A defect in BRI1-EMS-SUPPRESSOR 1 (bes1)-mediated brassinosteroid signaling increases photoinhibition and photo-oxidative stress during heat stress in Arabidopsis

Heat stress has negative effects on plant performance, especially in photosynthesis and photoprotection. To withstand heat stress, plants modulate steroid phytohormones, brassinosteroids (BRs). However, their regulation and functions in heat stress acclimation are still poorly understood. Plant growth, photoinhibition, photo-oxidative stress and endogenous contents of hormones (including not only BRs but also abscisic acid, salicylic acid and jasmonic acid) were evaluated during heat stress in Arabidopsis thaliana wild type plants and loss-of-function mutations in either BR biosynthetic or signaling genes. It was found that a defect in BRI1-EMS-SUPPRESSOR 1 (bes1)-mediated BR signaling showed the most sensitive characteristics to heat stress compared to wild type and other BR mutants. Sensitivity was associated with declined PSII photochemistry efficiency (F-v/F-m) together with increased carotenoid, tocopherol and lipid hydroperoxide contents, which evidences higher photoinhibition and photo-oxidative stress in the bes1 mutant under heat stress. Furthermore, the bes1 mutant showed greater contents of abscisic acid (ABA) after one day of exposure to heat stress. However, all heat stress symptoms in the bes1 mutant could be mitigated by the application of 24-epibrassinolide. Therefore, it is concluded that BES1 transcription factor plays a role in plants responses to heat stress, relieving photoinhibition and photo-oxidative stress, but that alternative BR signaling pathways to BES1 may also be effective in heat stress acclimation. Furthermore, this study emphasizes the complex interplay between BR and ABA in the heat acclimation process.