Different effects of two brassinosteroids on growth,auxin and cytokinin content in tobacco callus tissue

Two synthetic brassinosteroids, 24-epibrassinolide (24-epiBR) and 2,3, 17-trihydroxy-5-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 N⁶-(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.Abbreviations BR(s) brassinosteroid(s) - 24-epiBR 24-epibrassinolide - THA-BR 2,33, 17-trihydroxy-5-androstan-6-one - CK(s) cytokinin(s) - iP N⁶-(²-isopentenyl)adenine - [9R]iP N⁶-(²-isopentenyl)adenosine - Z trans-zeatin - [9R]Z ribosyl-trans-zeatin - ABA abscisic acid - IAA indole-3-acetic acid - NAA naphtalene-1-acetic acid - DEAE cellulose diethylaminoethyl cellulose - HPLC high performance liquid chromatography - ELISA enzyme linked immuno-sorbent assay