Control of ethylene-mediated processes in tomato at the level of receptors.

The plant hormone ethylene controls many aspects of development and response to the environment. In tomato, ethylene is an essential component of flower senescence, organ abscission, adventitious root initiation, and fruit ripening. Responses to ethylene are also critical for aspects of biotic and abiotic stress responses. Clearly, much of the control of these events occurs at the level of hormone synthesis. However, it is becoming apparent that levels of the ethylene receptors are also highly regulated. The tomato ethylene receptors are encoded by a family of six genes. Levels of expression of these genes are spatially and temporally controlled throughout development. Further, a subset of the receptor genes respond to external stimuli. Genetic and biochemical evidence supports a model in which the ethylene receptors act as negative regulators of downstream responses; in the absence of ethylene, receptors actively suppress expression of ethylene responsive genes. Consistent with this model, a reduction in the overall level of receptor increases ethylene responsiveness of a tissue while higher expression of receptor decreases ethylene sensitivity. Evidence to support this model will be presented.