PpXTH5 and PpXTH6, Xyloglucan Endotransglucosylases, Regulate Plasmodesmata Density and Structure in the Moss Physcomitrium Patens

Plant cell-cell communication plays a crucial role during growth and development. While plasmodesmata (PD) are well known in this communication, their biogenesis and regulation remain poorly understood. In this study, we demonstrate that xyloglucan endotransglucosylases (XETs), enzymes involved in transglycosylation of xyloglucan chains, could impact not only cellular growth but also PD development in the moss Physcomitrium patens (P. patens). Knockout mutant lines of two XET isoforms, named Ppxth5, Ppxth6, and Ppxth5,6dko, showed growth reductions in both protonemal and gametophytic stages. Subsequent analyses of these mutants indicated a decrease in PD density, abnormal clusters of xyloglucan and callose, and their colocalization, highlighting their potential involvement in PD formation. Further evaluation of these alterations using Dendra2 photoconvertible protein showed impaired intercellular communication, indicating reduced intercellular transport compared to WT. For PD structural changes, these mutants show higher ratios of incomplete simple PD and complex PD, highlighting the critical role of xyloglucan. Specifically, proper xyloglucan distribution, which is regulated by XET activities, is required for maintenance of PD density and structure, and for intercellular transport, possibly interacting with callose during cell plate formation.