ISOLATION AND CHARACTERIZATION OF ARABIDOPSIS MUTANTS DEFECTIVE IN THE INDUCTION OF ETHYLENE BIOSYNTHESIS BY CYTOKININ

Cytokinins elevate ethylene biosynthesis in etiolated Arabidopsis seed lings via a post-transcriptional modification of one isoform of the ke y biosynthetic enzyme ACC synthase. In order to begin to dissect the s ignaling events leading from cytokinin perception to this modification , we have isolated a series of mutants that lack the ethylene-mediated triple response in the presence of cytokinin due to their failure to increase ethylene biosynthesis. Analysis of genetic complementation an d mapping revealed that these Cin mutants (cytokinin-insensitive) repr esent four distinct complementation groups, one of which, cin4, is all elic to the constitutive photomorphogenic mutant fus9/cop10. The Cin m utants have subtle effects on the morphology of adult plants. We furth er characterized the Cin mutants by analyzing ethylene biosynthesis in response to various other inducers and in adult tissues, as well as b y assaying additional cytokinin responses. The cin3 mutant did not dis rupt ethylene biosynthesis under any other conditions, nor did it disr upt any other cytokinin responses. Only cin2 disrupted ethylene biosyn thesis in multiple circumstances. cin1 and cin2 made less anthocyanin in response to cytokinin. cin1 also displayed reduced shoot initiation in tissue culture in response to cytokinin, suggesting that it affect s a cytokinin signaling element.