Abstract
Wood is mainly composed of secondary walls, which constitute the most abundant stored carbon produced by vascular plants. Understanding the molecular mechanisms controlling secondary wall deposition during wood formation is not only an important issue in plant biology but also critical for providing molecular tools to custom-design wood composition suited for diverse end uses. Past molecular and genetic studies have revealed a transcriptional network encompassing a group of wood-associated NAC and MYB transcription factors that are involved in the regulation of the secondary wall biosynthetic program during wood formation in poplar trees. Here, we report the functional characterization of poplar orthologs of MYB46 and MYB83 that are known to be master switches of secondary wall biosynthesis in Arabidopsis. In addition to the two previously-described PtrMYB3 and PtrMYB20, two other MYBs, PtrMYB2 and PtrMYB21, were shown to be MYB46/MYB83 orthologs by complementation and overexpression studies in Arabidopsis. The functional roles of these PtrMYBs in regulating secondary wall biosynthesis were further demonstrated in transgenic poplar plants showing an ectopic deposition of secondary walls in PtrMYB overexpressors and a reduction of secondary wall thickening in their dominant repressors. Furthermore, PtrMYB2/3/20/21 together with two other tree MYBs, the Eucalyptus EgMYB2 and the pine PtMYB4, were shown to differentially bind to and activate the eight variants of the 7-bp SMRE consensus sequence, composed of ACC(A/T)A(A/C)(T/C). Together, our results indicate that the tree MYBs, PtrMYB2/3/20/21, EgMYB2 and PtMYB4, are master transcriptional switches that activate the SMRE sites in the promoters of target genes and thereby regulate secondary wall biosynthesis during wood formation.
Highlights
Wood is produced by the activity of the vascular cambium, and it encompasses a complex developmental program involving the differentiation of the vascular cambium into secondary xylem mother cells, cell elongation, secondary wall deposition, programmed cell death, and heartwood formation [1]
Our previous study of PtrWND functions showed that the expression of PtrMYB2 and PtrMYB21 is induced by PtrWNDs, indicating that PtrMYB2 and PtrMYB21 are PtrWND-regulated downstream transcription factors involved in transcriptional regulation of wood formation [15]
Phylogenetic analysis revealed that PtrMYB2 and PtrMYB21 are grouped together with the Arabidopsis MYB46 and MYB83 genes, two other poplar MYB genes (PtrMYB3 and PtrMYB20) that were previously shown to be functional orthologs of MYB46 [24], and many additional MYB46 homologs from other plant species (Fig. 1A)
Summary
Wood is produced by the activity of the vascular cambium, and it encompasses a complex developmental program involving the differentiation of the vascular cambium into secondary xylem mother cells, cell elongation, secondary wall deposition, programmed cell death, and heartwood formation [1]. Genomic studies of wood formation have revealed thousands of genes that are induced during wood formation [2,3,4,5,6,7], some of which are transcriptional regulators suggested to be involved in the regulation of various developmental steps of wood development, such as cambial activity and secondary xylem differentiation [8,9,10,11]. It was found that a group of wood-associated NAC domain transcription factors (WNDs) are the top master switches regulating the expression of a number of downstream transcription factors, which lead to the biosynthesis of secondary walls during wood formation in tree species [13,14,15,16,17]. Note that the vein in the myb myb mutant has little secondary wall thickening, which is rescued by the expression of PtrMYB2 or PtrMYB21. doi:10.1371/journal.pone.0069219.g001
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