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PLANT PHYSIOLOGY , Vol 108, Issue 4 1423-1430, Copyright © 1995 by American Society of Plant Biologists
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DEVELOPMENT AND GROWTH REGULATION |
The Effects of Cytokinin and Light on Hypocotyl Elongation in Arabidopsis Seedlings Are Independent and Additive
W. Su and S. H. Howell
Boyce Thompson Institute, Cornell University, Ithaca, New York 14853
Cytokinin has been reported to mimic some of the effects of light on
de-etiolation responses in dark-grown Arabidopsis seedlings. The
interaction between cytokinin and light was examined by analyzing cytokinin
dose and light fluence effects on hypocotyl elongation in wild-type and
mutant Arabidopsis seedlings with defects in light or hormone responses. It
was found that (a) cytokinin and light-response systems have independent
and additive effects on the inhibition of hypocotyl elongation and (b)
either cytokinin or light can saturate the morphogenic responses. As a
consequence, cytokinin has no effect on hypocotyl elongation under normal
growth conditions because light levels saturate the hypocotyl inhibition
response. To determine whether a functional light-response pathway is
required for cytokinin responses, light-insensitive long hypocotyl (hy)
mutants were tested for cytokinin responses. The hy mutants (hy1 to hy6)
had normal cytokinin responses, except phyB-1 (hy3-1), in which hypocotyl
elongation was insensitive to cytokinin. Cytokinin insensitivity in phyB-1
was attributed to an indirect effect of the mutation on cytokinin
responses. The effects of cytokinin on the inhibition of hypocotyl
elongation are largely mediated by ethylene, and blocking the
ethylene-response pathway through the action of a cytokinin-resistant,
ethylene-insensitive mutant (ckr1/ein2) had no effect on the light
inhibition of hypocotyl elongation. These results do not support the idea
that cytokinin mediates the action of light on hypocotyl elongation.
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