Phenolic compounds in ectomycorrhizal interaction of lignin modified silver birch

BioMed Central Page 1 of 15 (page number not for citation purposes) BMC Plant Biology Open Access Research article Phenolic compounds in ectomycorrhizal interaction of lignin modified silver birch Suvi Sutela*1, Karoliina Niemi2, Jaanika Edesi1, Tapio Laakso3, Pekka Saranpää3, Jaana Vuosku1, Riina Mäkelä1, Heidi Tiimonen4, Vincent L Chiang5, Janne Koskimäki1, Marja Suorsa1, Riitta Julkunen-Tiitto6 and Hely Häggman1 Address: 1Department of Biology, University of Oulu, PO Box 3000, 90014 Oulu, Finland, 2Department of Applied Biology, University of Helsinki, PO Box 27, 00014 Helsinki, Finland, 3Finnish Forest Research Institute, Vantaa Research Unit, Jokiniemenkuja 1, 01301 Vantaa, Finland, 4Finnish Forest Research Institute, Punkaharju Research Unit, Finlandiantie 18, 58450 Punkaharju, Finland, 5Forest Biotechnology Research Group, Department of Forestry and Environmental Resources, College of Natural Resources, North Carolina State University, Campus Box 7247, 2500, Partners II Building, Raleigh, NC 27695-7247, USA and 6Department of Biology, University of Joensuu, PO Box 111, 80101 Joensuu, Finland Email: Suvi Sutela* - [email protected]; Karoliina Niemi - karoliina.nie[email protected]; Jaanika Edesi - [email protected]; Tapio Laakso - [email protected]; Pekka Saranpää - [email protected]; Jaana Vuosku - jaan[email protected]; Riina Mäkelä - riina.mak[email protected]; Heidi Tiimonen - heidi.tii[email protected]; Vincent L Chiang - [email protected]; Janne Koskimäki - [email protected]; Marja Suorsa - [email protected]; Riitta Julkunen-Tiitto - riitta.julkunen- [email protected]; Hely Häggman - hely.haggm[email protected] * Corresponding author Abstract Background: The monolignol biosynthetic pathway interconnects with the biosynthesis of other secondary phenolic metabolites, such as cinnamic acid derivatives, flavonoids and condensed tannins. The objective of this study is to evaluate whether genetic modification of the monolignol pathway in silver birch (Betula pendula Roth.) would alter the metabolism of these phenolic compounds and how such alterations, if exist, would affect the ectomycorrhizal symbiosis. Results: Silver birch lines expressing quaking aspen (Populus tremuloides L.) caffeate/5hydroxyferulate O-methyltransferase (PtCOMT) under the 35S cauliflower mosaic virus (CaMV) promoter showed a reduction in the relative expression of a putative silver birch COMT (BpCOMT) gene and, consequently, a decrease in the lignin syringyl/guaiacyl composition ratio. Alterations were also detected in concentrations of certain phenolic compounds. All PtCOMT silver birch lines produced normal ectomycorrhizas with the ectomycorrhizal fungus Paxillus involutus (Batsch: Fr.), and the formation of symbiosis enhanced the growth of the transgenic plants. Conclusion: The down-regulation of BpCOMT in the 35S-PtCOMT lines caused a reduction in the syringyl/guaiacyl ratio of lignin, but no significant effect was seen in the composition or quantity of phenolic compounds that would have been caused by the expression of PtCOMT under the 35S or UbB1 promoter. Moreover, the detected alterations in the composition of lignin and secondary phenolic compounds had no effect on the interaction between silver birch and P. involutus. Published: 29 September 2009 BMC Plant Biology 2009, 9:124 doi:10.1186/1471-2229-9-124 Received: 20 February 2009 Accepted: 29 September 2009 This article is available from: http://www.biomedcentral.com/1471-2229/9/124 © 2009 Sutela et al; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. BMC Plant Biology 2009, 9:124 http://www.biomedcentral.com/1471-2229/9/124 Page 2 of 15 (page number not for citation purposes) Background The phenylpropanoid pathway gives rise to a variety of compounds that are used in the biosynthesis of cinnamic acid derivatives, lignin, flavonoids and condensed tannins. These phenolic compounds form a diverse group of secondary metabolites, exhibiting numerous biological functions in plants. In the Betula species, the phenolic compound concentrations vary according to the development phase of