Overexpression of OsAT-IV, a branched-chain amino acid aminotransferase gene, confers drought stress tolerance in rice

Abstract

Branched-chain amino acid aminotransferase (BCAT) proteins play crucial roles in the metabolism of branched-chain amino acids (BCAAs) leucine (Leu), isoleucine (Ile) and valine (Val). BCAAs have been reported to accumulate in response to stress which can then function as compatible solutes and/or as alternative source of respiratory substrates for the TCA cycle during stress. Though BCATs have been described in higher plants such as tomato, barley and Arabidopsis, information on roles of BCATs in rice remains unknown. Here, we show the roles of OsAT-IV, a BCAT in rice, in response to drought stress. Endogenous expression of OsAT-IV across different developmental stages was more dominant in root compared to shoot tissues. Under drought stress, OsAT-IV was up-regulated both in leaf and root tissues in an ABA-dependent manner. In silico analysis revealed 13 ABA-responsive cis¬-element motifs present within the 4-kb promoter region of OsAT-IV which could explain the ABA-dependent response of OsAT-IV. We also found that OsAT-IV was localized in cytosol as revealed through transient expression in rice protoplasts. In addition, overexpression of OsAT-IV in rice caused significant increase in BCAA concentration under normal conditions. Levels of Val, Ile and Leu were higher by 22.7, 35.6 and 29.6-fold in OsAT-IV overexpressing plants over WT, respectively. Upon exposure to drought stress, 100% of OsAT-IV overexpressing plants survived while those of WT plants were only 40%. Taken together, we show that overexpression of a BCAT, OsAT-IV, resulted in accumulation of BCAA leading to drought stress tolerance in rice.