Abstract
Comparative phylogenetic analyses of the R2R3-MYB transcription factor family revealed that five subgroups were preferentially found in woody species and were totally absent from Brassicaceae and monocots (Soler et al., 2015). Here, we analyzed one of these subgroups (WPS-I) for which no gene had been yet characterized. Most Eucalyptus members of WPS-I are preferentially expressed in the vascular cambium, the secondary meristem responsible for tree radial growth. We focused on EgMYB88, which is the most specifically and highly expressed in vascular tissues, and showed that it behaves as a transcriptional activator in yeast. Then, we functionally characterized EgMYB88 in both transgenic Arabidopsis and poplar plants overexpressing either the native or the dominant repression form (fused to the Ethylene-responsive element binding factor-associated Amphiphilic Repression motif, EAR). The transgenic Arabidopsis lines had no phenotype whereas the poplar lines overexpressing EgMYB88 exhibited a substantial increase in the levels of the flavonoid catechin and of some salicinoid phenolic glycosides (salicortin, salireposide, and tremulacin), in agreement with the increase of the transcript levels of landmark biosynthetic genes. A change in the lignin structure (increase in the syringyl vs. guaiacyl, S/G ratio) was also observed. Poplar lines overexpressing the EgMYB88 dominant repression form did not show a strict opposite phenotype. The level of catechin was reduced, but the levels of the salicinoid phenolic glycosides and the S/G ratio remained unchanged. In addition, they showed a reduction in soluble oligolignols containing sinapyl p-hydroxybenzoate accompanied by a mild reduction of the insoluble lignin content. Altogether, these results suggest that EgMYB88, and more largely members of the WPS-I group, could control in cambium and in the first layers of differentiating xylem the biosynthesis of some phenylpropanoid-derived secondary metabolites including lignin.
Highlights
Eucalypts have extremely high adaptive potential and a considerable capacity to produce lignocellulosic biomass, explaining why this genus is the most planted hardwood worldwide for many industrial uses (Myburg et al, 2014)
Based on the phylogeny of the whole R2R3-MYB family in E. grandis (Soler et al, 2015), we selected the genes from the Woody Preferential Subgroups (WPS)-I and from closely related subgroups (S4, S5, S6, S7, SAtMYB5, SAtMYB82, WPS-II, and III) that constitute a phenylpropanoid metabolism-related super-clade
WPS-I is supported by a high bootstrap value and is clearly separated from the other subgroups
Summary
Eucalypts have extremely high adaptive potential and a considerable capacity to produce lignocellulosic biomass, explaining why this genus is the most planted hardwood worldwide for many industrial uses (Myburg et al, 2014). Lignin is a complex phenolic polymer which ensures essential functions for terrestrial plants providing mechanical support and facilitating the water transport It is composed of mainly two units, syringyl (S) and guaiacyl (G), with minor amounts of p-hydroxyphenyl (H) units, all produced by the monolignolspecific branch of the general phenylpropanoid metabolism. In addition to lignin monomers, the phenylpropanoid metabolism serves as a starting point for a vast array of other important compounds, such as flavonoids (flavonols, anthocyans, and proanthocyanidins), coumarins (Vogt, 2010), or phenolic glycosides (Babst et al, 2010) All these phenylpropanoid-derived secondary metabolites are crucial for plant survival, contributing to all aspects of plant responses toward biotic and abiotic stimuli (Vogt, 2010)
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