SUMO E3 Ligase AtMMS21 Regulates Drought Tolerance in Arabidopsis thaliana

Post-translational modifications of proteins by small ubiquitin-like modifiers (SUMOs) play crucial roles in plant growth and development, and in stress responses. The MMS21 is a newly-identified Arabidopsis thaliana L. SUMO E3 ligase gene aside from the SIZ1, and its function requires further elucidation. Here, we show that MMS21 deficient plants display improved drought tolerance, and constitutive expression of MMS21 reduces drought tolerance. The expression of MMS21 was reduced by abscisic acid (ABA), polyethylene glycol (PEG) or drought stress. Under drought conditions, mms21 mutants showed the highest survival rate and the slowest water loss, and accumulated a higher level of free proline compared to wild-type (WT) and MMS21 over-expression plants. Stomatal aperture, seed germination and cotyledon greening analysis indicated that mms21 was hypersensitive to ABA. Molecular genetic analysis revealed that MMS21 deficiency led to elevated expression of a series of ABA-mediated stress-responsive genes, including COR15A, RD22, and P5CS1 The ABA and drought-induced stress-responsive genes, including RAB18, RD29A and RD29B, were inhibited by constitutive expression of MMS21. Moreover, ABA-induced accumulation of SUMO-protein conjugates was blocked in the mms21 mutant. We thus conclude that MMS21 plays a role in the drought stress response, likely through regulation of gene expression in an ABA-dependent pathway.