Two-component signal transduction is commonly used as a stimulus–response coupling mechanism to allow organisms such as eubacteria, archea, and a few eukaryotes to sense and respond to changes in many different environmental conditions (Stock et al., 2000). Typically, the histidine protein kinase senses extracellular stimuli by autophosphorylation and transfers a phosphoryl group to the response regulator, resulting in activation of downstream proteins that elicit a specific response. In Arabidopsis plants, a multi-step two-component system is well established as a key element of plant hormone cytokinin signaling (To and Kieber, 2008).