Morphological and physiological properties of plants are known to be regulated by various endogeneous plant hormones 1). Brassinosteroids(BRs) have been classified as a new group of plant hormones 2) and have unique biological activity in plant growth promotion 3). To elucidate the biological function of BRs, we have investigated the effect of brassinolide(BL) on epicotyl growth in pea seedlings using the wild-type of pea (Pisum sativum, cv Torsdag) and a BR biosynthesis mutant(lkb) 4, 5) that exhibits a dwarf phenotype. The dwarf phenotype of the lkb mutant became detectable by 53rd hour of imbibition and distinct at 89th hour. The dwarf phenotype was rescued most effectively by treatment with 1x10^-7 M BL for 1 hour at 29th hour of imibibition. The length of first internode with the BL treated lkb mutant was 2-fold longer than that of suggest that BL possesses the biological function on the elongation of the first internode in the lkb mutant and that the dwarf nature of lkb mutant results from not only a deficiency of BL but some other factor in the early development stage under the dark condition.
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