the plant [1,2], clone [2-4] or plant part [5] and to different environmental factors [2,4,5]. Moreover, tannins and specific flavonoids have been shown to play a role in defence against herbivory [6] and protection against UVB radiation [1,7-9]. In addition to the phenolic compound profiles of different Betula species, the general outline of the phenylpropanoid pathway of the species is also well known [4]. The secondary cell wall is essential for the conduction of water and the structural integrity of vascular plants as well as for defence against insect herbivores and pathogens. The secondary cell wall is composed of multiple layers of cellulose microfibrils embedded in a matrix of hemicellulose, lignin and pectin. Lignin, probably the most studied phenolic compound, is composed of monomers derived from the monolignol biosynthetic pathway [10]. In hardwoods, coniferyl, sinapyl and p-coumaryl alcohol are the main lignin monomers, giving rise to guaiacyl (G), syringyl (S) and p-hydroxyphenyl (H) phenylpropanoid units, respectively, when polymerized to the lignin molecule. These hydroxycinnamyl alcohols differ in their degree of methylation and, consequently, form varying linkage types in the lignin, determining the solubility of the polymer. In sinapyl alcohol, the C-5 position of the aromatic ring is methylated, which prevents the formation of strong linkage types that are typical for G units. Angiosperm lignin consists mainly of G and S monomers and is more easily delignified than the G unit rich gymnosperm lignin. The monolignol biosynthetic pathway is still under debate, partly because the enzymes involved in the pathway are multifunctional and exhibit broad substrate specificity, at least in vitro, making several alternative reaction orders possible. The most updated view of the angiosperm monolignol biosynthetic pathway is presented by Li et al. [11], Do et al. [12] and Vanholme et al. [13]. The caffeate/5-hydroxyferulate O-methyltransferase (COMT) (EC 2.1.1.68), also known as 5-hydroxyconiferyl aldehyde O-methyltransferase (AldOMT) [14] catalyses the methylation of the C-5 position of angiosperms' S precursors. COMT belongs to the plant Class II O-methyltransferases (OMTs) together with enzymes that methylate numerous phenolic compounds, such as phenylpropenes and flavonols [15,16]. Initially COMT was shown to use caffeic acid and 5-hydroxyferulate as substrates [17,18], but further studies demonstrated that COMT is especially involved in the biosynthesis of S lignin [19-22] and, furthermore, that the methylation occurs at 5-hydroxyconiferaldehyde and (or) 5-hydroxyconiferyl alcohol as shown with various angiosperm species [14,23-26]. However, the substrate preferences of COMT may vary between species being, for instance, relatively board in alfalfa (Medicago sativa L.) [24] and wheat (Triticum aestivum L.) [26]. Some of the enzymes having COMT activity are probably bifunctional as in the case of Arabidopsis thaliana OMT (At5g54160) which is involved in both lignin and flavonoid biosynthesis [12,27,28]. Silver birch (Betula pendula Roth) is one of the key species in boreal forest ecosystems and, in addition, economically the most important deciduous tree species in Nordic countries. In Finland, based on the national forest inventory performed during years 2004 through 2007 approximately 16% of growing stock was birch (363 mill. m3) [29]. The birch roundwood is used as a raw material in the chemical pulp industry but also in plywood production. Moreover, birch is an important source of energywood: in 2007 wood-based fuels covered one fifth (295PJ) of the total energy consumption in Finland [30]. Boreal forest trees live in a mutualistic association with ectomycorrhizal (ECM) fungi, which enables growth in the nutrient-poor, acidic soils. The formation of ECM symbiosis causes changes in the transcription levels of both partners [31-34], resulting in morphological and physiological alterations. The proliferation of root hairs is inhibited and the epidermal cells of feeder roots in angiosperms elongate radially as the fungus penetrates into the intercellular space of the epidermis. The fungal hyphae