Auxin-cytokinin interactions in wild-type and transgenic tobacco

Cytokinins and auxins are important regulators of plant growth and development, but there is incomplete and conflicting evidence that auxins affect cytokinin metabolism and vice versa, We have investigated these interactions in Nicotiana tabacum L. by separate in planta manipulation of levels of the hormones followed by analysis of the induced changes in the metabolism of the other hormone, Cytokinin-overproducing plants (expressing the Agrobacterium tumefaciens ipt gene) had lower than wild-type levels of free IAA, and reduced rates of IAA synthesis and turnover, but there were no differences in the profiles of metabolites they produced from fed IAA, Similarly, auxin-overproducing plants (expressing the A, tumefaciens iaaM and iaaH genes), had lower levels of the major cytokinins than wild-type plants and lower cytokinin oxidase activity, but there were no differences in the profiles of metabolites they produced from fed cytokinins, The data demonstrate that cytokinin or auxin overproduction decreases the content of the other hormone, apparently by decreasing its rate of synthesis and/or transport, rather than by increasing rates of turnover or conjugation. Implications for the importance of cytokinin:auxin ratios in plant development are considered.