Response of Rhododendron simsii and Rhododendron delavayi Superoxide Dismutase Family Genes to High-Temperature Stress

Superoxide dismutases (SODs) are the first line of defense in the antioxidant defense system, and they play an essential role in various adversity stress adaptations in Rhododendron. In this study, 9 Rhododendron simsii SODs (RsSODs) and 11 Rhododendron delavayi SODs (RdSODs) genes were identified in the genomes of R. simsii and R. delavayi. Phylogenetic relationship analysis classified SOD proteins from two Rhododendron species and other related species into three subfamilies. The results of gene structure and conserved motif analysis show that SOD proteins are strongly evolutionarily conserved, and SODs of the same subfamily have similar motif distributions, positions, and lengths. Twenty-two light-responsive elements, eight phytohormone regulatory elements, five adversity stress-related elements, and three growth and development regulatory elements were detected in the RsSOD and RdSOD promoters. Quantitative real-time fluorescence polymerase chain reaction analysis showed that among the 20 candidate genes, except for RdCSD5, the other SODs were expressed in at least one of four tissues, and all of these gene family members had high expression levels in the leaves. We then investigated the response of the RsSOD and RdSOD gene families to high-temperature stress in combination with the following specific stressors: abscisic acid, ethephon, and hydrogen peroxide treatments, followed by high-temperature stress. Different degrees of upregulated expression of the detected SOD gene family members were found for exogenous reagent treatments and different times of high-temperature stress. Thus, we provide a basis for the further functional characterization of SOD genes in R. simsii and R. delavayi in the future.