that cover feeder roots are also a source of an external hyphal net. These distinctive alterations in the symbiotic partners ensure the effective exchange of water and nutrients from the fungal partner to the carbohydrates of the plant [35]. ECM formation has also been observed to alter the expression levels of genes involved in the phenylpropanoid pathway [32,33,36] and the concentrations of phenolic compounds [37-43]. However, the results have been rather inconsistent. Transgenic plants have great potential for future agriculture, silviculture and biofuel production. Increasing the pest and disease resistance of plants as well as improving wood quality and enhancing wood production have been the targets of both conventional breeding and genetic engineering. From an industrial point of view, lignin quality and content are of particular interest. The removal of lignin in chemical pulping is a costly process which could be facilitated with more soluble lignin and lower lignin content [44]. A reduction in lignin content would also be beneficial for the production of bioethanol [45]. Other BMC Plant Biology 2009, 9:124 http://www.biomedcentral.com/1471-2229/9/124 Page 3 of 15 (page number not for citation purposes) processes related to the production of bioethanol could also be enhanced by modifications in the cell wall chemistry, as reviewed by Sticklen [46]. Lignin modifications using various gene constructs that are associated with the monolignol biosynthetic route have been conducted successfully on angiosperm tree species (reviewed in [10,11,13]). Changes in the primary as well as in the secondary metabolism of organisms are triggered by a variety of stimuli, such as changes in the developmental phase or environmental factors. Therefore, the pleiotropic or non-target effects of transgenes should also be studied in diverse environmental conditions. So far, only minor changes have been found in interactions between lignin modified trees and herbivores or soil microfauna [47-53]. Recent studies investigating possible changes in the secondary metabolism that are caused by genetic transformations have mostly been conducted on herbaceous species [5459] and without the involvement of ecological interactions. In the present study, we analyzed the phenolic compounds and lignin characteristics of PtCOMT silver birch lines (Betula pendula Roth.) in interaction with the ECM fungus Paxillus involutus (Batsch: Fr.) in order to determine the impact of the symbiosis on the phenylpropanoid route derived compounds and to detect possible unintended effects of transgene expression. Results Expression of PtCOMT and BpCOMT in roots The open reading frame of putative COMT (BpCOMT) and partial sequence (1536 bp) of PP2A (BpPP2A) of silver birch were sequenced. The putative BpCOMT was 72% identical to PtCOMT [EMBL: X62096] at the nucleotide level and 87% identical at the amino acid level (Additional file 1) and showed highest similarity to the castor bean (Ricinus communis) COMT [GenBank: EEF36570] (90%) and almond (Prunus dulcis) COMT [EMBL: CAA58218] (88%). The putative BpPP2A showed 91% similarity with Medicago sativa subsp. x varia [GenBank: AAG29593] and 90% similarity with A. thaliana [GenBank: NP_172790] PP2A at the amino acid level (Additional file 2). The expression of PtCOMT and the putative BpCOMT was studied from the non-inoculated and mycRT-PCR results of BpCOMT and PtCOMT in silver birch rootsFigure 1 RT-PCR results of BpCOMT and PtCOMT in silver birch roots. Relative expression of the endogenous putative caffeate/5-hydroxyferulate O-methyltransferase of silver birch (BpCOMT) (A) and the heterologous PtCOMT gene (B) normalized using atub and putative BpPP2A as reference genes in the non-inoculated and mycorrhizal roots of clone A and PtCOMTmodified lines 23, 44 and 65. Values are means ± standard error. Different letters above the columns denote significant (P < 0.05) difference between the PtCOMT lines and clone A within the treatments according to the two-sample t-test or the Wilcoxon rank sum test with the Bonferroni correction. Number of replicates 3-5. BMC Plant Biology 2009, 9:124 http://www.biomedcentral.com/1471-2229/9/124 