A conserved phospho-switch controls receptor kinase SDS2 activation and homeostasis in rice immunity

Plant surface immune receptors are tightly controlled to maintain homeostasis and prevent overactivation. However, the mechanisms coordinating the balance remain largely unknown. The monocot-specific receptor-like kinase SDS2 regulates cell death and immunity in rice. Here, we identify Thr671, a conserved phospho-switch in RD-type Ser/Thr kinases across kingdoms, as the central node that orchestrates both immune activation and turnover of SDS2. Autophosphorylation of Thr671 initiates signaling but also primes SDS2 for ubiquitination and degradation by the E3 ligase SPL11. This degradation is antagonized by the phosphatase SIPP1, which stabilizes SDS2 through Thr671 dephosphorylation to promote immunity. Strikingly, a second phosphatase, SIPP2, displaces SIPP1 from SDS2 via competitive binding, thereby enabling SPL11-dependent SDS2 degradation and ensuring timely immune attenuation. Thus, the SIPP1-SPL11-SIPP2 module establishes a dynamic equilibrium essential for SDS2 homeostasis, providing new insights into the sophisticated regulation of surface receptors.