OsPHO2 promotes the ubiquitin-dependent degradation of OsHMGB1 to fine-tune phosphate homeostasis in rice.

Phosphate (Pi) homeostasis is critical for rice growth and development. Previous studies have shown that OsHMGB1, a chromatin-associated protein, positively regulates Pi accumulation by modulating the chromatin accessibility of phosphate starvation-responsive (PSR) genes. OsHMGB1 is induced under Pi-deficient conditions, and its protein level declines more rapidly under Pi-sufficient conditions, suggesting the existence of post-translational regulation. However, the mechanism underlying OsHMGB1 protein degradation remains unclear. Here, we demonstrate that the degradation of OsHMGB1 under Pi-sufficient conditions depend on the ubiquitin-26S proteasome pathway. OsHMGB1 interacts with OsPHO2, an E2 ubiquitin-conjugating enzyme and a key negative regulator of Pi signaling in rice. The protein abundance of OsHMGB1 is increased in the pho2 mutant due to reduced protein degradation. Furthermore, in vivo ubiquitination assays indicate that OsPHO2 facilitates the ubiquitination of OsHMGB1. Genetic analysis shows that the hmgb1pho2 double mutant partially suppresses the excessive Pi accumulation phenotype of pho2 in older leaves, indicating a genetic interaction between OsHMGB1 and OsPHO2 in Pi homeostasis. In addition, transcriptome analysis reveals that OsPHO2 and OsHMGB1 oppositely regulate the expression of a series of PSR genes, as validated by RT-qPCR for OsPT1, OsPT4, and OsRNS3. Collectively, these findings suggest that OsPHO2 maintains Pi homeostasis in rice partially by mediating the ubiquitin-dependent degradation of OsHMGB1.