Page 4 of 15 (page number not for citation purposes) orrhizal roots of silver birch (Figure 1A, B). The relative expression of the putative BpCOMT was similar in both non-inoculated and mycorrhizal roots: 35S-PtCOMT lines 23 and 44 had lower average levels of BpCOMT transcripts than UbB1-PtCOMT line 65 and clone A (Figure 1A). However, significant differences (P < 0.05) in the relative expression of BpCOMT were only observed in the noninoculated roots between UbB1-PtCOMT line 65 and 35SPtCOMT line 23. The relative expression of PtCOMT was significantly (P < 0.05) higher in the non-inoculated roots of 35S-PtCOMT line 23 than in line 65, where the transgene was driven by the UbB1 promoter (Figure 1B). In mycorrhizal roots, the relative expression levels of PtCOMT between lines were comparable to those of noninoculated roots. Lignin distribution and composition Lignin content as a percentage of dry weight (DW) evaluated with the acetyl bromide method was 27.6% in the non-inoculated and 27.1% in the mycorrhizal roots of clone A. In PtCOMT lines the root lignin content varied between the highest value of 27.8% of mycorrhizal roots of line 65 and the lowest of 24.5% of mycorrhizal roots of line 23. The corresponding lignin contents of stem wood were more than 5 percentage units lower than the root lignin contents and varied between 19.5 and 23.5%. Neither the transgene nor the fungal treatment affected the lignin content. The GC-MS analyses of lignin units showed that the non-inoculated clone A had higher (P < 0.05) S/G ratios in both stem and root wood than the non-inoculated plants of PtCOMT line 44 (Figure 2A and 2B). In non-inoculated roots of PtCOMT line 23 the S/G ratio was lower (P < 0.05) than in the roots of clone A. The S/G ratio of stem and root wood of mycorrhizal PtCOMT line 44 was significantly reduced (P < 0.05) in comparison with the mycorrhizal clone A. In the stem and root wood of both non-inoculated and mycorrhizal PtCOMT line 65, the S/G ratios were at the same level as in clone A. Moreover, in the stem and root wood of non-inoculated PtCOMT line 65, the S/G ratios were significantly (P < 0.05) higher than in the corresponding non-inoculated PtCOMT lines 23 and 44. According to the Mäule assay the S lignin (i.e. the pink-red colouration) was only slightly reduced when the root and stem xylem sections of PtCOMT lines 23 (Figure 3F, N) and 44 (Figure 3G, O) The lignin syringyl/quiaicyl ratios of non-inoculated and mycorrhizal silver birchesFigure 2 The lignin syringyl/quiaicyl ratios of non-inoculated and mycorrhizal silver birches. The lignin syringyl/quiaicyl (S/G) ratios of stems (A) and roots (B) of non-transgenic clone A and PtCOMT-modified lines 23, 44 and 65. Values are means ± standard deviation. Different letters above the columns denote significant (P < 0.05) differences between the non-inoculated and mycorrhizal plants within the line/clone and between lines/clone within the fungal treatment according to the Wilcoxon rank sum test with the Benjamini & Hochberg correction or the two-sample t-test with the Benjamini & Hochberg correction. Number of replicates 3. BMC Plant Biology 2009, 9:124 http://www.biomedcentral.com/1471-2229/9/124 Page 5 of 15 (page number not for citation purposes) were compared to the xylem sections of clone A (3E, M) and PtCOMT line 65 (3H, P). Soluble phenolic compounds and condensed tannins No cloneor line-specific peaks were detected in the HPLC-DAD or HPLC-MS chromatograms and, moreover, all phenolic compounds were present in the non-inoculated and mycorrhizal samples of clone A and PtCOMT lines (Table 1 and Additional file 3). Acetylated kaempherol, myricetin and quercetin with rhamnoside moiety were found in all leaves of clone A and PtCOMT lines. Condensed tannin concentrations were high in the samples (Table 1). The tannin levels partly prevented the identification of soluble phenolic components, especially from the root samples (Additional file 3). In the leaves of mycorrhizal plants, significant differences (P < 0.05) were found in the concentrations of quercetin 3-arabinose and kaempherol 3-acetyl-glucoside between PtCOMT lines 