19 Ethylene: A Unique Plant Signaling Molecule

Mammalian neurobiologists and neurochemists have been fascinated to discover that their experimental systems use simple gases such as nitric oxide (NO) and carbon monoxide (CO) as neuronal messengers (Snyder 1992; Verma et al. 1993). Recent advances in neurobiology have revealed that humans use NO in a variety of physiological responses. NO plays a role in blood pressure maintenance as a vasodilator, helps kill foreign invaders in the immune response, is a major biochemical mediator of penile erection, and is probably a major biochemical component of long-term memory (Snyder 1992). Another simple gas, ethylene (C 2 H 4 , the simplest olefin), has fascinated plant biologists for more than a century by its spectacular effects on plant growth and development (Abeles et al. 1992; Theologis 1993). Plants do not have brains or sophisticated reproductive organs like humans, but the gas ethylene plays an important part in sex determination in some monoecious species (Abeles et al. 1992). This hydrocarbon gas, well known as the fruit-ripening hormone, is biologically active in trace amounts (as little as 10 nanoliters per liter of air). It promotes leaf and flower senescence and abscission, and causes the loss of geotropic sensitivity, root initiation, the onset of epinastic curvatures, the acceleration of respiration, and the modification of leaf and fruit pigments (Burg 1962; Abeles et al. 1992). Ethylene is thought to be a key regulator of the ability of rice to grow in the deepwater regions of southeast Asia and of most hypoxia-induced plant adaptations (Jackson 1985; Kende 1987). It also controls...