Xylem vessels have a distinct secondary cell wall (SCW) formation, where the metaxylem stage contains a characteristic pit pattern. The hemicellulose xylan is an important component of the SCW, functioning as a crosslinking polymer that coats cellulose microfibrils and interacts with other SCW components. However, the xylan initiation mechanism was unknown. Here, Wang et al. found that the IRX10 protein in rice (ortholog of a known Arabidopsis protein) and its homologues perform dual roles in xylan initiation and elongation during xylan synthesis, using in vitro enzyme studies. They demonstrate in both rice and Arabidopsis that xylan accumulates at the pit borders of the metaxylem, which is disrupted in irx10 mutants. Using various microscopy techniques, they show that the mutants have larger pit areas and smaller vessels, and reduced xylose content. Additionally, the cellulosic nanofibers in the SCW are not properly packed, and the SCW defects result in impaired xylem water transport function. (Summary by Jiawen Chen @Jiaaawen) Nature Plants 10.1038/s41477-022-01113-1.
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