The CRK1 receptor-like kinase gene of tobacco is negatively regulated by cytokinin

This report describes a novel receptor-like kinase gene of tobacco (Nicotiana tabacum L.) that, in cell culture, is rapidly regulated by very low concentrations of cytokinin. The steady-state transcript level of the CYTOKONIN-REGULATED KINASE 1 gene (CRK1) was strongly reduced 30 min after cytokinin treatment. At higher concentrations abscisic acid and auxin induced a similar response. None of the other plant hormones tested elicited this response. Further analyses of the cytokinin-dependent regulation showed that the reduction of transcript was transient, and the duration of the recovery phase was dependent on the hormone concentration. CRK1 is not a primary response gene as the simultaneous addition of cycloheximide inhibits its regulation by cytokinin. Inhibitor studies revealed that a protein phosphatase is likely involved in signalling processes upstream of CRK1. CRK1 is expressed at low levels in the leaves, stem and roots of tobacco. It is predicted that the CRK1 protein is located in the plasma membrane. It has in its N-terminal putative receptor sequence a signal peptide, a serine- and a proline-rich region, a six repeat motif similar to the CRINKLY4 protein of Zea mays and several regions homologous to purine-binding motifs. A single transmembrane domain is followed by a highly conserved intracellular Ser/Thr kinase domain. Therefore, CRK1 is a novel type of class I plant receptor kinase. We hypothesize that CRK1 is involved in an early step of hormone signalling and that transcript down-regulation reflects a desensitization step in reaction to the signalling molecule.