PtrCel9A6, an Endo-1,4--Glucanase, Is Required for Cell Wall Formation during Xylem Differentiation in Populus

This study characterizes a new type of endo-1,4--glucanase, PtrCel9A6, which is specifically expressed in developing xylem cells in Populus. The results demonstrate that PtrCel9A6 is localized on plasma membrane for remodeling the 1,4--glucan chains in the wall matrix and is required for cell wall thickening during xylem differentiation.Endo-1,4--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 PtrCel9A6 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--glucan chains in the wall matrix and is required for cell wall thickening during Populus xylem differentiation.