Highly conserved proteins that modify plant ethylene responses.

T he plant hormone ethylene controls many aspects of growth and development, including organ abscission, leaf and flower senescence, and, in certain species, fruit ripening. It also has a critical role in mediating many stress responses. The importance of ethylene to many fundamental processes in plants has resulted in a great deal of research on how plants perceive and respond to this hormone. Simple genetic screens have facilitated isolation of many of the genes encoding mediators of ethylene responses, most notably the receptors (1, 2). Plants have small gene families of ethylene receptors; Arabidopsis thaliana, for example, has five. These receptors are homologous to bacterial two-component regulators and have protein kinase activities (3, 4). Genetic analyses of the receptor family are consistent with a model in which receptors actively suppress ethylene responses in the absence of the hormone. There is redundancy built into the system: single loss-of-function mutants have no visible phenotype, whereas higher-order loss-of-function mutants exhibit varying degrees of enhanced ethylene responses (5). In contrast, single amino acid changes in the ethylene-binding domain of receptors frequently result in dominant ethylene insensitivity. This insensitivity is believed to be the consequence of an inability to recognize ethylene (6). Mechanistically, little is known about how the receptors bind to ethylene and transduce that information to the downstream signaling components. The only protein that is known to physically interact with the receptors is the RAF-like protein kinase CTR1 (7). In this issue of PNAS, two articles describe the identification of a new class of protein that genetically interacts with ethylene receptors to negatively regulate ethylene responses. Resnick et al. (8) started with a dominant ethylene insensitive Arabidopsis etr1 receptor mutant and screened for second-site suppressors that restore ethylene responsiveness. They identified a loss-of-function mutation in the REVERSION-TO-ETHYLENE SENSITIVITY1 (RTE1) gene that restores ethylene responses to close to wild-type levels, presumably because of inactivation of dominant etr1 signaling. In contrast, Barry and Giovannoni (9) characterized a tomato mutant in which fruits do not ripen. Tomato fruit ripening is absolutely dependent on ethylene responses, and this dominant mutant, Green ripe (Gr), had been previously Conflict of interest statement: No conflicts declared.