Investigating the regulatory mechanisms that govern plant growth is crucial for developing high-yield wood varieties. In this context, the KNOX gene family has been identified as a significant regulator of plant growth. Our study focuses on PagKNAT5a, a class II member of the KNOX gene family, which has been found to promote the growth of poplar. Transgenic plants overexpressing PagKNAT5a exhibited significant increases in both plant height and stem diameter compared to wild-type controls. Histochemical analyses revealed that these overexpression lines had elongated xylem vessels and fiber cells, which correlated with elevated auxin levels. Additionally, we observed thickened secondary cell walls (SCWs) and increased lignin content in the fiber cells of these transgenic lines. Further protein interaction assays indicated that PagKNAT5a physically interacts with MYB46, a crucial regulator of SCW biosynthesis. This interaction activates downstream secondary wall MYB-responsive elements (SMREs), leading to the upregulation of lignin biosynthesis genes driven by these cis-acting elements. Moreover, the increased photosynthetic rate observed in the overexpression lines is likely to significantly support overall plant development. Our findings suggest that PagKNAT5a facilitates the longitudinal elongation of vascular cells by modulating auxin levels while simultaneously promoting the radial growth of xylem tissue through the activation of the MYB46-mediated lignin biosynthesis pathway. The functional analysis of PagKNAT5a highlights its potential for improving wood yield in forestry applications.

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