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.