

8708

*

0.0076

NS

NS

177,743

461,377

632,706

1,403,084

3.5597

down

8801

*

0.0316

NS

NS

751,522

573,471

517,615

499,829

1.4519

up

8802

*

0.0034

NS

NS

511,552

398,150

269,962

349,834

1.8949

up

9302

*

0.0238

NS

NS

710,525

848,447

1,268,492

1,442,080

1.7853

down

9305

 

0.0085

SIG

SIG

301,789

462,883

368,200

614,918

1.2201

down

9401

 

0.0192

SIG

NS

717,956

641,915

419,853

410,107

1.7100

up

9402

 

0.0096

SIG

SIG

63,499

152,855

120,990

362,645

1.9054

down

9802

 

0.0021

SIG

SIG

112,023

263,531

8,004

5,915

13.9956

up

An asterisk to the right of the spot number indicates a spot with significant BBD effect and non-significant STAND and STANDxBBD interaction effect, and are the primary targets for biomarker development.

aBBD effect with q-value <0.05. q-value controls the False Discovery rate, and q<0.05 allows for no more than 26 false positives in the 425 spots tested.

bSTAND effect with p<0.0001 (Bonferroni adjusted critical value).

cSTANDxBBD interaction effect with p<0.0001 (Bonferroni adjusted critical value).

dGaussian image spot intensity after normalization expressed in counts. Mean is the mean over the three technical replicates per tree for all trees in that disease condition. SD is standard deviation.

eRatio is ratio of larger mean over smaller mean.

Mean = Gaussian image spot intensity after normalization expressed in counts. Mean is the mean over the three technical replicates for all trees in that disease condition.

SD = standard deviation.

fQ-values displayed as 0.0000 are positive values <0.0001. See additional table 1 for actual q-values.

Spot selection and LC-MS/MS analysis

Spots were selected for coring and sequencing based upon the BBD effect being significant (including some with other significant effects as well) and the location of the spot in the gel being conducive to excising a clean gel core (only one protein in the core, some areas were too densely packed to allow coring). The trees 1504 and CM02d were chosen for use in preparative gels because these trees contained the most proteins of interest (33 and 26 respectively). Attempts were made to sample all BBD significant spots in these trees, and images were carefully evaluated after spot cutting to verify the intended spot was recovered for analysis. In addition to the spots of interest, several well isolated spots were cored for quality control purposes. A total of 28 gel spots (20 BBD significant, 4 control, and 4 cut from both gels) were successfully recovered and analysed by LC-MS/MS. Of the 50 highest interest spots, 15 were successfully cored and sequenced. Resulting peptide spectra were identified by matching to NCBI datasets, or in a 2-phase matching strategy matched to beech EST's that were then matched to NCBI sequences. Of the 28 spots sequenced, 20 were identified (highest scoring matches or ties only, Table 4) based upon homology to known plant sequences, or homology of the matched EST to plant sequences (including all four control spots). Of the 15 sequenced from the 50 highest interest spots, 11 were identified by sequence homology. There are a few cases where spots were matched to more than one significant identification (spots 307, 2101, and 7408), but in two of them identical peptides returned multiple database entries with different annotations (spots 307 and 2101). The use of the EST database in spot identification greatly improved the success rate at identifying proteins, as over half of the identifications were made using the EST database and would have been unidentified had only Genbank been used. The majority of the spots that were identified based on sequence homology have been shown to be stress-related in other plant systems (Table 5).
Table 4

Protein spots analyzed by MS/MS

Spot

number

Tree & core

number

Protein ID or EST homology

[PRIDE accession: 17706] a

NCBI or EST Accession

Number b

Mascot

Score c

Percent

Coverage

Matching

peptides

Peptide sequence d

Peptide score e

m/z

Mr(expt)

109*f

1504_B6

polyphenol oxidase

ABWP2_088861_1753_2628

180

33

2

TEFAGSFVNVPSK

72.2

692.1743

1382.334

      

4

GVATIGGIK

47.2

408.4638

814.913

      

2

GVATIGGIK IQFVS

233.8

695.8282

1389.6418

      

2

IQFVS

26.9

593.343

592.3357

307

1504_H5

unnamed protein product

gi|157327346

735

44

2

NILFVISK PDVFK

71.3

760.6617

1519.3088

      

4

SPTSDTYVIFGEAK

73.9

758.1431

1514.2716

      

6

IEDLSSQLQTQAAEQFK

107.3

969.241

1936.4674

      

2

DIELVM TQAGVSR

75.7

718.1302

1434.2458

      

4

DIELVMTQAGVSR

98.1

709.7012

1417.3878

307

1504_H5

alpha nascent polypeptide

CCWP2_066819_3631_1428

255

52

2

NILFVISK PDVFK

71.3

760.6617

1519.3088

  

associated complex

   

4

SPTSDTYVIFGEAK

74.8

758.1431

1514.2716

      

5

IEDLSSQLQTQAAEQFK

108.5

969.241

1936.4674

505

1504_A6

none

none

       

1004*

CM02d_C1

thioredoxin

WOA_067025_0312_1563

120

25

2

VNTDESSSIATR

76.5

640.5275

1279.0404

      

2

STLTTSIEK

46.2

490.4947

978.9748

2101*

1504_G7

ribonuclease activity regulator

CCNHS_110619_3244_2001

214

24

2

VFEDNVLVR

57.1

546.048

1090.0814

      

2

VLVVDGGGSLR

67.6

537.0695

1072.1244

2101*

1504_G7

dimethylmenaquinone

gi|15232963

199

21

4

ALQPVFQIYGR

46.8

647.1753

1292.336

  

methyltransferase family protein

   

