Enhancement mechanism of chilling torelance induced by a novel steroidal phytohormone, brassinosteroid.

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This is study on a mechanism of chilling tolerance enhancement induced by treatment with brassinolide. Rice seedling was higher than that of cucumber seedling in sensitivity when searching a signal transduction of the chilling stress. Effect of brassinolide was appeared remarkably in the root growth under the chilling stress condition. The timing of brassinolide treatment was more effective with pre-treatment before the seedling was put in the chilling stress condition than with simultanious treatment or post- treatment after in the stress condition. Oxidation activity was strongly increased in the brassinolide-treated root tissue. This suggests that brassinolide promotes the growth of root and enhances nutrient absorption from root, so that a signal transduction system for the chilling stress response is activated. Humidity, especialy to keep it lower, was important to enhance the effect of brassinolide. Durability of brassinolide activity was short and it shows that brassinolide treated exogenously was rapidly metabolited in the seedling. In cucumber seedling brassinolide was easily epimerized to an isomer possessing the same molecular as brassinolide and then it was converted to conjugate coupled with a sugar and/or a fatty acid. The structures of the conjugate and its aglycone were undertaken to physicochemical analyses after chemical derivatization and also their spectral data were compared with several presumable compounds synthesized chemically and metabolites induced by cucumber cell suspension culture. In spite of them analyses determination of the structures have been not elicidated. On the other hand, several compounds of acid brassinolide with almost same activity as auxin in the lamina joint test. Chilling tolerance activity and its durability must be confirmed.