Mutual regulation of receptor-like kinase SIT1 and B'κ-PP2A shapes the early response of rice to salt stress

The receptor-like kinase SIT1 acts as a sensor in rice roots, relaying salt stress signals via elevated kinase activity to enhance salt sensitivity. Here, we demonstrate that Protein Phosphatase 2A (PP2A) regulatory subunit B9κ constrains SIT1 activity under salt stress. B9κ-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. B9κ overexpression suppresses the salt sensitivity of rice plants expressing high levels of SIT1, thereby contributing to salt tolerance. B9κ functions in a SIT1 kinase-dependent manner. During early salt stress, activated SIT1 phosphorylates B9κ; this not only enhances its binding with SIT1, it also promotes B9κ protein accumulation via Ser502 phosphorylation. Consequently, by blocking SIT1 phosphorylation, B9κ inhibits and fine tunes SIT1 activity to balance plant growth and stress adaptation.