Rice OsPUB16 modulates the 'SAPK9-OsMADS23-OsAOC' pathway to reduce plant water-deficit tolerance by repressing ABA and JA biosynthesis

Ubiquitin-mediated proteolysis plays crucial roles in plant responses to environmental stress. However, the mechanism by which E3 ubiquitin ligases modulate plant stress response still needs to be elucidated. In this study, we found that rice PLANT U-BOX PROTEIN 16 (OsPUB16), a U-box E3 ubiquitin ligase, negatively regulates rice drought response. Loss-of-function mutants of OsPUB16 generated through CRISPR/Cas9 system exhibited the markedly enhanced water-deficit tolerance, while OsPUB16 overexpression lines were hypersensitive to water deficit stress. Moreover, OsPUB16 negatively regulated ABA and JA response, and ospub16 mutants produced more endogenous ABA and JA than wild type when exposed to water deficit. Mechanistic investigations revealed that OsPUB16 mediated the ubiquitination and degradation of OsMADS23, which is the substrate of OSMOTIC STRESS/ABA-ACTIVATED PROTEIN KINASE 9 (SAPK9) and increases rice drought tolerance by promoting ABA biosynthesis. Further, the ChIP-qPCR analysis and transient transactivation activity assays demonstrated that OsMADS23 activated the expression of JA-biosynthetic gene OsAOC by binding to its promoter. Interestingly, SAPK9-mediated phosphorylation on OsMADS23 reduced its ubiquitination level by interfering with the OsPUB16-OsMADS23 interaction, which thus enhanced OsMADS23 stability and promoted OsAOC expression. Collectively, our findings establish that OsPUB16 reduces plant water-deficit tolerance by modulating the 'SAPK9-OsMADS23-OsAOC' pathway to repress ABA and JA biosynthesis. Author summary Plants face various adverse environmental stimuli during their growth, such as drought, salinity and extreme temperatures. Different from animals, plants cannot move, so they must endure abiotic stresses, which greatly limit the distribution of plants, alter their growth and development, and reduce crop productivity. To adapt to the major adverse environmental conditions, plants have developed multifaceted strategies at different levels. Among these, protein post-translational modifications play crucial roles. However, how protein post-translational modifications in response to environmental stress hormones modulate plant stress response still needs to be elucidated. Here, our results showed that the two types of post-translational modifications, phosphorylation and ubiquitination, regulate the balance between plant growth and stress response. Furthermore, there is the close link between the two types of post-translational modifications, which was affected by environmental stress hormones. The research will help us understand the mechanisms underlying the "trade-off" between plant growth and stress response, and engineer stress-resistant and high-yielding crops.