Effects of 2,4-epibrassinolide on photosynthesis and Rubisco activase gene expression in Triticum aestivum L. seedlings under a combination of drought and heat stress

We examined the effects of 2,4-epibrassinolide (EBR) application on photosynthesis, antioxidant enzyme activity, and Rubisco activase (RCA) gene expression in wheat (Triticum aestivum L.) seedlings under a combination of drought and heat stress. The net photosynthetic rates (P-n) of wheat seedlings decreased significantly, the photosynthetic capability was inhibited, and the activities of superoxide (SOD), peroxidase (POD), catalase (CAT), and RCA as well as the initial and total activity of Rubisco declined under the combined stress. These decreases and inhibitory effects were significantly ameliorated by exogenous EBR application. Three subunits (45-46, 41-42, 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 plants subjected to stressful conditions than in unstressed plants. Interestingly, a marked increase in 45-46 kDa RCA was observed under heat or heat combined with drought stress. The abundance of 38-39 kDa RCA in seedlings exposed to heat, drought, or 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), which is more thermostable and increased in response to moderate heat stress, and the smaller isoform (38-39 kDa) of RCA may play important roles in maintaining the photosynthetic capability by EBR under stress conditions.