Tomato RING Type E3 Ligases, SlRGLGs, Positively Regulate the Dehydration Stress Response

The ubiquitin-proteasomal protein degradation system is a key regulatory process mediating the dehydration stress response in plants, and RGLG proteins, a subfamily of the RING E3 ligases, are well known to modulate this response. In this study, we isolated four SlRGLG proteins (Solanum lycopersicum RING domain ligase) from tomato plants and characterized their functions at the molecular and biological levels. We found that these four SlRGLGs have the conserved VWA and RING domains and high amino acid sequence identities with RGLGs from Arabidopsis thaliana and pepper plants. The transcript levels of SlRGLGs were found to be responsive to several environmental stimuli, including dehydration, mannitol, and abscisic acid, which are believed to be associated with the presence of different stress-associated cis-regulatory elements in the respective promoter regions. Subcellular localization studies of SlRGLGs-GFP fusion proteins revealed distinct subcellular distribution patterns, and all four MBP-SlRGLGs recombinant proteins exhibited robust E3 ligase activities in vitro. To elucidate their biological roles in the dehydration stress response, we generated SlRGLGs-silenced tomato plants and SlRGLGs-overexpressing (OE) Arabidopsis plants. Notably, all SlRGLGs-silenced tomato plants were found to have dehydration-sensitive phenotypes with increased transpirational water loss and lipid peroxidation of cell membranes and decreased expression of dehydration stress-responsive genes. However, all SlRGLGs-OE Arabidopsis plants showed the dehydration-tolerant phenotypes, compared to control plants. Collectively, these findings indicate a positive role for all four SlRGLGs in the dehydration stress response of tomato.