COBRA-LIKE 9 modulates cotton cell wall development via regulating cellulose deposition.

Plant cell walls are complex and dynamic cellular structures critical for plant growth, development, physiology, and adaptation. Cellulose is one of the most important components of the cell wall. However, how cellulose microfibrils deposit and assemble into crystalline cellulose remains elusive. The COBRA-LIKE plant-specific protein family plays a vital role in modulating the deposition and orientation of cellulose microfibril in plant cell walls. Here, we investigate the role of GhCOBL9 in cotton (Gossypium hirsutum) fiber development, an ideal model for studying cell elongation and cell wall thickening. The expression period of GhCOBL9 is consistent with the thickening stage of the secondary wall of cotton fibers. Overexpression of GhCOBL9 results in increased cellulose content in the cell wall and produces shorter, thicker, and stronger fibers, while RNA interference (RNAi)-mediated downregulation of GhCOBL9 leads to the opposite phenotypes, indicating its crucial role in cell wall development. Subcellular localization and binding activity assays reveal that GhCOBL9 targets the cell wall and binds to crystalline cellulose with high affinity. Transcriptomic analysis of GhCOBL9 transgenic lines uncovers expression alterations in genes related to cellulose and monosaccharide biosynthesis. Furthermore, we identify a fasciclin-like arabinogalactan protein 9 (GhFLA9) as an interacting partner of GhCOBL9 to modulate cell wall development. Additionally, the R2R3-MYB transcription factor GhMYB46-5 activates GhCOBL9 expression by binding to the MYB46-responsive cis-regulatory element in the GhCOBL9 promoter. These findings broaden our knowledge of COBL function in modulating plant cell wall development.