Abscission: The Initial Effect of Ethylene Is in the Leaf Blade

Abstract

The leaf blade of cotton (Gossy-piu7n hirsutum L. cv. Stoneville 213) was investigated as the initial site of ethylene action in abscission. Ethylene applied at 14 ul/l to intact 3-week-old plants caused abscission of the third true leaf within 3 days. However, keeping only the leaf blade of this leaf in air during ethylene treatment of the rest of the plant completely prevented its abscission for up to 7 days. This inhibition of abscission was apparently the result of continued auxin production in the blade since (a) the application of an auxin transport inhibitor to the petiole of the air-treated leaf blade restored ethylene sensitivity to the leaf in terms of abscission; (b) repeated applications of naphthaleneacetic acid to the leaf blade of the third true leaf, when the entire plant was exposed to ethylene, had the same preventive effect on abscission of this leaf as keeping its leaf blade in air; and (c) the inhibitory effect of ethylene on auxin transport in the petiole, which is reduced by auxin treatment, was also reduced by placing the leaf blade in air.The reverse treatment of exposing only the leaf blade of the third true leaf to 14 ,l/l of ethylene, while the rest of the plant was kept in air, also did not cause abscission for up to 5 days. Auxin transport in the petioles of these leaves, however, was inhibited over 80% within 2 days and this effect presumably accounted for their increased sensitivity to ethylene during the subsequent exposures of the whole leaf to the gas.These results suggest that an initial and essential function of applied ethylene in abscission is to reduce the amount of auxin transported out of the leaf blade. This reduction together with the inhibitory effect of ethylene on auxin transport in the petiole reduces the auxin level at the abscission zone to a point where the cells in this region become responsive to the more direct action of the gas (e.g., enzyme induction and secretion). This sequence of events accounts for the lack of abscission unless ethylene is applied to both the leaf blade and the abscission zone.

Disruption of the Polar Auxin Transport System in Cotton Seedlings following Treatment with the Defoliant Thidiazuron

“…This reversal clearly demonstrates the specificity of this inhibitor in this instance and demonstrates the essentiality of ethylene in this response. The selective fumigation of only the leaf blade of cotton results in a marked inhibition of auxin transport through the untreated petiole (2). This observation together with the relative responses of leaf blades versus petioles noted above, suggests that endogenous ethylene production by the leaves exerts a strong inhibitory effect on petiolar auxin transport.…”

The Manipulation of Auxin in the Abscission Zone Cells of Arabidopsis Flowers Reveals That Indoleacetic Acid Signaling Is a Prerequisite for Organ Shedding

“…In intact plants, removal of the IAA source by deblading the leaf lamina or by application of auxin transport inhibitors distal to the AZ tissue accelerates shedding (Taylor and Whitelaw, 2001). Indeed, one of the initial effects of exposure to ethylene is to impede the polar transport of IAA into the AZ cells (Beyer, 1975), and it has been proposed that the auxin/ ethylene balance ultimately dictates the triggering of abscission and the rate at which it proceeds. However, much of the evidence of an involvement of auxin in the regulation of abscission is correlative and is based on manipulating hormone concentrations by tissue excision and auxin application.…”

Abscission Responses to Moisture Stress, Auxin Transport Inhibitors, and Ethephon

“…Ethephon and ATI applied at the beginning of the water stress treatment, rather than near the end, failed to promote stress-induced abscission, which suggests a preparatory event which increases sensitivity to ethylene. Thus, both water stress and ATI seem to deal with the preparatory event(s) which is related to auxin supply, inhibition of auxin transport through the petiole and leaf veins being a critical process to that supply (4)(5)(6). Ethephon or native ethylene also promotes the final events in abscission: induction of synthesis of hydrolytic enzymes and secretion of some of these enzymes into the cell wall (1,2,8,19).…”

“…First, there is growing evidence that endogenous ethylene regulates leaf abscission (9,18). The action of ethylene in abscission appears to be due in part to inhibition of auxin transport (4)(5)(6). Auxin transport inhibitors increase or hasten leaf abscission induced by exogenous ethylene or Ethephon (15,16).…”