MYB216 Is Required For The Cellulose Synthesis In Populus

Wood,as the most abundant renewable resources on the earth,is mainly composed of secondary cell wall,while Cellulose is the main chemical component of cell wall.Cellulose synthesis is important for the wood formation of woody plant.CESAs play important roles in the cellulose synthesis.Since that three kind of CESAs constitute a functional cellulose synthesis complex and five secondary wall CESAs existed in the poplar genome.The process of plant cell wall formation,controlled by a wide variety of factors,is very complex.Multiple experiments have identified a few transcription factors(TFs)that function in the transcriptional regulation of cellulose synthesis.However,only few of them functioned in the regulation of cellulose synthesis in woody plant.On the one hand,we examined the expression pattern and interaction of these CESAs in Populus to reveal the work model of them in secondary wall cellulose synthesis.On the other hand,we focus on the important role of PagMYB216 gene in the molecular mechanism of cellulose synthesis.The main results are as follows:The sequence analysis of PagMYB216 gene showed that it belongs to the glycosyltransferase GT2 gene family.Expression analysis showed that all the five secondary wall CESAs exhibited high expression in the mature stem and preferred to express in the later stage of stem development,suggesting that they involved in the secondary wall synthesis during wood formation.Firefly luciferase complementation assay showed that CESA7 A and CESA7 B could interact with CESA8 A and CESA8 B,indicating that they parallelly determine the final secondary wall cellulose synthesis complex.Promotor driving GUS expression assay of CESA4,CESA7 B,CESA8A and CESA8 B demonstrated high expression of these secondary wall CESAs in stem and leaf,and some differences,however,were existed in the detail expression pattern,especially in leaf veins.In addition,the expression of secondary wall CESAs were upregulated during GA3 and 6-BA treatment,while down-regulated during NAA,BR and ethylene treatment.The expression variation of these CESAs in response to different hormones were different.Our data suggest that poplar secondary wall CESAs could interact toeach other and form the cellulose synthesis complex in equal possibility,but the complex might be varied in different tissues and in response to different hormones,which might have an effect on the wood chemical composition.In this study,we identified and characterized a poplar MYB transcription factor PagMYB216,a homolog of OsMYB61,and found that it is involved in the regulation of cellulose biosynthesis.PagMYB216 is highly expressed in stem in poplar.Subcellular localization and transcriptional activation analysis suggest that PagMYB216 is a nuclear-localized transcriptional activator.Transient transcriptional assays revealed that PagMYB216 is an activator of secondary wall CESAs expression.Consistently,overexpression of PagMYB216 in poplar resulted in an increase in secondary cell wall thickness in stem and an increase in cellulose content.qRT-PCR results showed that PagMYB216 specifically activated the expression of secondary wall CESAs.Furthermore,the dominant repression of PagMYB216 transgenic plants demonstrated a decrease in cellulose content and down-regulation of secondary wall CESAs.Taken together,our results suggested that PagMYB216 is involved in the regulation of secondary cell wall formation in poplar by modulating the biosynthesis of cellulose.