The alternative oxidase pathway is involved in the BR-induced salt resistance in mustard

In this study, the role of Brassinosteroids (BRs) and the relationship between the mitochondrial alternative oxidase (AOX) and ROS in the BR-induced defence response to salt stress was studied in mustard plants. Salt stress induced a significant activation of AOX. Exogenous BR significantly enhanced the capacity of the cyanide-resistant pathway, and reduced the damage of cell membrane. Pretreatment with brassinazole (Brz, an inhibitor of the BR biosynthesis pathway) significantly blocked the capacity of the cyanide-resistant pathway. BR could partly recover the AOX inactivation under salicylhydroxamic acid (SHAM, an inhibitor of the cyanide-resistant pathway) pretreatment. It was also found that BR could enhance the ROS accumulation and the antioxidant enzyme activities, while the AOX could eliminate the excessive ROS and enhance the antioxidant enzyme activities. Furthermore, the suppression of the cyanide-resistant pathway significantly increased the MDA content and the electrolyte leakage in mustard leaves, and the suppression of the BR biosynthesis had little effect on their recovering. Taken together, the cyanide-resistant pathway was involved in BR-induced salt tolerance and played an important role in maintaining the permeability of the cell membrane.