2

VLVVDGGGSLR

78.9

537.0695

1072.1244

      

2

DVDEINGCDIGVR

66.5

731.6575

1461.3004

2101*

CM02d_C3

Eukaryotic translation initiation

gi|3024017

75

19

1

ELVFK EDGQEYAQVLR

45.4

642.4005

1924.1797

  

factor 1A

   

2

DYQDDK ADVILK

52.4

712.098

1422.1814

3101

CM02d_E5

heat shock protein

ROA_067935_0424_3607

120

22

2

ADLPGLK

39.2

357.4087

712.8028

  

[homologous match]

   

1

K EEVK VEVEEGR

32.3

477.7563

1430.2471

      

2

EEVK VEVEEGR

53.2

652.0981

1302.1816

4103

1504_F5

17.7 kDa heat shock protein

gi|1235898

238

26

2

ADLPGLK

39.7

357.3622

712.7098

      

7

K EEVK VEVEEGR

79.9

715.8541

1429.6936

      

5

EEVK VEVEEGR

58.7

652.0201

1302.0256

      

5

VLQISGER

30.3

451.5257

901.0368

      

5

AAMENGVLTVTVPK

32.4

723.6349

1445.2552

5001

CM02d_F5

stress and pathogenesis-related

gi|3901018

858

65

3

GVFTYESENTSVIPPAR

84.8

934.2761

1866.5376

  

protein

   

13

AFVLDADNLIPK

30.5

659.2544

1316.4942

      

2

AFVLDADNLIPK VAPQSIK

42.0

680.6581

2038.9525

      

7

STETLEGDGGPGTIK

44.6

731.3245

1460.6344

      

5

STETLEGDGGPGTIK K

66.3

795.6741

1589.3336

      

3

K ITFGEGSQFK

60.5

414.6845

1241.0317

      

4

ITFGEGSQFK

49.7

1113.4546

1112.4473

      

1

ISYEIK IVASPDGGSLLK

25.6

630.7996

1889.377

      

6

IVASPDGGSLLK

63.3

578.6285

1155.2424

      

2

GNHEIK EEEVK

55.8

656.6091

1311.2036

      

10

AVEAYLLAHPDAYN

47.7

774.1621

1546.3096

5002

1504_G5

superoxide dismutase

gi|38228697

263

40

1

HAGDLGNVNVGDDGTVSFTIIDK

72.8

782.4584

2344.3534

      

4

QIPLCGPNSIIGR

53.1

713.6349

1425.2552

      

4

AVVVHGDPDDLGK

55.5

661.611

1321.2074

      

2

STGNAGGRIACGIIGLQG

45.4

851.7599

1701.5052

      

4

IACGIIGLQG

45.6

1001.4746

1000.4673

5303*

1504_F7

triosephosphate isomerase

ABWP2_034459_1554_1107

250

52

3

VASPAQAQEVHFGLR

66.9

805.7179

1609.4212

      

3

K WLQVNTSPEVAATTR

48.9

601.354

1801.0402

      

1

IIYGGSVNGANCK

63.6

677.621

1353.2274

7107*

1504_B7

unknown, CBS domain protein

CCWP1_110480_2642_0865

274

53

2

VGDIM TEENK

49.3

576.5307

1151.0468

      

2

VGDIM TEENK LITVTLDTK

59.3

713.168

2136.4822

      

1

LITVTLDTK

45.3

502.5542

1003.0938

      

2

GM IGM VSIGDVVR

84.6

683.4301

1364.8456

      

2

LNAFIQGGY

38.5

982.3546

981.3473

7110*

1504_C7

unknown, CBS domain protein

CCWP1_110480_2642_0865

230

53

2

VGDIM TEENK

56.2

576.5154

1151.0162

      

2

VGDIM TEENK LITVTLDTK

61.3

713.1682

2136.4828

      

2

GM IGM VSIGDVVR

80.2

683.6461

1365.2776

      

2

LNAFIQGGY

40.0

982.4321

981.4248

7203

CM02d_H3

none

        

7408*

CM02d_A4

annexin

ACHS1n_159819_2640_1758

209

66

2

WTSSNQVLMEIACTR

84.0

906.6581

1811.3016

      

2

SSHDLLLAR

37.8

506.6234

1011.2322

      

1

SLEEDVAYHTTGDFR

28.8

580.6663

1738.9771

      

2

YEGDEVNMTLAK

60.4

693.4765

1384.9384

7408*

CM02d_A4

isoflavone reductase like

ABWP1_026351_0296_1851

203

37

1

SKILIIGGTGYIGK

57.7

710.562

1419.1094

      

2

ILIIGGTGYIGK

70.4

602.8907

1203.7668

      

2

SGHPTFALVR

36.0

542.7687

1083.5228

      

2

ESTVSDPVK

38.7

481.347

960.6794

7602*

CM02d_B4

dehydrin

ABWP1_045441_0451_3051

225

53

1

VFEKIPGAGNK

46.8

580.3038

1158.593

  

[homologous match]

   

1

IPGAGNKDR

40.9

465.1174

928.2202

      

2

DR VQGEQYR

41.0

576.0114

1150.0082

      

1

GAM DKVFEK

30.4

521.1149

1040.2152

      

2

IPGAGNKDK

43.2

450.2458

898.477

      

2

DKVQGDQYR

34.7

555.0321

1108.0496

8002

CM02d_E4

none

none

       

8104*

1504_C9

glycine-rich RNA binding protein

ABWP1_015278_0178_1196

100

24

2

AFSPYGEILESK

49.6

671.1248

1340.235

  

[homologous match]

   

