Brassinolide sustains photosynthesis during high-light stress by modulating mitochondrial electron transport and cellular ROS.

Hormones such as abscisic acid and brassinosteroids ameliorate stress tolerance in plants. The present investigation demonstrates the importance of brassinolide, an active form of brassinosteroids, in sustaining photosynthesis under high light (HL). The addition of brassinolide to mesophyll protoplasts activates the mitochondrial electron transport chain (mETC), particularly through an alternative oxidase (AOX) pathway. Brassinolide promotes both respiration and photosynthesis, including PSII activity, under HL. Total respiration was enhanced, but brassinolide caused a differential modulation of the components of mETC. The capacity of the AOX pathway was significantly enhanced, while the capacity of the cytochrome oxidase (COX) pathway was decreased in response to brassinolide under HL. Further, the transcripts of alternative oxidase1A (AOX1A) were elevated more than cytochrome oxidase subunit 15 (COX15), and cellular reactive oxygen species (ROS) were raised marginally upon treatment with brassinolide under HL. Brassinolide enhanced the capacity of the AOX pathway in mETC to ensure an optimal cellular ROS, which in turn sustains photosynthesis. Thus, mETC plays an important role in optimizing photosynthesis under HL stress and highlights the potential of brassinolide in enhancing plant stress tolerance.