Histidine kinase activity and the regulation of ethylene signal transduction

Ethylene is a gaseous hormone that regulates many aspects of plant growth and development. Although the effect of ethylene on plant growth was discovered a century ago, the key players in the ethylene response pathway were only identified over the last 15 years. In Arabidopsis, ethylene is perceived by a family of five receptors (ETR1, ETR2, ERS1, ERS2, and EIN4) that resemble two-component histidine kinases. Of these, only ETR1 and ERS1 contain all the conserved residues required for histidine kinase activity. The ethylene receptors appear to function primarily through CTR1, a serine/threonine kinase that actively suppresses ethylene responses in air (absence of ethylene). Despite recent progress toward understanding ethylene signal transduction, the role of the ethylene-receptor histidine-kinase activity remains unclear. This review considers the significance of histidine kinase activity in ethylene signaling and possible mechanisms by which it may modulate ethylene responses