Plant hormones: More than one way to detect ethylene

Plants use the gas ethylene (C2H4) as a signaling molecule for regulating a variety of developmental processes, including fruit and flower senescence, sex determination in some monoecious species, cell elongation and pathogen responses [1]. The final reactions of the ethylene biosynthetic pathway have been elucidated [2], and information on its signaling apparatus is beginning to emerge [3,4]. Molecular genetic analysis of the ethylene signaling pathway in Arabidopsis revealed that plants sense ethylene by a protein kinase cascade [3,4]. This revelation came from the cloning of CTR1 and ETR1, two key genes in the pathway. CTR1 mutations confer a constitutive ethylene response, whereas ETR1 mutations confer ethylene insensitivity. Ecker and colleagues [5] first cloned CTR1, which encodes a putative Raf-like serine/threonine protein kinase [5]. Soon after, Meyerowitz and colleagues [6] cloned ETR1, which encodes a putative histidine protein kinase similar to prokaryotic ‘two-component’ sensors (Fig. 1).