Interaction of a Brassinosteroid with IAA and GA3 in the Elongation of Cucumber Hypocotyl Sections

A synthetic brassinosteroid, 22,23(S,S)-homobrassinolide (hBR), was examined for its interaction with IAA and GA3 in the elongation of hypocotyl sections of light-grown cucumber (Cucumis salivus L. cv. Aonagajibai) seedlings. hBR alone was less active than IAA. Its optimal concentration was around 10 μm and the lowest effective concentration between 10 and 100 μm, which is more than 100 times higher than that of brassinolide. hBR was more active in sections from younger seedlings. Its growth-promoting effect was negated or greatly reduced by inhibitors of auxin-induced elongation such as p-chlorophenoxyisobutyric acid and kinetin. hBR acted synergistically with IAA and 2,4-D but not with GA3 showing only an additive effect. Sequential treatment of sections with hBR and then with IAA also resulted in synergistic enhancement of auxininduced elongation, but when the order of treatment was reversed, hBR was inactive. The synergistic effect was obtained with 1 h pretreatment with hBR and could be reduced by subsequent washing with water. There was no sequential interaction between hBR and GA3. The synergistic pretreatment effects of hBR and GA3 were simply additive to each other. A membrane-bound ATPase inhibitor, dicyclohexylcarbodiimide, inhibited the hBR-induced elongation, but did not affect GA3-induced elongation. The findings led to the conclusion that brassinosteroids enhance auxin action and possess growth-promoting activity which is independent of that of gibberellin.