9420775 Schaff Despite considerable effort, the process involved in the biosynthesis of endogenous cytokinin in higher plants is not fully understood. Currently, many researchers believe that plants synthesize cytokinins using the same process that Agrobacterium- transformed plants do. The accepted route for the conversion of 14C-adenine into cytokinin is that the adenine is first converted to AMP, which is then converted to cytokinin. To date, the enzyme has not been identified in intact higher plants. In contrast to this commonly held theory, Van Staden and Drewes (1993) consider that adenine may function as a substrate for the production of cytokinin. Their hypothesis predicts higher plant endogenous cytokinin synthase (EITase) condenses IPP and adenine to produce cytokinin. The proposed research project intends to develop direct support for this hypothesis using in vitro extracts from adenine phosphoribosyltransferase-less (APRT-) mutants of Arabidopsis to demonstrate the following: 1) the EITase uses adenine to form i6Ade, the first cytokinin and 2) the levels of adenine and cytokinins increase compared to the wild-type (APRT+), which results in a phenotype that represents an over-accumulation of cytokinin. ***
