Evidence for a Plastid Origin of Plant Ethylene Receptor Genes

Two genes in the newly complete genome sequence of the cyanobacterium Anabaena sp. strain PCC 7120 encode homologs of the ethylene hormone receptors of higher plants, and reanalysis of an ethylene-binding protein in the cyanobacterium Synechocystis sp. strain PCC 6803 reveals a third homolog. Here, we present analyses of these sequences and discuss the implications regarding the ancestry of ethylene receptor genes in higher plants. Ethylene gas, one of the major plant hormones, influences many aspects of growth and development throughout the plant life cycle. In higher plants, ethylene is perceived by a family of receptors with similarity to two-component signaling proteins (Bleecker, 1999; Chang and Stadler, 2001). The typical two-component system consists of a sensor His protein kinase and response regulator protein (Fig.1, top). These components, in various configurations, are widely used by bacteria in regulating responses to diverse stimuli (Parkinson and Kofoid, 1992). Several examples of this system have also been found in plants and fungi (Urao et al., 2001;Chang and Stewart, 1998), and include the recently discovered cytokinin hormone receptor in plants (Inoue et al., 2001). It is unclear whether the ethylene receptors signal by a typical two-component signaling mechanism, which involves His autophosphorylation followed by transfer of the phosphate to an Asp residue (Fig. 1, top). It is interesting that the pathway for ethylene signaling in higher plants appears to have evolved from distinct types of signaling components (Bleecker, 1999). That is, the ethylene receptors somehow transmit their signal to the CTR1 Raf-related Ser/Thr protein kinase for which there are animal but not bacterial homologs (Kieber et al., 1993).