ZEA3 - A NEGATIVE MODULATOR OF CYTOKININ RESPONSES IN PLANT SEEDLINGS

In Nicotiana plumbaginifolia cytokinins affect seedling development by inhibiting root growth and hypocotyl elongation and by stimulating co tyledon expansion. The zea3.1 mutant was selected for its inability to grow in conditions of low nitrogen and for its ability to grow indepe ndently on inhibitory concentrations of zeatin (J.D. Faure, M. Jullien , M. Caboche [1994] Plant J 5: 481-491). The zea3.1 growth response to cytokinins is reflected by an increase in cotyledon expansion due to cell division and by a swelling of the hypocotyl due to cell enlargeme nt. An analysis of the seedling's root length and fresh weight over a wide range of benzyladenine concentrations showed that zea3.1 plants e xhibit a higher sensitivity and an amplified response to cytokinins. A similar response of zea3.1 to benzyladenine was also seen in the expr ession of msr1, a cytokinin-regulated gene. Regulation of msr1 express ion by protein phosphorylation was unaffected by the zea3.1 mutation. No significant differences in cytokinin and auxin levels were found be tween zea3.1 and wild-type seedlings, suggesting that the mutant pheno type is not caused by an alteration of these hormone levels. The data presented suggest that ZEA3 negatively modulates cytokinin responses a nd may function as a broad regulator of seedling development.