2

NITVNEAQSR

50.5

566.5034

1130.9922

8104*

CM02d_F4

glycine-rich RNA-binding protein

ABWP1_015278_0178_1196

97

25

2

AFSPYGEILESK

50.4

671.1497

1340.2848

  

[homologous match]

   

2

GFGFVTFSNEK

46.6

617.2156

1232.4166

8202

1504_D9

none

none

       

8303

1504_H8

none

none

       

8309

1504_A9

none

none

       

8309

CM02d_A2

none

none

       

8506*

1504_F8

glyceraldehyde 3-phosphate

ROB_045015_1217_3381

176

57

2

AASFNIIPSSTGAAK

39.8

718.1123

1434.21

  

dehydrogenase, cytosolic

   

2

VPTVDVSVVDLTVR

82.6

750.6729

1499.3312

      

2

FGIVEGLM TTVHSITATQK

54.0

684.0022

2048.9848

8607

1504_E8

glyceraldehyde-3-phosphate

gi|120666

237

21

2

FGIVEGLMTTVHSITATQK

38.7

684.0699

2049.1879

  

dehydrogenase, cytosolic

   

2

AASFNIIPSSTGAAK

47.5

718.1649

1434.3152

      

2

VPTVDVSVVDLTVR

88.8

750.368

1498.7214

      

4

AGIALNDNFVK

37.9

581.4758

1160.937

      

2

LVSWYDNEWGYSTR

72.0

888.5875

1775.1604

8607

CM02d_C2

Ran binding protein

ABWP1_041027_0417_3079

146

33

4

LEEVAVTTGEEDETSILDLK

117.4

1096.213

2190.4114

      

2

FDKDGNQWK

28.9

569.5167

1137.0188

9302*

1504_G8

germin-like/unknown/predicted

CCNHS_162322_3610_0519

282

48

3

IPGLNTLGVSLSR

85.5

663.9008

1325.787

      

3

IDYAPGGLNPPHTHPR

77.0

872.1409

1742.2672

      

2

SIKKGEIFVFPK

66.0

697.1452

1392.2758

      

4

KGEIFVFPK

53.6

532.7322

1063.4498

9401

CM02d_B5

none

        

aIdentification of the NCBI protein match, or for EST peptide matches, the BLASTP identification of the EST. Mass spectrometry data and identifications have been submitted to the PRIDE database. See the field ‘gel spot identifier’ in the Identification Detail View to review individual spot spectra and identifications.

bNCBI number of the matching protein, or the Fagaceae genomics EST ID for peptided matching EST's.

cMASCOT score is a probability based score. Reported scores are sums of peptide identifications that are significant (except homologous where noted), and higher scores are equivalent to lower probability of an equal or better match occurring at random.

dUnderline indicates an oxidized residue. Bold indicates a missed trypsin cleavage.

ePeptide mascot score for each peptide (or the highest match for multiple peptides).

f * indicates spot with significant BBD effect and non-significant STAND and STANDxBBD interaction effect, and are the primary targets for biomarker development.

Table 5

Previously published stress response of proteins identified by MS/MS

Category

Protein (or protein family)

Spot(s)

Quantification in healthy trees relative to diseased trees

Previously published stress response

References

reactive oxygen species

polyphenol oxidase

109

up

wound, pathogen, insect

[1719]

glyceraldehyde-3-phosphate dehydrogenase

8506 8607, 1st ID

up

hypersensitive response mimic, hydrogen peroxide, wounding

[2023]

annexin

7408, 1st ID

up

oxidative stress

[2426]

thioredoxin

1004

down

oxidative stress

[2730]

stress and pathogenesis related proteins

dehydrins

7602

up

drought, low temperature, freezing salinity, abscisic acid treatment

[3133]

isoflavone reductase-like protein

7408, 2nd ID

up

lignification

[34]

germin-like proteins

9302

down

pathogen, aluminum, wounding

[3539]

transcription and translation

eukayrotic translation initiation factor (eif1)

2101

up

salt

[40, 41]

glycine-rich RNA binding protein

8104

up

hydrogen peroxide, flooding, cold, drought, salinity, abscisic acid treatment

[42, 43]

alpha nascent polypeptide associated complex 1 (αNAC1)

307

up

salt, systemic acquired resistance inducer

[4446]

Ran binding protein(RanBP)

8607, 2nd ID

 

auxin treatment

[47]

unknown, or previously metabolic only

triose-phosphate isomerase (TPI)

5303

down

systemic acquired resistance inducer, abscisic acid treatment, pathogen

[4649]

 

CBS domain containing protein

7107, 7110

up, down

stress responsive

[50]

Many of the proteins with a BBD effect that were identified by MS/MS are previously published as stress responsive in other plant systems.

Utility of the analysis to narrow the biomarker candidate pool

In order to illustrate the discriminatory power of our approach we have illustrated the spot set reduction strategy in Figure 3. Beginning with the 987 total protein spots identified, we show how at each step some spots are discarded from further consideration as a biomarker. The final set for continued biomarker considerations is eleven spots that have a BBD effect only (not confounded by stand effects or interactions between BBD and STAND) and are identified by their sequence homology.
https://static-content.springer.com/image/art%3A10.1186%2F1477-5956-11-2/MediaObjects/12953_2012_Article_405_Fig3_HTML.jpg
Figure 3

Flow chart illustrating the reduction in the number of protein spots of interest using our experimental and statistical approach. The flow chart shows the starting number of protein spots and how many of the spots were eliminated from further consideration at each step or decision point. The dramatic reduction in the number of spots of interest to only 11 spots demonstrates the discriminatory power of the experimental and statistical approach.

