Sci. Signal., 13 October 2009
Pathway Evolution Gas and Brakes Required
Annalisa M. VanHook
Science Signaling, AAAS, Washington, DC 20005, USA
Cytokinins are a class of hormones that regulate diverse physiological and developmental processes in land plants through a histidine kinase two-component signaling system. Cytokinins bind to the cyclase- and histidine kinase–associated sensory extracellular (CHASE) domain of membrane-localized hybrid histidine protein kinases, which then undergo an activating autophosphorylation reaction. Activated receptors transduce the signal intracellularly by phosphorylating the histidine phosphotransmitter proteins (HPts), which then translocate to the nucleus, where they activate the type-B response regulators (RRs), transcription factors that regulate the expression of various target genes, including those that encode the type-A RRs, which are a class of RRs that feed back to negatively regulate signaling. Pils and Heyl investigated the evolution of the cytokinin signaling pathway by comparing homologs of cytokinin signaling components present in a unicellular alga, a multicellular alga, a moss, a lycophyte (clubmoss), a monocot (rice), and two dicots (poplar and Arabidopsis). No CHASE domains were found in the genomes of either alga, but each land plant contained genes encoding at least two CHASE domain–containing proteins; phylogenetic analysis indicated that expansion of this family occurred early in land plant evolution. In contrast, at least one gene encoding an HPt and at least two RR-encoding genes were found in each species, although both HPts and RRs underwent expansion in the land plant lineage. Both type-B RRs and type-C RRs, which are not regulated by cytokinin signaling, were found in all species surveyed, but type-A RRs were found only in land plants. These findings indicate that both cytokinin receptors and negative regulators of cytokinin signaling appeared at about the same time early in the evolution of the land plants, suggesting that the ability to generate a signal in response to cytokinin stimulation was not useful without a negative regulator of the pathway to silence or fine-tune signaling.
Citation: A. M. VanHook, Gas and Brakes Required. Sci. Signal. 2, ec333 (2009).
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