PagMYB057 Activates <i>PagGT43B</i> Transcription to Enhance Xylan Biosynthesis in the Secondary Xylem of <i>Populus alba</i>  ×  <i>P. glandulosa</i> in Acclimation to Low Nitrogen Availability

Xylan is one of the main components of hemicellulose in the wood cell walls of Populus. Low nitrogen (LN) promotes cell wall thickening and hemicellulose biosynthesis in the wood. Glycosyltransferase 43B (GT43B) is a key gene in xylan biosynthesis. However, it remains unclear which specific gene regulates GT43B in xylan biosynthesis in acclimation to LN availability. Here, we report that a previously unrecognized PagMYB057-PagGT43B module regulates xylan biosynthesis in the secondary xylem of P. alba × P. glandulosa in acclimation to LN availability. PagGT43B was highly expressed in the secondary xylem and its expression was induced by LN. The thickness of fiber cell walls and xylan contents were increased in the secondary xylem of PagGT43B-overexpressing poplars compared with those of WT plants under LN conditions, whereas the opposite results were observed in PagGT43B-knockdown poplars compared to those of WT plants. Further molecular analyses revealed that PagMYB057 directly bound to the promoter sequence of PagGT43B to activate its expression. The phenotypes of PagMYB057-overexpressing poplars were similar to those of PagGT43B-overexpressing poplars under LN availability. These results suggest that PagMYB057 activates PagGT43B transcription to enhance xylan biosynthesis in the secondary xylem of P. alba × P. glandulosa in acclimation to LN availability.