Cellular glutathione redox homeostasis plays an important role in the brassinosteroid-induced increase in CO2 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 CO2 assimilation and cellular redox homeostasis was studied. The effects of BRs on CO2 assimilation were studied in cucumber (Cucumis sativus) through the analysis of the accumulation of H2O2 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 H2O2 accumulation, the ratio of reduced to oxidized glutathione (GSH:GSSG) and CO2 assimilation, whereas a BR biosynthetic inhibitor had the opposite effects. BR-induced CO2 assimilation was decreased by a H2O2 scavenger or inhibition of H2O2 generation, GSH biosynthesis and the NADPH-generating pentose phosphate pathway. BR-, H2O2- or GSH-induced CO2 assimilation was associated with increased activity of enzymes in the BensonCalvin 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 H2O2-mediated increase in the GSH:GSSG ratio, which may positively regulate the synthesis and activation of redox-sensitive enzymes in carbon fixation.