Comparative transcriptomics of the chilling stress response in two Asian mangrove species Bruguiera gymnorhiza and Rhizophora apiculata.

Low temperatures largely determine the geographic limits of plant species by reducing survival and growth. Interspecific differences in the geographic distribution of mangrove species have been associated with cold tolerance, with exclusively tropical species being highly cold-sensitive and subtropical species being relatively cold-tolerant. To identify species-specific adaptations to low temperatures, we compared the chilling stress response of two widespread Indo-West Pacific mangrove species from Rhizophoraceae with differing latitudinal range limits-Bruguiera gymnorhiza (subtropical range limit) and Rhizophora apiculata (tropical range limit). For both species, we measured the maximum photochemical efficiency of photosystem II (Fv/Fm) as a proxy for the physiological condition of the plants and examined gene expression profiles during chilling at 15 °C and 5 °C. At 15 °C, B. gymnorhiza maintained a significantly higher Fv/Fm than R. apiculata. However, at 5 °C, both species displayed equivalent Fv/Fm values. Thus, species-specific differences in chilling tolerance were only found at 15 °C, and both species were sensitive to chilling at 5 °C. At 15 °C, B. gymnorhiza down-regulated genes related to the light reactions of photosynthesis and up-regulated a gene involved in cyclic electron flow regulation, while R. apiculata down-regulated more RuBisCo related genes. At 5 °C, both species repressed genes related to CO2 assimilation. The down-regulation of genes related to light absorption and up-regulation of genes related to cyclic electron flow regulation are photoprotective mechanisms that likely contributed to the greater PSII photochecmical efficiency of B. gymnorhiza at 15 °C. The results of this study provide evidence that the distributional range limits and potentially the expansion rates of plant species are associated with differences in the regulation of photosynthesis and photoprotective mechanisms under low temperatures.