The tomato DWD motif-containing protein DDI1 interacts with the CUL4-DDB1-based ubiquitin ligase and plays a pivotal role in abiotic stress responses.

CULLIN4(CUL4)-DAMAGED DNA BINDING PROTEIN1 (DDB1)-based ubiquitin ligase plays significant roles in multiple physiological processes via ubiquitination-mediated degradation of relevant target proteins. The DDB1-CUL4-associated factor (DCAF) acts as substrate receptor in the CUL4-DDB1 ubiquitin ligase complex and determines substrate specificity. In this study, we identified a tomato (Solanum lycopersicum) DDB1-interacting (DDI1) protein as a DCAF protein involved in response to abiotic stresses, including UV radiation, high salinity and osmotic stress. Co-immunoprecipitation and bimolecular fluorescence complementation assay indicated that DDI1 associates with CUL4-DDB1 in the nucleus. Quantitative RT-PCR analysis indicated the DDI1 gene is induced by salt, mannitol and UV-C treatment. Moreover, transgenic tomato plants with overexpression or knockdown of the DDI1 gene exhibited enhanced or attenuated tolerance to salt/mannitol/UV-C, respectively. Thus, our data suggest that DDI1 functions as a substrate receptor of the CUL4-DDB1 ubiquitin ligase, positively regulating abiotic stress response in tomato.