Analysis of a salinity induced BjSOS3 protein from Brassica indicate it to be structurally and functionally related to its ortholog from Arabidopsis

Arabidopsis has been a favorite model system for plant biologist. It is anticipated that comparative analysis of this plant with other members of Brassicaceae may aid in identification of orthologs playing role as key genetic determinants for salinity response. In this endeavor, we have recently identified SOS family members from Brassica juncea in our laboratory and reported their salinity responsive transcriptional induction in seedlings of various diploid and amphidiploids species. In the present study, we have carried out detailed time kinetics for BjSOS3 expression in a salinity tolerant B. juncea var. CS52. Transcript analysis at the sensitive growth stages of plants viz. seedling and reproductive stage indicated clear differential transcriptional regulation of BjSOS3 under non-induced as well as salinity induced conditions in a time and organ specific manner, mirroring their respective tolerance physiology. Similar to its ortholog from Arabidopsis thaliana, the modeled BjSOS3 protein show typical features of a Ca2+ binding protein with four conserved EF-hands. We have also attempted to study the binding of SOS3 protein with the modeled SOS2 protein. It has been established that SOS3 protein senses Ca2+ though the binding is very weak; we show the down regulation of BjSOS3 mRNA in presence of calcium chelator – EGTA under the various stress conditions including ABA. In situ localization of BjSOS3-GFP fusion protein in onion peel has shown its presence strongly in plasma membrane as well as cytosol. The leads presented in the paper will assist in understanding and establishing the SOS signaling machinery in B. juncea.