DIFFERENT EFFECTS OF TWO BRASSINOSTEROIDS ON GROWTH, AUXIN AND CYTOKININ CONTENT IN TOBACCO CALLUS TISSUE

摘要

Two synthetic brassinosteroids, 24-epibrassinolide (24-epiBR) and 2 alpha,3 alpha,17 beta-trihydroxy-5 alpha-androstan-6-one (THA-BR), exhibit different effects on growth of tobacco callus tissue. When added to a culture medium containing growth-limiting amounts of auxin, 24-epiBR reduced and THA-BR increased the fresh weight yield of tissue up to 53% and 207%, respectively, after 6 weeks of cultivation. The stimulatory and inhibitory effects of the two brassinosteroids on tissue growth occurred over a broad range of concentrations without a pronounced maximum corresponding to the 'yes or no' type of response. Different effects of 24-epiBR and THA-BR on tissue growth were inversely proportional to the content of endogenous cytokinins. Maximum contents of predominant cytokinins N-6-(Delta(2)-isopentenyl)adenine (iP) and trans-zeatin (Z) in tissues supplied with 24-epiBR in growth-inhibiting concentrations were up to 3.7 fold and 3.4 fold higher, respectively, as compared to tissues grown on media containing growth-stimulating concentrations of THA-BR. Stimulation of tissue growth by THA-BR correlated with content of endogenous IAA and an inverse correlation was found between the content of endogenous IAA and cytokinins in tissues supplied with 24-epiBR. THA-BR exhibited weak cytokinin-like activity in a bioassay based on stimulation of growth of lateral buds of pea while 24-epiBR was inactive. Results indicate that the qualitatively different effects of the two brassinosteroids on growth of tobacco tissue may reflect their different influence on content of endogenous cytokinin. [References: 26]