WUSCHEL-RELATED HOMEOBOX genes are crucial for normal vascular organization and wood formation in poplar

Vascular cambium in tree species is a cylindrical domain of meristematic cells that are responsible for producing secondary xylem (also called wood) inward and secondary phloem outward. The poplar (Populus trichocarpa) WUSCHEL (WUS)-RELATED HOMEOBOX (WOX) family members, PtrWUSa and PtrWOX13b, were previously shown to be expressed in vascular cambium and differentiating xylem cells in poplar stems, but their functions remain unknown. Here, we investigated roles of PtrWUSa, PtrWOX13b and their close homologs in vascular organization and wood formation. Expression analysis showed that like PtrWUSa and PtrWOX13b, their close homologs, PtrWUSb, PtrWUS4a/b and PtrWOX13a/c, were also expressed in vascular cambium and differentiating xylem cells in poplar stems. PtrWUSa also exhibited a high level of expression in developing phloem fibers. Expression of PtrWUSa fused with the dominant EAR repression domain (PtrWUSa-DR) in transgenic poplar caused retarded growth of plants with twisted stems and curled leaves and a severe disruption of vascular organization. In PtrWUSa-DR stems, a drastic proliferation of cells occurred in the phloem region between vascular cambium and phloem fibers and they formed islands of ectopic vascular tissues or phloem fiber-like sclerenchyma cells. A similar proliferation of cells was also observed in PtrWUSa-DR leaf petioles and midveins. On the other hand, overexpression of PtrWOX4a-DR caused ectopic formation of vascular bundles in the cortical region, and overexpression of PtrWOX13a-DR and PtrWOX13b-DR led to a reduction in wood formation without affecting vascular organization in transgenic poplar plants. Together, these findings indicate crucial roles of PtrWUSa and PtrWOX13a/b in regulating vascular organization and wood formation, which furthers our understanding of the functions of WOX genes in regulating vascular cambium activity in tree species.