AUXIN-CYTOKININ INTERACTIONS IN WILD-TYPE AND TRANSGENIC TOBACCO

Cytokinins and auxins are important regulators of plant growth and dev elopment, but there is incomplete and conflicting evidence that auxins affect cytokinin metabolism and vice versa, We have investigated thes e interactions in Nicotiana tabacum L. by separate in planta manipulat ion of levels of the hormones followed by analysis of the induced chan ges in the metabolism of the other hormone, Cytokinin-overproducing pl ants (expressing the Agrobacterium tumefaciens ipt gene) had lower tha n wild-type levels of free IAA, and reduced rates of IAA synthesis and turnover, but there were no differences in the profiles of metabolite s they produced from fed IAA, Similarly, auxin-overproducing plants (e xpressing the A, tumefaciens iaaM and iaaH genes), had lower levels of the major cytokinins than wild-type plants and lower cytokinin oxidas e activity, but there were no differences in the profiles of metabolit es they produced from fed cytokinins, The data demonstrate that cytoki nin or auxin overproduction decreases the content of the other hormone , apparently by decreasing its rate of synthesis and/or transport, rat her than by increasing rates of turnover or conjugation. Implications for the importance of cytokinin:auxin ratios in plant development are considered.