Increased endogenous cytokinin in the Arabidopsis amp1 mutant corresponds with de-etiolation responses

The high-cytokinin Arabidopsis thaliana (L.) Heynh. mutant amp1 of has been further characterised. We extend our previous work on the cytokinin level in the amp1 mutant and show that it contains high levels of endogenous cytokinins in both light- and dark-grown plants, and that amp1 can be phenocopied in the dark by growing wild-type plants on exogenous cytokinin. Previously we showed that dark-grown amp1 plants display de-etiolated characteristics such as short unhooked hypocotyls, opened cotyledons and formation of leaves. We further show that amp1 mutants display de-etiolated plastid morphology and increased levels of transcripts of light-regulated genes, indicating that in this mutant light-mediated processes are partially induced in the dark. The amp1 mutant also shows a reduced level of expression of several light-regulated genes compared with the wild type when grown in light, and has an altered dark-adaptation response when compared with the wild type. These results demonstrate an association between high cytokinin levels and de-etiolation, and we infer that cytokinin itself or a cytokinin-mediated process is involved in regulation of etiolation. The map location, phenotypes and de-etiolation responses in the amp1 mutant are different from those of previously described de-etiolated mutants such as det1, det2, cop1 and cop9. We propose a model in which cytokinin acts as a component of the induction of photomorphogenic processes via a signal transduction pathway which is independent of light.