Cellular glutathione redox homeostasis plays an important role in the brassinosteroid‐induced increase in CO 2 assimilation in Cucumis sativus

Brassinosteroids (BRs) play a vital role in plant growth, stress tolerance and productivity. Here, the involvement of BRs in the regulation of CO(2) assimilation and cellular redox homeostasis was studied. The effects of BRs on CO(2) assimilation were studied in cucumber (Cucumis sativus) through the analysis of the accumulation of H(2)O(2) and glutathione and photosynthesis-related enzyme activities using histochemical and cytochemical detection or a spectrophotometric assay, and Rubisco activase (RCA) using western blot analysis and immunogold labeling. Exogenous BR increased apoplastic H(2)O(2) accumulation, the ratio of reduced to oxidized glutathione (GSH:GSSG) and CO(2) assimilation, whereas a BR biosynthetic inhibitor had the opposite effects. BR-induced CO(2) assimilation was decreased by a H(2)O(2) scavenger or inhibition of H(2)O(2) generation, GSH biosynthesis and the NADPH-generating pentose phosphate pathway. BR-, H(2)O(2) - or GSH-induced CO(2) assimilation was associated with increased activity of enzymes in the Benson-Calvin cycle. Immunogold labeling and western blotting showed that BR increased the content of RCA and this effect was blocked by inhibitors of redox homeostasis. These results strongly suggest that BR-induced photosynthesis involves an H(2)O(2) -mediated increase in the GSH:GSSG ratio, which may positively regulate the synthesis and activation of redox-sensitive enzymes in carbon fixation.