A NAC domain protein family contributing to the regulation of wood formation in poplar

Wood harvested from trees is one of the most widely utilized natural materials on our planet. Recent environmental issues have prompted an increase in the demand for wood, especially as a cost-effective and renewable resource for industry and energy, so it is important to understand the process of wood formation. In the present study, we focused on poplar (Populus trichocarpa) NAC domain protein genes which are homologous to well-known Arabidopsis transcription factors regulating the differentiation of xylem vessels and fiber cells. From phylogenetic analysis, we isolated 16 poplar NAC domain protein genes, and named them PtVNS (VND-, NST/SND- and SMB-related proteins) genes. Expression analysis revealed that 12 PtVNS (also called PtrWND) genes including both VND and NST groups were expressed in developing xylem tissue and phloem fiber, whereas in primary xylem vessels, only PtVNS/PtrWND genes of the VND group were expressed. By using the post-translational induction system of Arabidopsis VND7, a master regulator of xylem vessel element differentiation, many poplar genes functioning in xylem vessel differentiation downstream from NAC domain protein genes were identified. Transient expression assays showed the variation in PtVNS/PtrWND transactivation activity toward downstream genes, even between duplicate gene pairs. Furthermore, overexpression of PtVNS/PtrWND genes induced ectopic secondary wall thickening in poplar leaves as well as in Arabidopsis seedlings with different levels of induction efficiency according to the gene. These results suggest that wood formation in poplar is regulated by cooperative functions of the NAC domain proteins.