Discussion

Challenges of proteomic investigation of forest trees

In general, protein extraction from plant tissue is technically challenging due to the high proportion of contaminants relative to the low concentration of protein. Proteomics in forest trees is further complicated by the complexity of working with trees as an experimental system due to factors such as their large size, long life cycle, and large genome [51]. In contrast to most proteomics studies conducted on model organisms, our subjects are wild, unrelated, mature trees selected from multiple stands. Like many forest trees, American beech is wind-pollinated and has a low self-pollination rate [52], resulting in high heterozygosity among trees within stands [53]. We selected trees from eight non-contiguous stands, further decreasing any chance of relatedness between trees across the study and likely increasing the number of alleles per locus sampled [54]. These factors lead to our study having a much higher degree of genetic complexity in the sampling units than is generally encountered in proteomics work where the use of inbred lines, clones, or pooling across genotypes is common. In addition, the multi-component nature of beech bark disease also adds to the complexity of protein patterns. Due to BBD having both an insect and a fungal component, both wound/insect and pathogen responsive genes are likely to be detected in diseased trees. In addition, BBD develops over a time scale of months or years, rather than the time course of days often studied in wound (insect), gene-for-gene, or viral pathosystems. BBD develops as a chronic disease, with significant associated bark damage including cracking, callous formation [1], and likely secondary local stress effects such as dehydration or nutrient and photosynthate transport disruptions. These bark stress factors may induce other, poorly understood sets of stress responses. The signalling pathways triggered by these multiple sources of stress are also likely interconnected through their regulatory pathways, which in other plant systems have been shown to be antagonistic, further adding to the complexity and variation of the response of the tree to the causal components of beech bark disease [55].

A high level of genetic diversity across geographical regions was previously reported in maritime pine [54]. In our study, genetic diversity across geographical regions is combined with the complexity of host responses to a dual pathogen disease system making the high number of spots identified in samples (up to 522 per tree, total of 987) not unexpected. Resolution of similar ranges of spot numbers (from 451 up to 753) have been previously reported in other plant species using similar supplies, equipment, and technical approaches [5660]. Technical effects were significant for only six spots indicating that the observed differences between gels and samples were accounted for by the experimental design and analysis. Bark samples were anecdotally different for some visible features (such as chlorophyll content) that may vary by season, micro-site, or sample. Several spots that were not identified by tandem mass spectrometry had some (non-significant) homology to Fagaceae EST's, suggesting matching of unidentified peptides may be possible as additional woody plant, bark, and particularly American beech sequences become available. The low number of identified proteins in forest tree proteomic studies in general is a direct reflection of the lack of genomic DNA, EST and protein sequence data entries in public databases [51] for forest trees and for woody tissues.

The genetic complexity of the sample unit, the sampling across a wide geographic area, and the complexity of the BBD phenotype all contribute to possible protein differences between trees in the study. This complexity, especially combined with typical technical sources of variation, required careful study design and more elaborate statistical considerations than many proteomics studies. Identification of up to 101 protein spots unique to an individual tree emphasizes the genetic diversity captured in our study. One-hundred-twenty protein spots (22.5% of the matched spots) were identified as BBD significant despite the experimental complexity, so the experiment was effective at finding proteins of interest. We controlled the false discovery rate to 5% (using the q-value), so we would expect only six of the 120 proteins identified as differentially expressed to be erroneous.

Sequenced spots have homology to known stress, insect, and pathogen related proteins in other plants

Most of the proteins identified by MS/MS in this study have homology to proteins known to be involved in stress responses in other plants (Table 5). The variety of biological responses to which these proteins are linked are consistent with the complexity of BBD. Since BBD has both an insect and a fungal component, it is not unexpected that both insect and pathogen related proteins would be found. BBD is also a long-term disease with both bark damage and significant healing as part of the physiology of the disease. Wounding, reactive oxygen species and drought responsive proteins are also expected. The variety of stress responses apparently influenced by BBD along with the identification of proteins involved both in transcription and translation control and basic metabolic responses, supports a model that beech trees have an active physiological response to BBD. These proteins are good targets for further research to understand the pathways involved in response to BBD.

The proteins identified in this study also expand on recent studies of the proteome of European Beech (Fagus sylvatica L.) [6163]. While our study is the first publication of the proteome of bark tissue or any tissue from mature trees from Fagus, a few proteins identified in our study were also identified in other studies in Fagus. Of particular interest, Valcu et al. [63] identified triose-phosphate isomerase (TPI) as having a lower protein level in wounded leaves, while we found the protein to have higher expression in the bark of BBD diseased trees. Beech trees afflicted with beech bark disease would be responding to both scale feeding and pathogen infection (Neonectria spp.). The higher expression of TPI in diseased trees is consistent with the reports of higher expression of TPI in response to a fungal pathogen in Brassica carinata [48]. Further study of TPI to understand its role in defense in different tissues and different stages of beech bark disease infestation is certainly warranted.

Biomarker candidates for further analysis

The long term goal of our research program is to identify broadly useful markers for BBD resistance that forest managers can use to plan for and mitigate BBD damage as it spreads to new regions and stands. A resistance biomarker could also be used to expedite the selection and breeding of scale-resistant trees in on-going tree improvement programs [10].

Twelve of the sequence identified spots (marked with an asterisk in both Table 3 and Table 4) showed a significant BBD effect and no effect of stand or BBDxStand interaction effect. Proteins different in healthy versus diseased trees regardless of stand (i.e. across stands) are the best biomarker candidates. These proteins are the most likely to be linked to resistance or susceptibility across broad geographic and genetic ranges, and so are the highest priority for follow-up study and biomarker development. Quantitation of these proteins in additional trees, especially in trees from other stands and regions, will be important in determining if these proteins can be used as biomarkers. Additional 2-DE gel studies, or development of antibody based (western blot, or ELISA) methods will facilitate this. RNA expression studies may also be helpful in understanding which proteins (genes) are the best markers for BBD resistant trees. An additional five spots that weren't identified by sequence homology fit into the same class and may be identified in the future as additional sequence becomes available. Often spots unidentified by MS/MS are dropped from further study. But given both the low amount of Fagus sequence available for comparison and the low amount of sequences in the database from bark tissues and forest trees in general, these spots are still good candidates for further study for biomarker development.

