Abstract: The relative expressions of stress genes Hsp70, Hsp90 and choloplast psaA, psbA (SAM as reference gene) in Galaxea astreata endosymbiotic zooxanthella (Symbiodinium spp.) under heat stress were detected by using real-time PCR technique. All samples were exposed to elevated temperature at 32 ℃ over a 7-day period, and then the temperature was reduced to 28 ℃ gradually, with the temperature change from 28 ℃ to 32 ℃ at a rate of 1 ℃ per 24 hours. The results showed that the expression patterns of Hsp70 and Hsp90 were similar. The induced expression levels were upregulated but showed no significant difference compared to the control group (p>0.05) during the early stage of temperature climbing, subsequently reaching the maximize level after 168h tolerance period at 32 ℃. The lowest abundance of the gene products was detected when returned to the normal condition of 28 ℃ after 5-day recovery. Moreover, we examined the expression levels of chloroplast psaA and psbA genes stimulated by heat. Both were increased at the initial stage of thermal stress, and reached maximize at 31 ℃ for 24 h, with 1.2 and 1.9 folds compared to the level of the untreated group, respectively. Subsequently at 32 ℃, psbA transcripts maintained the high level until 24 h consecutive hours of stress, while psaA immediately decreased significantly (p<0.01). When temperature recovery reached 28 ℃, the transcripts of the two genes were significantly lower than the initial level (p<0.01), which indicates that the photosynthetic function of endosymbiotic zooxanthella may be injured (or inhibited).

Key words: zooxanthella, gene expression, thermal stress