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Abstract
The main evidence for a single mechanism of ethylene action is the observation that nearly all responses to the gas have the same dose response curve (8), suggesting a single type of receptor molecule. If so, the case is similar to that of phytochrome where one biochemical change produces a multitude of secondary changes resulting in a variety of physiological responses depending upon the tissue involved. We have chosen to use the etiolated pea seedling to investigate the primary and secondary actions of the gas because all parts of this plant have been extensively studied and are highly responsive to ethylene. When this seedling is exposed to ethylene stem growth slows, the hook tightens, the subapex swells and nutates horizontally, root growth slows and the zone of elongation swells, root hairs form, lateral root formation is inhibited, and the root tip bends plageotropically. The causes of these changes are to be found in the effects of ethylene on cell division, cell expansion, and auxin transport.