Brassinolide promotes chloroplasts mitochondria interactions during the optimization of photosynthesis by mitochondrial electron transport chain in mesophyll protoplasts of arabidopsis thaliana

Abstract

In plant mitochondrial electron transport chain (mETC), the movement of electrons bifurcates at ubiquinone to follow either the cyanide-sensitive cytochrome oxidase (COX) pathway or cyanide-resistant alternative oxidase (AOX) pathway, respectively, and reduces the molecular O2 into the H2O molecules. The electron transport through the COX pathway generates a proton gradient and synthesizes ATP. In contrast, electron transport through the AOX pathway is not coupled to ATP synthesis (bypasses complex III and IV), and energy is dissipated as heat. Further, the COX and AOX pathways of mETC is well known to optimize photosynthesis by oxidizing the excess chloroplastic reducing equivalents received via malate-valve and maintaining reactive oxygen species (ROS) at low levels by coordinating with antioxidant systems. newlineIn addition, brassinolide, an active form of brassinosteroids, is a polyhydroxy steroidal phytohormone. They play a very important role in plant growth and development. Their external application is known to support photosynthesis under stress conditions with the help of an antioxidant system. However, the information on the impact of brassinosteroids in promoting the interactions between chloroplasts and mitochondria to optimize photosynthesis via COX or AOX pathways of mETC is limited under both normal and stress conditions. newlineTherefore, the present study employs the advantage of using mesophyll protoplasts and leaf discs to ascertain the importance of brassinolide in promoting the beneficial interactions between chloroplasts and mitochondria to optimize photosynthesis through the COX or AOX pathways of mETC using inhibitor or reverse genetic approaches under normal light or high light, or oxidative stress. The various parameters associated with respiration, photosynthesis, redox, and ROS at biochemical and transcript levels were monitored under those conditions. The result of the present study suggests that (i) brassinolide promotes photosynthesis through active participation of the COX and AOX pathway of m