Regulation Effect Of Exogenous Epibrassinolide,and Nitric Oxide On Expression Of Rubisco Activase Gene And Funciton Of Photosystem Ⅱ In Wheat(Triticum Asetinum L.) Leaves Under Combination Of Heat And Drought Stress

Rubisco,a key enzyme in photosynthesis,is dependent on catalysis of Rubisco activase to acquire activation.In addition,Rubisco activase is thermo-sensitive and be influenced to drought stress.Brassinolide（BR） and nitric oxide（NO） are two important signal molecules involved in enhancing resistance to stresses,improving photosynthesis.However,the previous studies were paid more attention to the effect of individual stress of heat and drought on crops,and little attention to the combined stress of heat and drought,especially the effects of brassinolide and nitric oxide on photosynthetic potential under combined heat and drought stress in wheat.Therefore,we examined the effects of 2,4-epibrassinolide（EBR） and nitric oxide application on photosynthesis,antioxidant enzyme activity,and Rubisco activase（RCA） gene expression in wheat seedlings under a combination of drought and heat stress.1.Effects of EBR and NO on antioxidant enzymes in wheat leaves under drought or heat stressActivities of superoxide（SOD）,peroxidase（POD）,catalase（CAT） in wheat leaves were significantly decreased under drought or cross stress of drought and heat,but the reduced trend were inhibited by EBR and NO.However,the antioxidant enzymes were increased after heat stress,and were repressed in the leaves treated with EBR and NO.2.Effects of EBR and NO on expression of Rca gene in wheat leaves under drought or heat stressThe transcriptional level of Rca a in wheat leaves was decreased under drought,heat and their cross stress.For Rca b gene,its transcriptional level was declined significantly under drought stress condition,and was enhanced by heat or cross stress heat and drought.Three subunits（45–46 kDa,41–42 kDa,and 38–39 kDa） of RCA were observed in wheat seedlings.The abundances of the 38–39 kDa and 41–42 kDa subunits were significantly lower in all plants subjected to stressful conditions than that in unstressed plants.Interestingly,a marked increase in 45–46 kDa RCA was observed under heat and cross stress heat and drought.The abundance of 38–39 kDa RCA in seedlings exposed to heat,drought,and their combination was significantly enhanced by EBR pretreatment,which paralleled the changes in initial Rubisco activity and P_n,but was not consistent with observed mRNA abundance.These results indicated that the larger subunit of RCA（45–46 kDa） is more thermostable and increased in response to moderate heat stress,and the smaller isoform（38–39 kDa） of RCA may play important role in maintaining the photosynthetic capability of EBR under stress conditions.NO could enhance the gene transcript expression of Rca under drought stress in wheat,but it was not observed under heat and the combined stress.However,the abundance of three RCA subunits in wheat increased after NO pretreatment under drought,heat and their combined stress conditions,especially the 41-42 kDa and 38-39 kDa subunits.It is indicated that the regulation of NO to Rca is post-transcriptional.3.Effects of EBR and NO on RCA and Rubisco activity in wheat leaves under drought or heat stressThe RCA activity of wheat seedlings was decreased as well as the initial and total activity of Rubisco declined under drought,heat and their combined stress.The decreases and inhibitory effects were significantly ameliorated by exogenous EBR and NO application.4.Effects of EBR and NO on PS Ⅱ and photosynthetic rate in wheat leaves under drought or heat stressDrought,heat and their combined stress resulted in a significant decrease in F_v/F_m、PI_abs、F_v/F_o.The decrease under drought and combined stress was alleviated by EBR and NO,while the opposite result was observed after heat stress.The net photosynthetic rates（P_n） decreased significantly under all stresses condition,and it was improved by EBR treatment.NO could enhance the Pn of wheat leaves exposed to drought and combined stress,but it was not the case under heat stress.In a word,EBR and NO improved the photosynthetic capability of PS Ⅱ and photosynthetic rate through increasing in the activities of antioxidant system,Rubisco,enhancing expression of RCA under cross stress of drought and heat.The study further enriched the possible physiological and molecular mechanism about the regulation of EBR and NO to photosynthetic performance of wheat leaves under drought and heat stresses.Meanwhile,it is useful reference to cope with adversity stresses in crop production.