Jumping between histidine kinase receptors: the cytokinin transduction pathway

Abstract

Cytokinins are a class of plant hormones that play a major role in the regulation of cell division and differentiation. Different lines of evidence suggest that they are detected by a two-component system. First, overexpression of a histidine kinase gene, CKI1, induces typical cytokinin responses and second, several genes belonging to a set of response regulators of a two-component system can be induced by cytokinins. Two component systems, common in microorganisms and plants, use a histidine kinase as a sensor and transduce the signal by a phosphorylation cascade.Tsutomo Inoue et al.1xIdentification of CRE1 as a cytokinin receptor from Arabidopsis. Inoue, T. et al. Nature. 2001; 409: 1060–1063Crossref | PubMed | Scopus (542)See all References1 now report important progress in understanding cytokinin perception in Arabidopsis. The group screened mutagenized Arabidopsis for mutants that were impaired in cytokinin responses, including shoot formation in tissue culture and rapid cell proliferation. Using this genetic strategy, they isolated a mutant designated cytokinin response 1-1 (cre1-1), which exhibited reduced responses to cytokinin. The mutated gene cre1 encodes a histidine kinase. Furthermore, Arabidopsis has genes for two products, AHK2 and AHK3, which share high sequence similarity to CRE1. In addition, cre1 expression conferred a cytokinin-dependent growth phenotype on a yeast mutant that lacked the endogenous histidine kinase, SLN1.This research has identified CRE1 as a cytokinin receptor in Arabidopsis, and also shows a possible regulatory mechanism, whereby the cytokinin cascade can be started by the activation of CRE1, initiating phosphorelay signalling. The two CRE homologues might also function as cytokinin receptors, which could explain why defects in cre1 did not cause more phenotypes related to cytokinin functions. These data suggest a scenario where multiple histidine kinase receptors are likely to function in cytokinin signal transduction. As a multiplicity of cytokinin-dependent signalling pathways has been shown in Arabidopsis, it will be important to uncover the range of responses involving CRE-related genes. How does cytokinin activate a specific set of genes? Does cre overexpression affect plant development? Inoue et al. have identified the first new cytokinin receptor that has the potential to modulate many cytokinin-responsive genes in Arabidopsis, a significant step in understanding how plants harmonize developmental responses during vegetative growth.