The protein phosphatase qGL3/OsPPKL1 self-regulates its degradation to orchestrate brassinosteroid signaling in rice.

Brassinosteroids (BRs) are a class of phytohormones that regulate plant growth and development. In previous studies, we cloned and identified PROTEIN PHOSPHATASE WITH KELCH-LIKE1 (OsPPKL1) as the causal gene for the quantitative trait locus GRAIN LENGTH3 (qGL3) in rice (Oryza sativa). We also showed that qGL3/OsPPKL1 is mainly located in the cytoplasm and nucleus and negatively regulates BR signaling and grain length. Because qGL3 is a negative regulator of BR signaling, its turnover is critical for rapid response to changes in BRs. Here, we demonstrate that qGL3 interacts with the WD40-domain-containing protein WD40-REPEAT PROTEIN48 (OsWDR48), which contains a nuclear export signal (NES). The NES signal is crucial for the cytosolic localization of OsWDR48 and also functions in the self-turnover of qGL3. We show that OsWDR48 physically interacts with and genetically acts through qGL3 to modulate BR signaling. Moreover, qGL3 may indirectly promote the phosphorylation of OsWDR48 at the Ser-379 and Ser-386 sites. Substitutions of both phosphorylation sites in OsWDR48 to non-phosphorylatable alanine enhanced the strength of the OsWDR48-qGL3 interaction. Furthermore, we found that brassinolide can promote the accumulation of non-phosphorylated OsWDR48, leading to strong interaction intensity between qGL3 and OsWDR48. Taken together, our results show that OsWDR48 facilitates qGL3 retention and induces degradation of qGL3 in the cytoplasm. These findings suggest that qGL3 self-modulates its turnover by binding to OsWDR48 to regulate its cytoplasmic localization and stability, leading to efficient orchestration of BR signal transduction in rice.