The Arabidopsis AHK4 histidine kinase is a cytokinin-binding receptor thattransduces cytokinin signals across the membrane

Common histidine-to-aspartate (His --> Asp) phosphorelay is a paradigm of s ignal transduction in both prokaryotes and eukaryotes for the propagation o f certain environmental stimuli, in which histidine (His)-kinases play cent ral roles as sensors for environmental signals. For the higher plant, Arabi dopsis thaliana, it was recently suggested that the His-kinase (AHK4 / CRE1 / WOL) is a sensor for cytokinins, which are a class of plant hormones imp ortant for the regulation of cell division and differentiation. Interesting ly, AHK4 is capable of functioning as a cytokinin sensor in the eubacterium , Escherichia coli (Suzuki et al. 2001, Plant Cell Physiol. 42: 107). Here we further show that AHK4 is a primary receptor that directly binds a varie ty of natural and synthetic cytokinins (e.g. not only N-6-substituted amino purines such as isopentenyl-adenine, trans-zeatin, benzyl-adenine, but also diphenylurea derivatives such as thidiazuron), in a highly specific manner (K-d = 4.55 +/- 0.48x10(-9) M). AHK4 has a presumed extracellular domain, within which a single amino acid substitution (Thr-301 to Ile) was shown to result in loss of its ability to bind cytokinins. This particular mutation corresponds to the previously reported wol allele (wooden leg) that causes a striking phenotype defective in vascular morphogenesis. Collectively, ev idence is presented that AHK4 and its homologues (AHK3 and possibly AHK2) a re receptor kinases that can transduce cytokinin signals across the plasma membrane of A. thaliana.