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 isopentenyl transferase (ipt) gene was constructed and used to transform Nicotiana tabacum L. Several primary transformants were obtained that displayed a range of phenotypes characteristic of cytokinin over-production. Two of the transformants with moderately altered phenotypes, both of 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 zeatin-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 step in the regulation of zeatin (Z) levels. Very sharp gradients in concentration of cytokinin riboside and ribotides, related to age of tissue and distance from the apex, were found in both wild type and transfomed plants, which could be important in developmental regulation and could also account for some of the discrepancies between reported cytokinin levels in various plants. Intriguingly, however, although the combined level of zeatin riboside and ribotide was much higher in the transformed plants than in wild type, the combined level of isopentenyl riboside and ribotide was lower.