CYTOKININ METABOLITES AND GRADIENTS IN WILD-TYPE AND TRANSGENIC TOBACCO WITH MODERATE CYTOKININ OVER-PRODUCTION

A binary T-DNA plant expression vector carrying a promoterless isopent enyl transferase (ipt) gene was constructed and used to transform Nico tiana tabacum L. Several primary transformants were obtained that disp layed a range of phenotypes characteristic of cytokinin over-productio n. Two of the transformants with moderately altered phenotypes, both o f which produced viable offspring and expressed the ipt gene at a low level, were selected for use in studies of the regulation of cytokinin metabolism. Both lines were found to contain high concentrations of z eatin-7-glucoside (Z7G), indicating that Z7G can accumulate in plants even when the rate of endogenous over-production of cytokinins is low. This supports the hypothesis that 7-glucosidation is an important ste p in the regulation of zeatin (Z) levels. Very sharp gradients in conc entration of cytokinin riboside and ribotides, related to age of tissu e and distance from the apex, were found in both wild type and transfo med plants, which could be important in developmental regulation and c ould also account for some of the discrepancies between reported cytok inin levels in various plants. Intriguingly, however, although the com bined level of zeatin riboside and ribotide was much higher in the tra nsformed plants than in wild type, the combined level of isopentenyl r iboside and ribotide was lower.