44 and 65 (Additional file 3). Significant differences between clone A and the PtCOMT lines were found in the concentration of p-OH-cinnamic acid derivates, individual cinnamic acid derivatives 3 and 4 and chlorogenic acid and chlorogenic acid derivative. A significant difference (P < 0.05) was detected in the amount of (+)-catechin in the leaves of the mycorrhizal and noninoculated plants of clone A. In stems, the cinnamic acid derivatives were at a higher (P < 0.05) level in clone A than in PtCOMT lines 23 and 44 (Table 1). Of individual components, the concentration Histochemical localization of lignin in non-inoculated silver birchesFigure 3 Histochemical localization of lignin in non-inoculated silver birches. Cross-sections of stem and root of non-inoculated clone A (A, E, I, M) and PtCOMT-modified lines 23 (B, F, J, N), 44 (C, G, K, O) and 65 (D, H, L, P). Stems (A-H) and roots (I-P). Lignin stained pink-red in the phloroglucinol-HCL stained sections (A-D, I-L). In the Mäule stainings (E-H, M-P), syringyl lignin pink-red and guaiacyl lignin light brown to dark brown. xf, xylem fibre; xr, xylem ray; xv, xylem vessel. BMC Plant Biology 2009, 9:124 http://www.biomedcentral.com/1471-2229/9/124 Page 6 of 15 (page number not for citation purposes) of p-OH-cinnamic acid glucoside was higher (P < 0.05) in the stems of the clone A than in the stems of PtCOMT lines 23 and 44 (Additional file 3). Significant differences were also detected between the PtCOMT lines and clone A in the concentrations of gallocatechin, 3,4'-dihydroxypropiophenone 3-glucoside (DHPPG) and (+)-catechin, which was at a higher level in the stems of clone A than in the stems of PtCOMT lines. The concentration of condensed tannin precursors was significantly (P < 0.05) higher in the mycorrhizal roots of line 23 than in the roots of lines 44 and 65 (Table 1). A Table 1: Concentrations of phenolic compounds and condensed tannins in non-inoculated and mycorrhizal silver birches Leaves Clone Lines T A 23 44 65 Cinnamic acid derivatives c 2.48 ± 0.44 a 2.12 ± 0.57 a 2.21 ± 0.37 a 2.30 ± 0.43 a ECM 2.84 ± 0.71 a 2.35 ± 0.72 a 2.35 ± 0.41 a 2.50 ± 0.54 a Flavonoids c 26.14 ± 4.00 a 30.67 ± 8.64 a 28.85 ± 2.14 a 30.87 ± 8.68 a ECM 27.07 ± 7.36 a 25.38 ± 9.78 a 25.80 ± 5.25 a 29.71 ± 5.80 a Apigenin derivatives c 0.20 ± 0.12 a 0.31 ± 0.10 a 0.31 ± 0.09 a 0.43 ± 0.25 a ECM 0.40 ± 0.21 a 0.19 ± 0.11 a 0.24 ± 0.07 a 0.39 ± 0.12 a Kaempherol derivatives c 0.85 ± 0.13 a 0.71 ± 0.23 a 0.90 ± 0.19 a 0.68 ± 0.20 a ECM 0.83 ± 0.14 a 0.73 ± 0.22 a 0.97 ± 0.13 a 0.69 ± 0.06 a Myricetin derivatives c 18.31 ± 3.58 a 22.81 ± 6.34 a 21.77 ± 1.75 a 21.66 ± 6.11 a ECM 18.83 ± 5.04 a 18.40 ± 8.20 a 17.90 ± 4.30 a 20.17 ± 4.54 a Quercetin derivatives c 5.19 ± 1.34 a 6.34 ± 2.12 a 5.43 ± 1.45 a 7.05 ± 1.64 a ECM 6.38 ± 1.52 a 5.52 ± 1.98 a 5.84 ± 1.18 a 7.20 ± 1.19 a Condensed tannins c 160.41 ± 24.97 a 142.87 ± 47.63 a 127.54 ± 34.23 a 154.82 ± 28.44 a ECM 142.40 ± 68.98 a 106.7 ± 22.57 a 149.21 ± 24.76 a 142.32 ± 62.23 a p-OH-cinnamic acid derivatives c 3.21 ± 0.47 a 1.84 ± 0.49 b 2.46 ± 0,32 ab 2.71 ± 0.61 ab ECM 3.10 ± 0.36 ac 2.22 ± 0.71 bc 2.42 ± 0.32 ac 2.42 ± 0.38 ab Stems Clone Lines T A 23 44 65 Cinnamic acid derivatives c 2.68 ± 0.68 a 1.37 ± 0.28 b 1.33 ± 0.25 b 1.65 ± 0.37 bc ECM 2.33 ± 0.33 ac 1.46 ± 0.18 b 1.27 ± 0.30 b 1.18 ± 0.20 b Flavonoids c 14.33 ± 1.74 ab 11.43 ± 1.49 ab 10.23 ± 1.43 b 13.79 ± 2.81 ab ECM 14.60 ± 2.12 a 12.38 ± 1.89 ab 10.77 ± 2.37 ab 11.18 ± 1.47 ab Phenolic glycosides c 16.86 ± 1.46 ab 19.48 ± 0.81 ab 18.18 ± 2.22 ab 22.67 ± 5.31 b ECM 15.40 ± 2.74 a 20.61 ± 2.29 ab 20.61 ± 2.29 ab 17.37 ± 5.29 ab Condensed tannins c 108.57 ± 50.76 a 138.57 ± 13.52 a 137.43 ± 13.72 a 141.47 ± 16.70 a ECM 144.82 ± 19.68 a 132.86 ± 11.23 a 131.90 ± 9.26 a 130.32 ± 12.55 a Roots Clone Lines T A 23 44 65 Cinnamic acid derivatives c 0.71 ± 0.33 a 0.17 ± 00.05 a 0.17 ± 0.10 a 0.37 ± 0.23 a ECM 0.53 ± 0.30 a 0.19 ± 0.02 a 0.20 ± 0.13 a 0.24 ± 0.14 a Flavonoids c 11.39 ± 0.62 a 9.45 ± 1.83 a 8.41 ± 1.93 a 8.26 ± 2.26 a ECM 11.24 ± 2.78 a 12.10 ± 2.55 a 7.88 ± 2.91 a 7.21 ± 1.25 a Gallo/Ellagitannins c 0.06 ± 0.01 a 0.32 ± 0.24 a 0.28 ± 0.09 a 0.12 ± 