Plant hormones known as cytokinins, which stimulate growth and cell division, are detected by three receptors with hormone-induced histidine kinase activity. The loss of cytokinin-binding ability of one of these receptor kinases, CRE1, stunts root growth. This phenotype is suppressed, the group now finds, by mutations elsewhere within CRE1. Based on this result, they hypothesized that hormone-free CRE1 inhibits its immediate downstream elements, which can be phosphorylated by multiple histidine kinases.
A logical countermeasure to a kinase is a phosphatase. Indeed, the group shows, CRE1 is a phosphatase when not bound to cytokinins. Perhaps this bidirectional phosphorelay activity creates more rapid changes in signaling than when kinases and phosphatase are separate. Replacing Arabidopsis CRE1 with a kinase-only homologue resulted in cytokinin hypersensitivity.
The cytokinin phosphorelay pathway is analogous to bacterial two-component systems, which are absent from the animal kingdom. Plant light and ethylene receptors are also part of two-component–like networks, although no built-in phosphatase activity is known for these receptors.