An RGLG E3 Ubiquitin Ligase CaSIRE1 Negatively Regulates ABA-Dependent Drought Tolerance by Destabilizing CaSAP14 in Pepper.

Plants respond to drought stress primarily through the abscisic acid (ABA) signalling pathway, which is finely regulated by post-translational modifications. Among these, the ubiquitin-proteasome system plays a central role in controlling protein turnover, with E3 ligases conferring substrate specificity. Here, we identify CaSIRE1 (Capsicum annuum SAP14 Interacting RING-type E3 ligase 1), which encodes a RING-domain ligase belonging to the C3HC4 RING-type E3 ligase family, as a negative regulator of drought stress response and ABA signalling in pepper. Functional analyses revealed that silencing of CaSIRE1 in pepper enhances drought tolerance and ABA sensitivity, whereas overexpression of CaSIRE1 in Arabidopsis compromises these responses. CaSIRE1 physically interacts with CaSAP14, a stress-associated protein previously characterized as a positive regulator of drought stress response, and promotes its ubiquitination and subsequent degradation via the 26S proteasome pathway. Importantly, CaSIRE1-dependent CaSAP14 degradation is attenuated under drought conditions, resulting in increased CaSAP14 stability. Genetic analysis further demonstrated that CaSAP14/CaSIRE1 double-silenced pepper plants displayed drought-sensitive phenotypes comparable to CaSAP14-silenced plants, indicating that CaSIRE1 acts upstream of CaSAP14 in ABA signalling and drought stress response. Together, our findings uncover a CaSIRE1-CaSAP14 regulatory module that dynamically modulates protein stability to fine-tune ABA-mediated drought stress response in pepper.