Physiological and transcriptome analyses provide new insights into the mechanism mediating the enhanced tolerance of melatonin-treated rhododendron plants to heat stress

Rhododendron is a well-known genus consisting of commercially valuable ornamental woody plant species. Heat stress is a major environmental factor that affects rhododendron growth. Melatonin was recently reported to alleviate the effects of abiotic stress on plants. However, the role of melatonin in rhododendron plants is unknown. In this study, the effect of melatonin on rhododendron plants exposed to heat stress and the potential underlying mechanism were investigated. Analyses of morphological characteristics and chlorophyll a fluorescence indicated 200 µmol L–1 was the optimal melatonin concentration for protecting rhododendron plants from heat stress. To elucidate how melatonin limits the adverse effects of high temperatures, melatonin contents, photosynthetic indices, Rubisco activity, and adenosine triphosphate (ATP) contents were analyzed at 25, 35, and 40°C, respectively. Compared with the control, exogenous application of melatonin improved the melatonin contents, electron transport rate, photosystem II and I activities, Rubisco activity, and ATP contents under heat stress. The transcriptome analysis revealed many of the heat-induced differentially expressed genes were associated with the photosynthetic pathway; the expression of most of these genes was down-regulated by heat stress more in the melatonin-free plants than in the melatonin-treated plants. We identified RhPGR5A, RhATPB, RhLHCB3, and RhRbsA as key genes. Thus, we speculate that melatonin promotes photosynthetic electron transport, improves Calvin cycle enzyme activities, and increases ATP production. These changes lead to increased photosynthetic efficiency and CO2 assimilation under heat stress conditions via the regulated expression of specific genes, including RhRbsA. Therefore, the application of exogenous melatonin may increase the tolerance of rhododendron to heat stress.