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Identification of CRE1 as a Cytokinin Receptor from Arabidopsis


Identification of a Receptor for the Plant Hormone Cytokinins.



Cytokinins are central to the regulation of numerous physiological processes including cell division and differentiation in plants. Cytokinins are necessary for cell division, induce formation of leaves and buds on a callus (an undifferentiated mass of cells)1, break lateral bud dormancy, regulate nutrient allocation and delay senescence2. Despite the central importance of cytokinins in plant development, no cytokinin receptor has been identified yet.

Identification of an Arabidopsis cytokinin-resistant mutant

To identify important genes for cytokinin signal transduction, we have screened for mutants that are altered in cytokinin responses. Usually tissue segments excised from Arabidopsis grow rapidly with a green color reflecting chloroplast development, in the presence of a cytokinin and an auxin. Plants were grown from seeds of Arabidopsis that had been mutagenized with ethyl methane sulfonate, self-pollinated and then allowed to set seeds (M2 seeds). Hypocotyl segments were excised from M2 seedlings and cultured on plates that contain a cytokinin at a level that normally induce rapid growth and chloroplast development. From about 19,000 hypocotyl segments, we found a mutant that did not exhibit cytokinin responses. This mutant was designated cre1-1 for cytokinin response 1-1.

Characterization of the cre1-1 mutant

The cre1-1 mutant was recessive. We tested the responses of homozygous cre1-1 to auxin and cytokinin in tissue culture, using naphthalene acetic acid as an auxin, and kinetin as a cytokinin. Wild-type explants responded to increasing levels of kinetin with rapid proliferation, greening and formation of shoots (Fig.1a). By contrast, such cytokinin responses were not evident in cre1-1 (Fig.1b). This mutant was also less responsive to other cytokinins, including trans-zeatin, benzyl adenine, and a phenylurea-type synthetic cytokinin, thidiazuron. Cytokinin responsiveness was also tested in another assay system. Cytokinins, auxins, ethylene and abscisic acid, which are all plant hormones, inhibit elongation of root cells. The cre1-1 mutant was less responsive specifically to cytokinins for inhibition of root elongation.


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