0.06 a ECM 0.11 ± 0.06 a 0.36 ± 0.08 a 0.23 ± 0.12 a 0.15 ± 0.13 a Condensed tannins c 130.50 ± 22.43 a 113.87 ± 17.86 a 106.92 ± 27.75 a 112.24 ± 8.94 a ECM 126.09 ± 6.80 a 116.24 ± 10.11 a 108.47 ± 5.62 a 100.46 ± 2.24 a Condensed tannin precursors c 35.07 ± 5.81 ab 31.44 ± 7.15 ab 28.60 ± 5.50 ab 28.50 ± 3.81 ab ECM 37.20 ± 9.43 ac 41.92 ± 10.62 a 20.38 ± 6.70 b 25.44 ± 2.21 bc Concentrations (mg/DW g) of phenolic compounds and condensed tannins in the leaf, stem and root samples of silver birch clone A and PtCOMTmodified lines 23, 44 and 65 after 8 weeks in co-culture with P. involutus. Values are means ± standard deviations in the presence (ECM) or absence (c) of the fungus. Different letters following the values denote significant differences (P < 0.05) between the non-inoculated and mycorrhizal plants within the line/clone and between lines/clone within the fungal treatment according to the Kruskal-Wallis test combined with the Wilcoxon rank sum test with the Benjamini & Hochberg correction or the one-way or two-way Anova combined with Tukey's honestly significant difference test or with the two-sample t-test with the Benjamini & Hochberg correction. For statistical testing the leaf apigenin derivatives were square root, stem cinnamic acid derivatives log 10 and root condensed tannins square transformed. Number of replicates 4-7. BMC Plant Biology 2009, 9:124 http://www.biomedcentral.com/1471-2229/9/124 Page 7 of 15 (page number not for citation purposes) small amount of ellagic acid was found in PtCOMT lines 23 and 44, where the PtCOMT was driven by the 35S promoter, but not in lines 65 and clone A (Additional file 3). An ellagic acid derivative was also found in line 65 and in the mycorrhizal roots of clone A. Formation of ECMs and growth characteristics of silver birches All PtCOMT lines were able to form ECM symbiosis with P. involutus, and inoculation resulted in slightly higher survival percentages in clone A and PtCOMT lines 23 and 44 (Table 2). The mycorrhizal percentages of inoculated plants varied considerably between PtCOMT lines and clone A. No differences were detected in the number of ECMs per root systems (Table 2) or in the morphology of the mycorrhizas between lines: well-developed hyphal mantle covered the root tips and the epidermal cells were radially elongated and surrounded by fungal Hartig net (Figure 4A-D). Compared with the fresh weights (FWs) of the PtCOMT-modified lines, plants of clone A had lower FWs, but the growth rate (i.e. final FW/initial FW ratio) of the clone A plants in both treatments corresponded to the growth of the transgenic lines (Table 3). Inoculation of PtCOMT-modified lines with P. involutus enhanced their growth, resulting in significantly (P < 0.05) higher FWs than that of the non-inoculated plants (Table 3). The root/shoot ratios of plants increased significantly (P < 0.05) as a result of inoculation in clone A and PtCOMT line 23. Inoculation had no effect on the number or length of adventitious roots. Discussion In the present study, no changes were found in phenolic compounds of PtCOMT-modified silver birch lines that would have been caused by the formation of ECM symbiosis with P. involutus. The only difference between the mycorrhizal and non-inoculated plants was observed in the catechin concentration in the leaves of the non-transgenic clone A. Münzenberger et al. [39,40] observed a reduction in various phenolic compounds (e.g. phydroxybenzoic acid glucoside, picein and catechin) in the mycorrhizal fine roots of European larch (Larix decidua Mill.) and Norway spruce [Picea abies (L.) Karst.] when compared with the non-mycorrhizal roots. Similar results were obtained with European beech (Fagus sylvatica L.) ECM roots which contained less catechin [41]. By contrast, an increase in catechin concentration was detected in the ECM roots of European larch [42] and the needles and stems of mycorrhizal Scots pine (Pinus sylvestris L.) [43]. Furthermore, enhanced levels of phenolic compounds have been observed in Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] [60] and brown barrel (Eucalyptus fastigata Deane and Maiden) [37]. The discrepancy of