Published online 26 June 2006. doi:10.1083/jcb.1741rr2
The Rockefeller University Press, 0021-9525 $8.00
JCB, Volume 174, Number 1, 5-5
Phosphate give and take
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A CRE1 mutant that cannot bind cytokinins has short roots (left) because it is locked in a phosphatase form.
MÄHÖNEN/ELSEVIER
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A plant hormone receptor has a split personality, revealing its kinase or phosphatase side depending on whether hormone is bound, say Ari Mähönen, Ykä Helariutta (University of Helsinki, Finland), Masayuki Higuchi, Tatsuo Kakimoto (Osaka University, Japan), and colleagues.
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. 
Reference:
Mähönen, A.P., et al. 2006. Curr. Biol. 16:1116–1122.[CrossRef][Medline]
Nicole LeBrasseur
lebrasn{at}rockefeller.edu
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