Proteins for which BBD effect is significant along with stand, interaction, or both require more careful interpretation, but may still become useful biomarkers. In combination with significant stand and BBD effects, a significant interaction effect means that the direction of association of a BBD effect (e.g. increased protein in healthy trees) is not consistent in the different stands. When STAND and BBD effects are both significant (but with a non-significant interaction effect), the protein may not be consistently distributed in American beech and may be present in only some of the stands managers wish to screen. Both of these groups of proteins are less attractive as biomarker candidates, but interpretation of them along with other higher priority candidate proteins may be insightful.

Most of the proteins identified by sequence have been found to be responsive to stress, insect, or pathogens in other plant systems, and are differentially expressed between the healthy and diseased trees. A small number of these proteins will be selected for further study, with priority given to those that are higher in the healthy trees and are predicted to be involved in insect resistance because of the known scale-insect resistance of the healthy trees and the requirement of scale feeding induced bark wounding to provide subsequent entryway for Neonectria infection [1, 2, 7]. Polyphenol oxidase and glyceraldehyde dehydrogenase both fall into this class and are considered high priority for further analysis. It will be important to continue validation of expression results using additional trees from the same stands, and additional stands from new geographic regions. In addition, the best biomarker protein may not be the one identified in this study, but rather a protein acting upstream in a response pathway, or regulating a response pathway(s). Further characterization of the biochemical pathways, and their induction through time, season, and spatially through the tree will be important. It is possible the proteins identified in this study will coincide with quantitative trait loci for scale resistance.

Conclusions

American beech is an ecologically important species in many North American forests, only a portion of which are currently impacted by BBD [64]. Development of management options to reduce the economic losses and ecological costs of BBD are critically needed. This study has identified protein spots differentially expressed in the bark of healthy, scale-resistant trees and BBD-susceptible trees. This identification suggests that American beech has an active physiological response to BBD. Confirming this response is an important first step in understanding how BBD may progress physiologically and mechanistically in BBD susceptible trees, and how BBD resistance may be manifested. Additionally the results of this study should support and complement on-going strategies to find biomarkers for BBD resistance.

Methods

Selection of beech trees and collection of bark samples

Ten healthy trees were identified in seven stands in Fredericton, New Brunswick, Canada (Table 1). Healthy trees comprised only 5% of the beech trees in this area and were included in this study only if they were greater than 15 cm DBH [16]. All of these stands have been under attack by both Cryptotoccus fagisuga and Neonectria spp. since the early 1930’s [1] and would be considered an aftermath forest. After initial mortality waves, the remaining trees in an aftermath forest are primarily heavily cankered and a lower density persistent scale infestation is present in the stand. Diseased trees were selected along with healthy trees in five of the stands. A healthy tree (greater than 10 cm DBH) and a susceptible tree from Ludington, MI., USA were also included in this study. Beech scale is estimated to have been established in Ludington as early as 1990 and the presence of Neonectria was confirmed in 2001 [11]. At the time of tissue collection, 2004, this was considered a killing front. All diseased trees selected for this study showed visible signs of Neonectria infection such as cankers or the presence of perithecia and scale infestation.

The experimental sampling is unbalanced with respect to disease resistance because the primary interest is in resistant genotypes for breeding (more resistant trees are selected for grafting, testing, and planting). Modern statistical algorithms and computer power are sufficient to allow significant imbalance in experiments to be modelled, and we take advantage of this in our experimental design. All trees selected were tested for resistance to the beech scale insect in studies reported previously [16, 65] and summarized in Table 1. These tests demonstrated that all the healthy trees were resistant to the beech scale insect and all of the diseased trees were susceptible to the beech scale insect.

Branches, 1–2 cm in diameter, were removed from the crown of each of the selected trees in October, 2006 in the New Brunswick stands and in September 2004 in Ludington, MI. Bark was peeled from the branches with a potato peeler and bark strips were placed in labelled 50mL falcon tubes, flash frozen and stored in liquid Nitrogen or a dry ice ethanol bath on site. Peeled bark collected from each tree was divided among three tubes and transferred to a −80°C freezer for storage either at the Natural Resources Canada Lab in Fredericton, New Brunswick, Canada or the US Forest Service (USFS) Lab at Delaware, Ohio, USA. In February of 2007, samples from New Brunswick, Canada were shipped overnight on dry ice to Delaware, OH, USA.

