Abstract
The receptor-like kinase SIT1 acts as a sensor in rice (Oryza sativa) roots, relaying salt stress signals via elevated kinase activity to enhance salt sensitivity. Here, we demonstrate that Protein Phosphatase 2A (PP2A) regulatory subunit B'kappa constrains SIT1 activity under salt stress. B'kappa-PP2A deactivates SIT1 directly by dephosphorylating the kinase at Thr515/516, a salt-induced phosphorylation site in the activation loop that is essential for SIT1 activity. B'kappa overexpression suppresses the salt sensitivity of rice plants expressing high levels of SIT1, thereby contributing to salt tolerance. B'kappa functions in a SIT1 kinase-dependent manner. During early salt stress, activated SIT1 phosphorylates B'kappa; this not only enhances its binding with SIT1, it also promotes B'kappa protein accumulation via Ser502 phosphorylation. Consequently, by blocking SIT1 phosphorylation, B'kappa inhibits and fine-tunes SIT1 activity to balance plant growth and stress adaptation.