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 cytoki nin level in the amp1 mutant and show that it contains high levels of endogenous cytokinins in both light- and dark-grown plants, and that a mp1 can be phenocopied in the dark by growing wild-type plants on exog enous cytokinin. Previously we showed that dark-grown amp1 plants disp lay de-etiolated characteristics such as short unhooked hypocotyls, op ened cotyledons and formation of leaves. We further show that amp1 mut ants display de-etiolated plastid morphology and increased levels of t ranscripts of light-regulated genes, indicating that in this mutant li ght-mediated processes are partially induced in the dark. The amp1 mut ant also shows a reduced level of expression of several light-regulate d genes compared with the wild type when grown in light, and has an al tered 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-media ted process is involved in regulation of etiolation. The map location, phenotypes and de-etiolation responses in the amp1 mutant are differe nt from those of previously described de-etiolated mutants such as det 1, det2, cop1 and cop9. We propose a model in which cytokinin acts as a component of the induction of photomorphogenic processes via a signa l transduction pathway which is independent of light.