Ethylene biosynthesis and action: a case of conservation

Ethylene is one of the simplest organic molecules with biological activity. At concentrations as low as 0.1 ppm in air, it has been shown to have dramatic effects on plant growth and development [1]. Neljubov [78] was the first to show that ethylene has three major effects in etiolated pea seedlings called the triple response: (1) diageotropic growth, (2) thickening of stem and inhibition of stem elongation, and (3) exaggeration of apical hook curvature. Since then, numerous ethylene effects have been described in light-grown plants such as sex determination in curcurbits, fruit ripening in climacteric fruits, epinastic curvature, flower senescence, and root initiation [1]. Interestingly, ethylene has also been shown to have opposite effects in some plants; for instance, it inhibits stem elongation in most dicots, whereas in some aquatic dicots and rice, it stimulates growth [1, 45, 72].