GhTOPP4aD and GhRAF36 inversely regulate cotton (Gossypium hirsutum) response to ABA and salt stress through reversible phosphorylation of GhABI1

The post-translational phosphorylation modification of stress-related proteins regulated by kinases and phosphatases is one of the crucial regulatory mechanisms for plants in response to salt stress. However, the paired kinases and phosphatases of the same substrate that participate in response to salt tolerance in crops, especially in cotton, remain to be elucidated. Here, we identified GhTOPP4aD as a negative regulator of salt-stress response in cotton. GhTOPP4aD interacted with Raf-like kinase 36 (GhRAF36) and ABA Insensitive 1 (GhABI1) respectively, thereby inhibiting the phosphorylation activity of GhRAF36 and directly dephosphorylating GhABI1 to counteract GhRAF36 regulation. The phosphatase activity of GhABI1 was inhibited by GhRAF36-mediated phosphorylation at two unique residues Thr124 and Ser357 in cotton, whereas it was compromised by GhTOPP4aD. GhTOPP4aD thereby limited ABA signal transduction and orchestrated ABA-responsive gene expression. Together, modulation of the phosphorylation dynamics of GhABI1 by GhRAF36 kinase and GhTOPP4aD phosphatase constitutes an essential mechanism for ABA response and salt tolerance in cotton.