Protein extraction

Protein was extracted according to Bona et al. [66] with minor modifications to account for the high soluble phenolic content of tree bark and phloem tissues. Bark tissue from each tree was combined with dry ice and ground to a course powder in a standard household coffee grinder and then transferred to a −80°C freezer. Three technical replicates were produced from the tissue from each tree. For each replicate, 2g of powdered tissue (after dry ice sublimated off) were combined with 2g of frozen polyvinyl-polypyrrolidone and 20mL of lysis buffer (as per Bona et al. [66] except that 1% Sigma Plant Proteinase inhibitor cocktail, P-9599, was used in place of 2% phenylmethylsulfonyl floride/dimethyl sulphoxide) and homogenized using a tissue homogenizer (Janke & Kunkel, IKA Labortecknik, Ultra-Turrax T25 with 18N tip). The resulting homogenate was centrifuged at 26,000gn for 10 minutes at 4°C to pellet solids. The supernatant (generally 10 mL) was combined with 10 mL of tris(hydroxymethyl)aminomethane (Tris–HCl, pH 8.8) saturated phenol and mixed for one hour at room temperature. The phenolic phase was separated by centrifugation and rinsed with another 10 mL of lysis buffer, followed by further centrifugation to separate the phenolic phase. The final phenolic phase was recovered and proteins were precipitated by adding five volumes of methanol/0.1M ammonium acetate and incubating overnight at −20°C. Proteins were pelleted by centrifuging at 26,000gn for 20 minutes and the resulting pellet rinsed three times with cold methanol, once with cold acetone, and dried under vacuum. The pellet was resolubilized in 450uL of resolubilization buffer (Biorad ReadyPrep sequential extraction reagent II (8M Urea, 4% 3-[(3-cholamidopropyl)dimethylamonio]-1-propanesulphonate (CHAPS), 40mM Tris, 0.2% Bio-Lyte 3/10 ampholytes) plus 1% tris-butyl phosphate (TBP, Sigma T-7567) and 1% plant proteinase inhibitor cocktail). Proteins were quantified (in triplicate) using the Biorad RC DC protein assay kit (Biorad 500–0118) microfuge tube assay protocol with the optional second wash. Protein quality was checked by running 40μg of protein on a denaturing polyacrylamide gel and staining with coomassie stain as per standard protocols (Biorad mini-protean-3 cell Instruction Manual).

Two-dimensional electrophoresis (2-DE)

2-DE was conducted at the Plant Microbe Genomics Facility at The Ohio State University (OSU). Isoelectric focusing (IEF) was performed using 11cm pH 3–10 immobilized pH gradient strips (Biorad, 163–2014) in the Protean IEF Cell (Biorad). For quantitative gels, 100 μg of protein was mixed with rehydration buffer (7.0M Urea, 2.0M Thiourea, 2.0% w/v CHAPS, 2.0% w/v sulfo-betain 10 and focused for 55 kVH at 25°C. After IEF, the strips were treated according to the ReadyPrep Reduction-Alkylation kit (Biorad, 163–2090) which uses TBP for reduction, and iodoacetamide for alkylation. Second dimension separation was carried out on Criterion 8-16% Tris–HCl gels (Biorad 161–1394) using a Criterion Dodeca cell so that all eight gels in the replicate could be run in parallel. Gels were run at 200V for 60 minutes and then fixed for 30 minutes in a solution of 10% methanol and 6% acetic acid. Gels were then stained with 1x SYPRO-Ruby (Biorad, 170–3138) following manufacturer's instructions. Post staining, gels were de-stained for 1 hour in identical solution as that used for fixation. Preparative gels for spot cutting to recover proteins were prepared in the same way, except that 450 μg of protein was used per sample and gels were stained with Coomassie stain (Biorad, 161–0787) following manufacturer's instructions.

Image analysis and quantification

Gels were imaged on a VersaDoc imager (Biorad), and the software program PDQuest (version 8.0, Biorad) was used to conduct the image analysis, spot identification and quantification. Gel images were digitally cropped along the outer edge to remove the molecular size marker and gel edges, and to standardize image size, but both pI fronts and the full size resolving area were retained.

The spot selection and gel matching were conducted in two stages, first a separate master gel was created for each tree by auto-matching the three replicate gels using the 'create experiment' dialog boxes of PDQuest. For these tree master gels, the spot detection and automated spot matching are conducted as part of the same procedure. For spot detection we used the spot detection wizard with vertical streak reduction on, and selecting the user chosen reference spot for small spot, faint spot, and large spot cluster from the same region of the gel for all gels. In addition we selected the local area regression method of normalization, which is proprietary but appears to be based on similar microarray normalization methods (see Quackenbush [67]). For spot matching, we defined no groups and spots were added to the master image only if present in two of three gels. Auto-matched spots were manually checked and corrected by dividing the gel area into 81 quadrants and hand marking landmark spots in each quadrant present in all three gels. All of the matches were hand checked based upon these landmark spots, and manual corrections to the spot detection and auto-matching were made, including removal of spots detected on the unresolved pI fronts and gel edges.

The second phase of image analysis was to create a 'compare experiments' analysis including all sixteen individual tree 'master gels' (CM02d was software selected as the base master gel). Automated matching was used to create the initial master file, then all matches were manually checked. Additional spots were added to the master manually if they were present in two or more tree masters. We applied the same hand check quality control as for individual tree masters (manually checking and correcting each of 81 quadrants using landmark spots) and applied the same normalization method (local area regression model option). Of note, we did not incorporate an additional scaling factor and the normalization method doesn’t scale the data, so the final spot quantities still have the original unit of counts.

Once the compare experiment master gel was fully checked, a quantitative dataset was created. The quantitative dataset was output from PDQuest using the function Report| Quantity Table Report, with the settings: all matched spots checked, configuration set to 'individual gels', missing spots set to 'estimate', and saturated spots set to 'estimate'. Spot quantities were estimated so that analysis options that require balanced and nonzero datasets could be used. PDQuest estimates saturated spots by fitting a Gaussian spot to the edges only and extrapolating the peak, then calculating the estimated volume from the extrapolated value. PDQuest estimates missing spots as the value of a minimum detectable spot. The resulting report contained spot quantities for all spots in the master gel across all 48 experimental gels. Graphical analysis of the spot quantities by spot were deemed sufficiently normally distributed to proceed with modelling.