results may reflect the fluctuation of transcriptome patterns during ECM formation, as seen in various microarray experiments [32-34], diverse biological material and experimental designs. In the present work, all PtCOMT lines were able to form symbiosis with morphologically normal ECMs. Moreover, the mycorrhizal interaction increased FWs in all PtCOMT lines. Similar results have been obtained with silver birches expressing sugar beet chitinase IV [61] and 4-coumarate: coenzyme A ligase (4CL) [52] and PtCOMT [53]. In all of these studies, transgenic silver birches were capable of forming ECM symbiosis although 4CL expressing silver birches had changes in their growth characteristics [52] and two PtCOMT silver birch lines had altered ECM morphology in vitro [53]. In silver birch-P. involutus interaction, Feugey et al. [62] observed a transient increase in phenylalanine ammonialyase (PAL) activity, but in the micro-array studies [31,33] PAL was not differentially expressed in ECM roots compared with non-inoculated roots. Instead, Le Quéré et al. [33] found an increase in genes coding monolignol biosynthesis route associated products: Arabidopsis caffeoyl-coA 3-O-methyltransferase (CCoAOMT) homolog, dirigent protein homolog and sinapyl alcohol dehydrogenase (SAD) homolog. CCoAOMT expression was consistent after 4 days of inoculation to 14 days, whereas the expression of dirigent protein homolog and SAD homolog was at its highest after 2 days of inoculation and then again 14 days after the start of the co-cultivation. Our results indicate that ECM formation had no drastic effect Table 2: Survival and ECM characteristics of silver birches ECM categories Clone/ Line Survival % of non-inoculated plants Survival % of inoculated plants ECM % I II III IV V A74 763132121 23 92 95 83 11 8 3 4 3 44 95 100 74 14 2 6 3 3 65 92 87 58 8 5 3 1 2 Survival percentages of non-inoculated and inoculated silver birches of clone A and PtCOMT-modified lines 23, 44 and 65, percentages of the ECM plants of all inoculated plants (ECM %) and number of ECM root tips in root systems classified to five categories: I = 1-20 ECMs, II = 20-30 ECMs, III = 30-50 ECMs, IV = 50-100 ECMs, V ≥ 100 ECMs. BMC Plant Biology 2009, 9:124 http://www.biomedcentral.com/1471-2229/9/124 Page 8 of 15 (page number not for citation purposes) on the lignin or the phenolic compound biosynthesis in stems or roots. The expression of PtCOMT under control of the 35S promoter resulted in lower S/G ratios in the stem and root wood when compared with clone A, as observed in previous studies [48,53,63]. By contrast, when PtCOMT was under the UbB1 promoter, no changes were detected in the lignin characteristics. We have previously shown [48,63] that there are multiple copies of the PtCOMT gene in lines 23, 44 and 65 and that the UbB1-PtCOMT-transcript is bigger than the 35S-PtCOMT-transcript. In the present study, the relative expression of the heterologous PtCOMT seemed to be higher in the roots of 35S-PtCOMT lines 23 and 44 than in those of UbB1-PtCOMT line 65. Conversely, the BpCOMT mRNA transcript levels were more decreased in 35S-PtCOMT lines 23 and 44 than in UbB1-PtCOMT-line 65. The homology between BpCOMT and PtCOMT at the nucleotide level was quite high and it is therefore possible that the heterologous PtCOMT expression resulted in RNAi-mediated partial silencing of the endogenous BpCOMT. The relative expression levels of BpCOMT and PtCOMT possibly indicate that the 35S promoter generated a higher number of mRNA transcripts of PtCOMT than UbB1 and, as a consequence, decreased the number of BpCOMT transcripts more intensively in 35S-PtCOMT lines 23 and 44, thus causing a reduction in the lignin S/G ratio. The monolignol biosynthetic pathway crosstalks with other cell wall associated pathways [64,65] and also with the biosynthetic pathways of various phenolic compounds [66-70] which share the same precursors. Consequently, the altered expression of monolignol biosynthetic pathway genes may result in changes in the lignin content and phenolic compound profiles as shown with suppressed COMT and CCoAOMT (EC 2.1.1.104) [66], cinnamoyl-CoA reductase (CCR; EC 