Phosphorylation of LTF1, an MYB Transcription Factor in Populus, Acts as a Sensory Switch Regulating Lignin Biosynthesis in Wood Cells

Lignin is specifically deposited in plant secondary cell walls,and initiation of lignin biosynthesis is regulated by a variety of developmental and environmental signals.However,the mechanisms governing the regulation of lignin biosynthesis remain to be elucidated.In this study,we identified a lignin biosynthesis-associated transcription factor(LTF)from Populus,LTF1,which binds the promoter of a key lignin biosynthetic gene encoding 4-coumarate-CoA ligase(4CL).We showed that LTF1 in its unphosphorylated state functions as a regulator restraining lignin biosynthesis.When LTF1 becomes phosphorylated by PdMPK6 in response to external stimuli such as wounding,it undergoes degradation through a proteasome pathway,resulting in activation of lignification.Expression of a phosphorylation-null mutant version of LTF1 led to stable protein accumulation and persistent attenuation of lignification in wood cells.Taken together,our study reveals a mechanism whereby LTF1 phosphorylation acts as a sensory switch to regulate lignin biosynthesis in response to environmental stimuli.The discovery of novel modulators and mechanisms modifying lignin biosynthesis has important implications for improving the utilization of cell-wall biomass.

PtrCel9A6, an Endo-l,4-β-Glucanase, Is Required for Cell Wall Formation during Xylem Differentiation in Populus

ABSTRACT Endo-l,4-β-glucanases （EGases） are involved in many aspects of plant growth. Our previous study found that an EGase, PtrCel9A6, is specifically expressed in differentiating xylem cells during Populus secondary growth. In this study, the xylem-specific PtrCel9A6 was characterized for its role in xylem differentiation. The EGase is localized on the plasma membrane with catalytic domain toward the outside cell wall, hydrolyzing amorphous cellulose. Suppression of PtrCel9A6 expression caused secondary cell wall defects in xylem cells and significant cellulose reduction in Populus. Heterologous expression of PtrCelgA6 in Arabidopsis enhanced plant growth as well as increased fiber cell length. In addition, introduction of PtrCel9A6 into Arabidopsis resulted in male sterility due to defects in anther dehiscence. Together, these results demonstrate that PtrCel9A6 plays a critical role in remodeling the 1,4-β-glucan chains in the wall matrix and is required for cell wall thickening during Populus xylem differentiation.

TwoMADS-boxgenes regulate vascularcambium activity and secondary growth by modulating auxin homeostasis in Populus

In trees,stem secondary growth depends on vascular cambium proliferation activity and subsequent cell differentiation,in which an auxin concentration gradient across the cambium area plays a crucial role in regulating the process.However,the underlying molecular mechanismfor the establishment of auxin concentration is not fully understood.In this study,we identified two function-unknown MADS-box genes,VCM1 and VCM2,which are expressed specifically in the vascular cambium and modulate the subcellular homeostasis of auxin.Simultaneous knockdown of both VCM1 and VCM2 enhanced vascular cambium proliferation activity and subsequent xylem differentiation.Overexpression of VCM1 suppressed vascular cambium activity and wood formation by regulating PIN5 expression,which tuned the soluble auxin concentration in the vascular cambium area.This study reveals the role of VCM1 and VCM2 in regulating the proliferation activity of the vascular cambium and secondary growth by modulating the subcellular auxin homeostasis in Populus.