THE EFFECTS OF CYTOKININ AND LIGHT ON HYPOCOTYL ELONGATION IN ARABIDOPSIS SEEDLINGS ARE INDEPENDENT AND ADDITIVE

Cytokinin has been reported to mimic some of the effects of light on d e-etiolation responses in dark-grown Arabidopsis seedlings. The intera ction between cytokinin and light was examined by analyzing cytokinin dose and light fluence effects on hypocotyl elongation in wild-type an d mutant Arabidopsis seedlings with defects in light or hormone respon ses. it was found that (a) cytokinin and light-response systems have i ndependent and additive effects on the inhibition of hypocotyl elongat ion and (b) either cytokinin or light can saturate the morphogenic res ponses. As a consequence, cytokinin has no effect on hypocotyl elongat ion under normal growth conditions because light levels saturate the h ypocotyl inhibition response. To determine whether a functional light- response pathway is required for cytokinin responses, light-insensitiv e 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 t he inhibition of hypocotyl elongation are largely mediated by ethylene , and blocking the ethylene-response pathway through the action of a c ytokinin-resistant, ethylene-insensitive mutant (ckr1/ein2) had no eff ect on the light inhibition of hypocotyl elongation. These results do not support the idea that cytokinin mediates the action of light on hy pocotyl elongation.