1.2.1.44) [68], hydroxycinnamoyl-CoA shikimate/quinate hydroxycinnamoyl transferase (HCT; EC 2.3.1.133) [69] and cinnamate 4-hydroxylase (C4H; 1.14.13.11) [67,70]. In the present study, chemical changes were detected between the PtCOMT lines and the non-transgenic clone A in the concentrations of phenolic compounds in the roots, stems and leaves of both non-inoculated and mycorrhizal plants. The detected changes were probably not direct results of the transgene because 35S-PtCOMT lines 23 and 44 displayed differences in phenolic compound profiles. Furthermore, the changes in the phenolic profiles of leaves, stems and roots are within the natural variation of phenolic compounds within silver birch [1-3,5,7]. Conclusion In the present study, the down-regulation of BpCOMT in the 35S-PtCOMT lines caused no shift of monolignol pathway intermediates to the biosynthesis of the phenolic secondary compounds. Moreover, no apparent effect in the composition or quantity of phenolic compounds caused by the expression of PtCOMT under the 35S or UbB1 promoter could be found. To conclude, our results indicate that the present lignin modification in the PtCOMT lines does not affect phenolic profiles or the symbiotic relationship between silver birch and P. involutus. Methods Plant and fungal material Silver birch (Betula pendula Roth.) lines 23, 44 and 65 expressing the caffeate/5-hydroxyferulate O-methyltransferase (PtCOMT) gene [EMBL: X62096] of quaking aspen Table 3: Growth characteristics of non-inoculated and mycorrhizal silver birches Clone/ Line T Initial FW (g) Final FW (g) Ratio of final and initial FW Root/shoot FW ratio Number of adventitous roots Length of adventitous roots (cm) A c 0.08 ± 0.04 a 1.75 ± 0.76 a 25.79 ± 19.90 a 0.80 ± 0.25 a 4.00 ± 1.33 a 16.70 ± 2.83 a ECM 0.07 ± 0.02 a 2.03 ± 0.76 a 34.49 ± 17.44 a 0.97 ± 0.24 b 4.00 ± 1.31 a 17.14 ± 1.04 a 23 c 0.15 ± 0.08 a 2.76 ± 0.88 a 23.75 ± 16.23 a 1.39 ± 0.40 a 4.97 ± 1.49 a 17.55 ± 3.78 a ECM 0.16 ± 0.09 a 3.06 ± 0.86 b 23.06 ± 13.63 a 1.59 ± 0.48 b 5.50 ± 1.72 a 17.59 ± 3.35 a 44 c 0.14 ± 0.06 a 2.70 ± 0.37 a 22.93 ± 9.28 a 1.40 ± 0.26 a 5.15 ± 1.54 a 18.64 ± 2.00 a ECM 0.15 ± 0.08 a 2.89 ± 0.51 b 23.28 ± 12.30 a 1.43 ± 0.29 a 7.68 ± 3.16 a 19.07 ± 2.10 a 65 c 0.17 ± 0.08 a 2.59 ± 0.43 a 17.70 ± 7.03 a 1.19 ± 0.20 a 4.03 ± 0.85 a 19.52 ± 2.05 a ECM 0.18 ± 0.09 a 2.83 ± 0.47 b 20.73 ± 11.90 b 1.26 ± 0.31 a 4.53 ± 1.22 a 18.52 ± 1.85 a Effects of the mycorrhiza formation on the growth of silver birch control clone A and PtCOMT-modified lines 23, 44 and 65 after 8 weeks in coculture with P. involutus in a greenhouse. Initial fresh weights (FWs) and final FWs of plants, root/shoot ratios, number and length of adventitious roots. Values are means ± standard deviations in the presence (ECM) or absence (c) of the fungus. Different letters following the values denote a significant difference (P < 0.05) between the non-inoculated and inoculated plants with mycorrhizas within each line/clone according to the Wilcoxon rank sum test or the two-sample t-test. Number of replicates 9-35. BMC Plant Biology 2009, 9:124 http://www.biomedcentral.com/1471-2229/9/124 Page 9 of 15 (page number not for citation purposes) (Populus tremuloides L.) [17] were generated as described by Aronen et al. [63] and Tiimonen et al. [48]. The PtCOMT encodes Class II methyltransferase (EC 2.1.1.68), which uses 5-hydroxyconiferyl aldehyde as a primary substrate [14]. All transgenic lines were produced from clone A as described in Valjakka et al. [71], originating in Punkaharju, Eastern Finland (61°48' N, 29°17' E). In PtCOMT lines 23 and 44, the transgene was driven by the 35S cauliflower mosaic virus (CaMV) promoter and in line 65 by the sunflower polyubiquitin (UbB1) promoter. The gene constructs were pRT99/35S-PtCOMT and pRT99/UbB1-PtCOMT, respectively. The plants of all lines ECM roots of silver birchFigure 4 ECM roots of silver birch. Cross-sections (5-10 μm) of silver birch clone A (A) and PtCOMT lines 23 (B), 44 (C) and 65 (D) roots after 8 weeks of co-cultivation with P. involutus. Arrow, Hartig net; triangle, mycelium of P. involutus; e, epidermal cell of silver birch root. Bars = 20 μm.