Physiological and molecular effects of 24-epibrassinolide, a brassinosteroid on thermotolerance of tomato

Brassinosteroids are naturally occurring plant growth regulators, which exhibit structural similarities to animal steroid hormones. Recent studies have indicated that besides an essential role in plant growth and development, brassinosteroids also exert anti-stress effects on plants. We show here that tomato plants treated with 24-epibrassinolide (EBR) are more tolerant to high temperature than untreated plants. An analysis of mitochondrial small heat shock proteins (MT-sHSP) in tomato leaves by western blotting revealed that the MT-sHSP did not preferentially accumulate in EBR treated plants at 25 degrees C. However, treatment of plants at 38 degrees C induced much more accumulation of MT-sHSP in EBR treated than in untreated plants. Results of this study provide the first direct evidence for EBR induced expression of MT-sHSP, which possibly induced thermotolerance in tomato plants. EBR treated tomato plants had better photosynthetic efficiency. We also observed significantly higher in vitro pollen germination, enhanced pollen tube growth and low pollen bursting in the presence of EBR at 35 degrees C, a temperature high enough to induce heat-stress symptoms in tomato, indicating a possible role of EBR during plant reproduction.