UBIQUITIN-CONJUGATING ENZYME34 Mediates Pyrophosphatase AVP1 Turnover and Regulates Abiotic Stress Responses in Arabidopsis

Understanding the molecular mechanisms of abiotic stress responses in plants is instrumental for the development of climate-resilient crops. Key factors in abiotic stress responses, such as the proton-pumping pyrophosphatase (AVP1), have been identified, but their function and regulation remain elusive. Here, we explored the post-translational regulation of AVP1 by the ubiquitin-conjugating enzyme UBC34 and its relevance in the salt stress and phosphate starvation responses of Arabidopsis (Arabidopsis thaliana). Through in vitro and in vivo assays, we established that UBC34 interacts with and ubiquitylates AVP1. Mutant lines in which UBC34 was downregulated showed higher tolerance to salt and low inorganic phosphate (Pi) stresses, while we observed the opposite for plants overexpressing UBC34. Our results showed that UBC34 co-localizes with AVP1, and AVP1 activity is enhanced in the plasma membrane fractions of ubc34 mutants, indicating that UBC34 mediates the turnover of plasma membrane-localized AVP1. We also observed that UBC34 affects the apoplastic pH but not the vacuolar pH of root cells. Based on our results, we propose a mechanistic model in which UBC34 mediates AVP1 turnover at the plasma membrane of root epidermal cells. Downregulation of UBC34 under salt and phosphate starvation conditions enhances AVP1 activity, leading to a higher proton gradient available for sodium sequestration and phosphate uptake.