The Osmotin-Like Protein Gene Is Involved in Secondary Cell Wall Biosynthesis during Wood Formation in Poplar.

Osmotin-like proteins (OLPs) mediate defenses against abiotic and biotic stresses and fungal pathogens in plants. However, no OLPs have been functionally elucidated in poplar. Here, we report an osmotin-like protein designated from (Marsh.). Expression analysis showed that transcripts were mainly present in immature xylem and immature phloem during vascular tissue development in . . We conducted phenotypic, anatomical, and molecular analyses of -overexpressing lines and the -downregulated hybrid poplar 84K ( × ) (Hybrid poplar 84K PagOLP1, PagOLP2, PagOLP3 and PagOLP4 are highly homologous to , and are downregulated in -downregulated hybrid poplar 84K). The overexpression of led to a reduction in the radial width and cell layer number in the xylem and phloem zones, in expression of genes involved in lignin biosynthesis, and in the fibers and vessels of xylem cell walls in the overexpressing lines. Additionally, the xylem vessels and fibers of -downregulated poplar exhibited increased secondary cell wall thickness. Elevated expression of secondary wall biosynthetic genes was accompanied by increases in lignin content, dry weight biomass, and carbon storage in -downregulated lines. A coexpression network was constructed and showed that was coexpressed with a large number of genes involved in secondary cell wall biosynthesis and wood development in poplar. Moreover, based on transcriptional activation assays, PtobZIP5 and PtobHLH7 activated the promoter, whereas PtoBLH8 and PtoWRKY40 repressed it. A yeast one-hybrid (Y1H) assay confirmed interaction of PtoBLH8, PtoMYB3, and PtoWRKY40 with the 1 promoter in vivo. Together, our results suggest that is a negative regulator of secondary wall biosynthesis and may be valuable for manipulating secondary cell wall deposition to improve carbon fixation efficiency in tree species.