To be sure the unmatched spots that are unique to one tree were not artifacts related to low spot intensity or variance in protein quantification making it difficult to match them, a random check of the intensity distribution of unmatched spots was conducted. Four trees were selected at random, then the spot intensity output for all spots on the tree master for these four trees was examined both visually by inspecting spot intensities in the gels and by comparing the distribution of intensities of the unique spots to the matched spots. Comparisons were made both graphically and by using summary statistics (mean, minimum, Q1, median, Q3, and maximum values).

Experimental design and statistical analysis

The experiment included 16 trees, each of which had a location (stand) code and a disease condition code. Three replicate extractions (for three separate gels) were run per tree. For each replicate there were a total of 16 extractions, one per tree. Each protein extraction was assigned first to one of four extraction sets (three healthy and one diseased tree per set), then extraction sets paired to form gel sets (two gel sets per replicate). Samples in an extraction set were extracted in parallel, and gel sets were run in parallel for both IEF and polyacrylamide gel separation. Hence, each extraction had a full list of factors assigned: tree, stand, disease state, replicate, extraction set, gel set. The full dataset included these factors and spot quantities for each spot on the master gel for each of the 48 gels (see additional files: Additional file 2, full study design; Additional file 3, full set of spot quantities). Calculated spot quantities were from the normalized gel images, and were evaluated and determined to need no additional transformation. An ANOVA approach to statistical analysis was used to so that multiple effects and interactions could be included in the same model to better control error variance, and because the other biological effects (i.e. stand and its interaction with BBD) will be informative in selecting proteins for future study.

Statistical analysis was generated using SAS® software version 9.2 of the SAS system for Windows, copyright 2002–2008 SAS Institute Inc. SAS and all other SAS Institute Inc. products or service names are registered trademarks or trademarks of SAS Institute, Cary, NC, USA.

A series of tests were used to categorize each protein spot, to arrive at a list of candidate spots for further analysis (summarized in Figure 3). The first phase of the analysis sought to exclude constitutive proteins that did not differ between any trees, and assess the significance of the technical factors. The following model was fit using the General Linear Model procedure of SAS:
intensity = μ + t j + g k 1 + ε ijk 1

where μ is an overall average, tj is the effect of the jth tree (tree effect), gk(l) is the effect of the lth extraction set nested within the kth gel set (technical effect), and εijk(l) is a random error term. The model was fit for each spot, and the test of significant effects computed using the type III sums of squares. Model fit was evaluated by verifying that the overall model fit had a significant F value (p-value < 0.05) and by examination of standardized residuals (especially by plotting against the levels of effects). For each model, careful assessment of residual plots confirmed model assumptions about error distribution and equal variances were sufficiently met (i.e. the spot quantities did not deviate from the assumed normal distribution enough to warrant additional transformation). Degrees of freedom (df) were the same for each spot model: tree effect has 15 df, technical effect had 11 df, and error df = 21 (total df=47). For the technical effect, a Bonferroni adjustment was used to determine significance level, but for the tree effect a p<= 0.05 was considered significant. This permissive cut off is appropriate since the goal of the test was to eliminate constitutive proteins (and reduce the physical size of the dataset to ease computations) and because a false acceptance of the null is more problematic than a false rejection at this point in the analysis. Any spots that are not significantly different in at least one tree (tree effect p<0.05) were dropped from the dataset. Technical effects were found to be not significant and were dropped from further analysis for all but six spots that were dropped from the dataset.

The second phase of the analysis was designed to determine how spots differed between trees. Technical factors were dropped and stand and disease state factors were added (there were insufficient degrees of freedom to analyse technical and biological factors in the same model, partly due to imbalance in the number of trees selected per stand). The following model was fit using the General Linear Model procedure of SAS:
intensity = μ + s j + d k + sd jk + ε ijk

where μ is an overall average, sj is the effect of the jth stand (stand effect), dk is the effect of the kth disease state (BBD effect), sdjk is the interaction effect of stand and disease state, and εijk is a random error term. Model fit was evaluated by verifying that the overall model fit had a significant F value (p-value < 0.05) and by examination of standardized residuals (especially by plotting against the levels of effects). For each model, careful assessment of residual plots confirmed model assumptions about error distribution and equal variances were sufficiently met (i.e. the spot quantities did not deviate from the assumed normal distribution enough to warrant additional transformation). Degrees of freedom were the same for each spot model: BBD effect has 1df, STAND effect has 7 df, BBDxSTAND interaction effect has 5 df, and error has 34 df (total df=47). The model was fit for each spot, and the test of significant effects computed using the type III sums of squares. Interaction effects and stand effect were tested using the conservative Bonferroni correction. For the BBD effect, p-values from the tests were output to a new dataset and the package 'qvalue' (q-value 1.1) for the statistical program R (R-2.4.0-win32, [68]) was used to compute the associated q-value for each test. Significance was determined using q-values while controlling the false discovery rate at 5% [69]. False discovery rate controls the percentage of null hypothesis rejected in error (false positives) rather than the overall error rate, and is an accepted and typical statistical analysis for large genomic and proteomic datasets [70].

Spot selection and cutting

All spots with a significant effect for the disease state factor were considered for spot cutting and sequencing. Spot quantities were evaluated in all of the trees and trees ranked as the best trees were those having the most BBD significant spots at the highest spot densities. The two top trees were used for preparative gels and spot cutting. All BBD significant spots in the two selected trees were evaluated on the gel images to determine if the spot could be excised cleanly and was sufficiently intense to support sequencing. Spots were excised from the preparative gels at the PMGF using the Protean 2-D spot cutter (Biorad Laboratories). Several constitutive spots were also selected as sequencing reference spots. High resolution pre-cut and post-cut images of preparative gels were captured on the VersaDoc imager and evaluated for quality control. Only protein spots that were cleanly excised and had no evidence of contamination from adjacent spots were sent for MS/MS analysis.

