Protective Roles of Brassinolide on Rice Seedlings under High Temperature Stress

Two indica rice (Oryza sativa L.) materials, Xieqingzao B (sensitive to heat stress) and 082 (tolerant to heat stress), were used to study the role of brassinolide (BR) in protection of rice seedlings from heat stress. Young seedlings were subjected to high temperature (38 degrees C/30 degrees C) and sprayed with 0.005 mg/L of BR. Analysis was conducted on the contents of chlorophyll, protein and malondialdehyde (MDA), the leakage of electrolyte, the activities of peroxidase (POD) and superoxide dismutase (SOD) and their isozymes expression levels in leaves. Under the high temperature treatment, application of BR significantly increased the contents of chlorophyll and protein, and the activities of POD and SOD, and reduced the content of MDA and the leakage of electrolyte in the leaves of the heat-sensitive material Xieqingzao B, whereas BR had less effect on those of the heat-tolerant material 082 relatively. The BR treatment enhanced the expression of POD isozymes in the leaves of both materials. Under the high temperature stress and BR treatment, the expression of four SOD isozymes reduced in 082, but the expression of two SOD isozymes increased in Xieqingzao B. This suggests that BR plays an important role in protection of rice seedlings from heat stress by enhancing the activities or expression level of protective enzymes in the leaves. The materials with various heat-tolerance might differ in the mechanism of response to heat stress with BR application.