Functional Identification of OsHk6 as a Homotypic Cytokinin Receptor in Rice with Preferential Affinity for iP

Cytokinins are involved in key developmental processes in rice (Oryza sativa), including the regulation of cell proliferation and grain yield. However, the in vivo action of histidine kinases (OsHks), putative cytokinin receptors, in rice cytokinin signaling remains elusive. This study examined the function and characteristics of OsHk3, 4 and 6 in rice. OsHk6 was highly sensitive to isopentenyladenine (iP) and was capable of restoring cytokinin-dependent ARR6 reporter expression in the ahk2 ahk3 Arabidopsis mutant upon treatment with 1 nM iP. OsHk4 recognized trans-zeatin (tZ) and iP, while OsHk3 scarcely induced cytokinin signaling activity. OsHk4 and OsHk6 mediated the canonical two-component signaling cascade of Arabidopsis to induce phosphorylation of ARR2. OsHk4 and OsHk6 were highly expressed in spikelets, suggesting that tZ and iP might play key roles in grain development. OsHk6 formed a self-interacting homomer in rice protoplasts, although the trans-phosphorylation activity between subunits was much lower than the intra-molecular trans-phosphorylation activity. This indicates that the action mechanism of OsHks is evolutionarily diverged from bacterial histidine kinases. Ectopic expression of OsHk6 in rice calli promoted green pigmentation and subsequent shoot induction, further supporting an OsHk6 in planta function as a cytokinin receptor. From the results of this study, OsHks are homomeric cytokinin receptors with distinctive cytokinin preferences in rice.