Mass spectrometry

Mass spectrometry was performed at the OSU Campus Chemical Instrumentation Center. Gel pieces were washed twice in 50% methanol/5% acetic acid for one hour each, followed by dehydration in acetonitrile. Cysteines were reduced by rehydrating and incubating in dithiothreitol (DTT) solution (5mg/mL in 100 mM ammonium bicarbonate) for 30 minutes. Cysteins were alkylated by the addition of 15mg/mL iodoacetamide in 100 mM ammonium bicarbonate solution, and incubation in the dark for 30 min. The gel cores were washed again with cycles of acetonitrile and ammonium bicarbonate (100mM) in 5 min increments, then dried under vacuum. Protein was digested in Multiscreen Solvinert Filter Plates from Millipore (Bedford, MA) with sequencing grade modified trypsin (Promega, Madison WI) overnight. The peptides were extracted from the polyacrylamide by washing several times with 50% acetonitrile and 5% formic acid, pooled, and concentrated under vacuum to ~30 uL.

Capillary-liquid chromatography-nanospray tandem mass spectrometry (Nano-LC/MS/MS) was performed on a Thermo Finnigan LTQ mass spectrometer equipped with a nanospray source operated in positive ion mode. The LC system was an UltiMate™ 3000 system from Dionex (Sunnyvale, CA). Five microliters of each sample were first injected on to the micro-Precolumn Cartridge (Dionex, Sunnyvale, CA), and washed with 50 mM acetic acid. The injector port was switched to inject and the peptides were eluted off of the trap onto the column. A 5 cm 75 μm ID ProteoPep II C18 column (New Objective, Inc. Woburn, MA) packed directly in the nanospray tip was used for chromatographic separations. Peptides were eluted directly off the column into the LTQ system using a gradient of 2-80% acetonitrile over 45 minutes, with a flow rate of 300 nl/min and total run time was 65 minutes. The MS/MS was acquired using a nanospray source operated with a spray voltage of 3 KV and a capillary temperature of 200°C. The analysis was programmed for a full scan recorded between 350 – 2000 Da, and a MS/MS scan to generate product ion spectra to determine amino acid sequence in consecutive instrument scans of the ten most abundant peaks in the spectrum. The CID fragmentation energy was set to 35%. Dynamic exclusion is enabled with a repeat count of 30s, exclusion duration of 350s and a low mass width of 0.5 Da and high mass width of 1.50 Da.

Sequence data processing and matching

Sequence information from the MS/MS data were searched using Mascot Daemon (version 2.2.1 Matrix Scientific, Boston, MA) [71] against several databases (detailed below). The search parameters were: mass accuracy of the precursor ions = 2.0, fragment mass accuracy = 0.5 Da, considered (variable) modifications = methionine oxidation and carbamidomethyl cysteine, missed cleavages = 2–4. Due to the low representation of woody plant and bark tissue sequences in the databases, the search was conducted against several databases. Searching against the full SwissProt database version 54.1 (283454 sequences; 104030551 residues) was unproductive (only procedural peptides identified, data not shown). A second search was conducted restricting the search set to taxon Viridiplantae (version, sequences, residues). The Fagaceae genomics project [72] has constructed EST libraries from American Beech, Red Oak (Quercus rubra L.), White Oak (Quercus alba L.), American chestnut (Castanea dentata (Marsh.) Borkh) and Chinese chestnut (Castanea mollissima Blume) including libraries constructed from both healthy and diseased stem tissues. Twenty-four individual EST libraries (#13696, 8-21-2009, 10691208 sequences; 751178460 residues) were compiled into a custom database and searched. Peptide matches were checked manually and only those identifications with a Mascot score of 50 or higher and having two or more unique peptides of five or more residues were accepted. For EST matches, peptides were matched to EST's (by the same criteria), then EST's searched against GenBank (BLASTP, default settings [73] to obtain protein identifications. Analysis data is available in the PRIDE database [74, 75] under the accession numbers 17706. The data was converted using the PRIDE Converter [76, 77].

Abbreviations

2-DE: 

2 Dimensional Electrophoresis

ANOVA: 

Analysis Of Variance

BBD: 

Beech Bark Disease, or its effect in a statistical model

CHAPS: 

3-[(3-Cholamidopropyl)Dimethylamonio]-1-Propanesulphonate

DF: 

Degrees of Freedom in a statistical model or test

IEF: 

Iso-Electric Focusing

INT: 

The Interaction Effect (Bbdxstand) In Statistical Models

OSU: 

The Ohio State University

STAND: 

The Effect Of Stand In The Statistical Models

TBP: 

Tris-Butyl Phosphate

Tris–HCl: 

Tris(Hydroxymethyl)Aminomethane.

Declarations

Acknowledgements

We would like to thank Donnie McPhee and Kathleen Forbes for their assistance with collecting material, Tannis Beardmore for practical advice, and The Fagaceae Genomics Tools project [72] for access to EST data from American Beech. We thank John Carlson, Sasha Popova-Butler, and Sourav Chakraborty for helpful comments on a draft version of this manuscript. And we are grateful to Dave Carey for his excellent technical work and advice on all aspects of the beech bark disease research project.

Authors’ Affiliations

(1)
Department of Entomology, Ohio Agricultural Research and Development Center, The Ohio State University
(2)
US Forest Service, Northern Research Station
(3)
Faculty of Forestry and Environmental Management, University of New Brunswick
(4)
Formerly: Canadian Forest Service, Natural Resources Canada
(5)
Bioversity International

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