CYTOKININS IN PLANT-PATHOGENIC BACTERIA AND DEVELOPING CEREAL-GRAINS

Cytokinin analysis by immunoaffinity chromatography (IAC), high-perfor mance liquid chromatography (HPLC) and radioimmunoassay (RIA) or enzym e-linked immunosorption assay (ELISA) has been used to study two separ ate topics: the role of tRNA in bacterial cytokinin biosynthesis and t he changes in cytokinin concentration which occur during cereal grain development. Transfer RNA isopentenylation in the gall-forming plant p athogen Agrobacterium tumefaciens is encoded by the chromosomal miaA l ocus. Mutation of miaA reduces tRNA isopentenylation significantly and preliminary data suggest that turnover of isopentenylated tRNA is res ponsible for low level secretion of free N-6-isopentenyladenine (iP) b y the bacteria. However, the major route of cytokinin biosynthesis by gall-forming plant pathogenic bacteria is not via tRNA turnover but by direct biosynthesis mediated by dimethylallylpyrophosphate:5'-AMP tra nsferase (DMAPP:AMP transferase) encoded by such genes as ipt, tzs (fr om A. tumefaciens) or ptz (from Pseudomonas savastanoi). Analysis of c ytokinin levels in developing wheat and rice grains in the period imme diately following pollination showed large transient increases in zeat in (Z) and zeatin riboside (ZR) which coincided with the period of max imum endosperm cell division reported by others. Detailed analyses of maize kernels, where development can be staged readily, showed that Z and ZR concentrations peaked 9 days after pollination (DAP). During th e period 8-10 DAP, cytokinin oxidase underwent a significant increase in specific activity, indicating that cytokinin catabolism was enhance d as endosperm cell division ended.