2 GENES WITH SIMILARITY TO BACTERIAL RESPONSE REGULATORS ARE RAPIDLY AND SPECIFICALLY INDUCED BY CYTOKININ IN ARABIDOPSIS

Cytokinins are central regulators of plant growth and development, but little is known about their mode of action. By using differential dis play, we identified a gene, IBC6 (for induced by cytokinin), from etio lated Arabidopsis seedlings, that is induced rapidly by cytokinin. The steady state level of IBC6 mRNA was elevated within 10 min by the exo genous application of cytokinin, and this induction did not require de novo protein synthesis. IBC6 was not induced by other plant hormones or by light. A second Arabidopsis gene with a sequence highly similar to IBC6 was identified. This IBC7 gene also was induced by cytokinin, although with somewhat slower kinetics and to a lesser extent. The pat tern of expression of the two genes was similar, with higher expressio n in leaves, rachises, and flowers and lower transcript levels in root s and siliques. Sequence analysis revealed that IBC6 and IBC7 are simi lar to the receiver domain of bacterial two-component response regulat ors. This homology, coupled with previously published work on the CKI1 histidine kinase homolog, suggests that these proteins may play a rol e in early cytokinin signaling.