THE RESPONSE OF A CYTOKININ RESISTANT MUTANT IS HIGHLY SPECIFIC AND PERMITS A NEW CYTOKININ BIOASSAY

We have previously isolated six independent cytokinin-resistant mutant s of Nicotiana plumbaginifolia which define three complementation grou ps, zea1, zea2 and zea3. We report here the characterization of the ph enotypic response to cytokinin treatment of the mutant 1-64, belonging to the zeal group, and the result of the study of the specificity of this response. The phenotype of this mutant grown in the presence of c ytokinin concentrations higher than 0.1 mu M is characterized by a hyp ertrophy of the cotyledons and hypocotyl which results in an increase of plantlet fresh weight. This hypertrophy is correlated to cytokinin concentration in a range between 0.01 to 10 mu M. The specificity of t his response has been verified by using adenine and urea type cytokini ns, as well as enantiomers of methylzeatin and methylbenzyladenine whi ch differ widely in their cytokinin activities. We show that the high specificity of the hypertrophic response to cytokinins can be used as a convenient bioassay to screen the cytokinin activity of adenine or u rea type molecules.