Functional Characterization Of Two Members Of ERD15 Gene Family From Solanum Pennellii

Plants are constantly confronted with a number of abiotic stresses, such as drought, salinity and extreme temperatures, which threaten their survival. There occurs physiological and metabolic flexibility, among the plants, to overcome these challenges. One of the key responses to stresses is the alteration in gene expression leading to changes in cellular functions, regulated by a complex network of transcription factors and associated regulatory proteins. All these intricate processes involve protein-protein interactions and/or post-translational modifications, resulting in the production of beneficial substances, such as sugars, compatible solutes, and antioxidants and/or degradation of toxic substances like reactive oxygen species.This work focuses on the two members of the Early Responsive to Dehydration 15 (ERD15) gene family, which are important regulators of developmental-and abiotic stress-related responses in tomato. The two genes, named as SpERD15-1 and SpERD15-2 because these were amplified from Solanum pennellii, were transcriptionally responsive to environmental signals and showed more or less same expression profile. Both were induced by dehydration, salinity, low and high temperature, paraquat toxicity, ABA, GA and ethylene treatments. SpERD15-1 transcript was abundant in roots, stems, and old leaves while, SpERD15-2 transcript level was high in roots, stems, flower and fruit as compared to leaves of S. pennellii.Both genes had sequence similarity with each other and also share some similarity with ERD15 from Arabidopsis (AtERD15). Both shares a PAM2 motif, that was highly conserved domain and common among the genes that interact with the C-terminal of poly(A)-binding protein (PABP). This domain was also functional in SpERD15-2 and therefore, SpERD15-2 interacted with PABP, as evident from the results of Bimolecular Fluorescence Complementation (BiFC) assay. Besides over-expression, SpERD15-1 also induced co-suppression in transgenic tobacco plants because it shares more than 90% similarity with the ERD15 from tobacco. The constitutive over-expression of both genes conferred tolerance to transgenic plants against drought, osmotic and oxidative stress. SpERD15-1 transgenic plants also displayed tolerance to salinity, but SpERD15-2 plants were sensitive. There was contradiction in salinity tolerance of SpERD15-2 in different growing conditions, tolerant in closed environment in MS medium and sensitive in soil. Results of salinity stress for SpERD15-2 suggest that salinity experiments must be performed in conditions where plant can transpire properly. SpERD15-1 over-expression increased the soluble sugars and proline contents and decreased the lipid peroxidation, which was closely related with the change in transcript level of stress-related genes. These solutes work in a variety of ways, such as protection of cellular structures, detoxification of the enzymes, and scavenging of ROS alone or in combination with other defense-related enzyme systems. These compounds confer integrity to the membranes and keep the photosynthetic system functioning, as evidenced by the higher Fv/Fm values observed for over-expression plants compared with Wt. Reduced photoinhibition in co-suppression lines might be due to lower proline levels. The accumulation of osmolytes under cold stress helped the over-expression lines in retention of sensing and transference of stress signals from the plasmalemma. Moreover, accumulation of these solutes helped to overcome the osmotic component of the salinity and resulted in the continued growth of SpERD15-1 over-expression plants, which provided additional surface area to sequester toxic ions and yielded significantly higher biomass accumulation and leaf area.In conclusion, ERD15 genes induce stress tolerance mainly through enhancement of osmotic adjustment, co-coordinated by altered expression of stress related genes. The identification of tomato ERD15 genes, most of whose functions are still unknown, opens new avenues for studies on plant developmental as well as stress responses.