Heat shock protein modulates cell expansion via <scp>ROS</scp> homeostasis

Heat shock proteins (HSPs) are evolutionarily conserved, yet their functions in plant growth and development remain incompletely characterized. Here, we demonstrate that a HSP90 co-chaperone PpNudC6 is essential for directional cell expansion in the moss Physcomitrium patens. We generated ppnudc6 mutants and characterized their phenotypes. Dysregulation of PpNudC6 disrupts cellulose microfibril organization and cell wall stiffness gradients, as shown by scanning electron microscopy and atomic force microscopy, ultimately resulting in shortened and thickened protonemal cells. Mechanistically, this phenotype is mediated by disrupted reactive oxygen species (ROS) homeostasis. Loss of PpNudC6 function induces ectopic activity of the NADPH oxidase PpRbohD in protonemata, leading to abnormal ROS accumulation. Pharmacological inhibition of NADPH oxidases by diphenyleneiodonium rescues mutant phenotypes, confirming ROS overproduction as the primary driver of developmental defects. Furthermore, PpNudC6 interacts with the scaffold protein PpRACK1B and the co-chaperone PpSGT1, suggesting a multisubunit complex that modulates respiratory burst oxidase homolog (Rboh) activity. In summary, our findings reveal a chaperone-mediated regulatory module that mediates the production of ROS, thereby maintaining cell wall mechanical anisotropy required for directional expansion. This work provides insights into a novel role of HSP complexes in regulating directional cell expansion and links redox homeostasis to cell wall mechanics during moss development.