Comparative phosphoproteomic analysis reveals phosphorylation of sucrose synthase GhSUS2 by Ca 2+-dependent protein kinases GhCPK84/93 affects fiber development of cotton.

Cotton fiber elongation is a critical growth phase that affects final fiber length. Morphological analysis indicated an asynchronous fiber elongation pattern between two cotton varieties J7-1 and J14-1. Through phosphoproteomic analysis, total 89 differentially-phosphorylated proteins (DPPs) were identified in elongating fibers between J7-1 and J14-1. Gene ontology (GO) analysis showed that these DPPs were mainly enriched in sucrose synthase activity, transferase activity and UDP-glycosyltransferase activity. In J14-1, the phosphorylation level of GhSUS2, a key sucrose synthase in sucrose metabolism pathway, is significantly higher than that in J7-1. We further revealed that GhSUS2 positively regulates fiber elongation, and GhSUS2-silenced transgenic cotton displayed the phenotype of "short fibers" compared with the controls. During fiber development, the residue Ser11 in GhSUS2 protein is phosphorylated by Ca 2+-dependent protein kinases GhCPK84 and GhCPK93. The phosphorylated GhSUS2 is localized in the cytoplasm, while unphosphorylated GhSUS2 is localized in the plasma membrane. Moreover, abscisic acid (ABA) could promote the transcription and translation of GhCPK84 and GhCPK93, thereby enhancing the phosphorylation of GhSUS2 to impede fiber elongation. Thus, our data demonstrated that GhSUS2 plays a positive role in fiber development, but its phosphorylation by GhCPK84 and GhCPK93 hinders fiber elongation of cotton.