Auxin-Induced Ethylene Production as Related to Auxin Metabolism in Leaf Discs of Tobacco and Sugar Beet

Abstract

ABSTRACIExogenously supplied indole-3-acetic acid (IAA) stimulated ethylene production in tobacco (Nicotiana glauca) leaf discs but not in those of sugar beet (Beta vulgaris L.). The stimulatory effect of IAA in tobacco was relatively small during the first 24 hours of incubation but became greater during the next 24 hours. It was found that leaf discs of these two species metabolized I1-'4CIIAA quite differently. The rate of decarboxylation in sugar beet discs was much higher than in tobacco. The latter contained much less free IAA but a markedly higher level of IAA conjugates. The major conjugate in the sugar beet extracts was indole-3-acetylaspartic acid, whereas tobacco extracts contained mainly three polar IAA conjuptes which were not found in the sugar beet extracts. The accumulation of the unidentified conjuptes corresponded with the rise of ethylene production in the tobacco leaf discs. Reapplication of all the extracted IAA conjugates resulted in a great stimulation of ethylene production by tobacco leaf discs which was accompanied by decarboxylation of the IAA conjugates. The results suggest that in tobacco IAAtreated leaf discs the IAA conjugates could stimulate ethylene production by a slow release of free IAA. The inability of the exogenously supplied IAA to stimulate ethylene production in the sugar beet leaf discs was not due to a deficiency of free IAA within the tissue but rather to the lack of responsiveness of this tissue to IAA, probably because of an autoinhibitory mechanism existing in the sugar beet leaf discs.In vegetative tissues, the rate ofethylene production is thought to be regulated by the endogenous level of free auxin (9, 18). Recently, Yu et al. (24, 25) demonstrated that IAA stimulates ethylene production by inducing the synthesis of the ACC2 synthase, which is the rate-limiting enzyme in the pathway of ethylene biosynthesis (23). Evidence has been presented, showing that the rate of IAA-induced ethylene production was parallel to the level of free IAA retained in the tissue (15, 17,18). The level of the latter depends on the rate of IAA uptake, decarboxylation, conjugation, and IAA transport out of tissue.In this study, leaf discs of tree tobacco and sugar beet were investigated. Tobacco ylene which lasts for several days, and exogenous IAA enhances it. The second system, leaf discs of sugar beet, produces a small amount ofethylene (after the amount ofwound-induced ethylene subsidies) and added IAA fails to induce ethylene production. Rates of uptake and metabolism of ['4C]IAA in relation to ethylene production were studied. MATERIALS AND METHODSPlant Material and Pretreatment. Experiments were conducted with discs taken from fully expanded leaves of sugar beet (Beta vulgaris L. cv Saccarifera) and of tree tobacco (Nicotiana glauca). Sugar beet was grown in a greenhouse under natural light at temperatures between 20 and 30°C. Tree tobacco was grown outdoors on the campuses of the University of California, Davis, and of the Volcani Center, Rehovot, Israel. Leaves were washed...

Physiological Aspects of Free-Radical Reactions

“…It may be emphasized from the physiological point of view that in the presence of oxygen the peroxidase-IAA system yields strong oxidizing species. Since it has been reported that the fornation of ethylene (plant hornone) is promoted by the addition of IAA (137)(138)(139)(140)(141), there is a possibility that the ethylene formation is promoted by such oxidizing species.…”

Carbohydrates Stimulate Ethylene Production in Tobacco Leaf Discs

“…Recent studies of auxin metabolism in tobacco leaves (4,18) have suggested that the rate of ethylene production in the IAAtreated tissue is dependent on the formation of IAA conjugates and their ability to release free IAA (15). In this respect, it was reported that the synthetic IAA conjugate, IAA la, which can induce ethylene production in vegetative tissues (15,16), was much more active when applied with sucrose (3).…”

“…Although the findings reported herein and elsewhere (3,4,18) suggest IAA mediation in the sucrose-stimulated ethylene production, other mechanisms to account for the effect of sugars should also be taken into consideration (19). …”

“…To elucidate the mechanism by which carbohydrates can stimulate IAA activity, we studied IAA uptake, decarboxylation, and the possibility of reversible IAA conjugation. It was found previously (4,18) that the two IAA conjugates, as well as metabolite-1 (Fig. 7), could induce increased ethylene production when reapplied to tobacco leaf discs in the presence of sucrose.…”

“…Thus, after a 4-h pulse of IAA, the discs were transferred to an IAA-free medium in the absence (Fig. 6A) duction for 48 h, even when IAA was removed from the medium after 4 h. Such an extended IAA effect might occur by activation of a bound IAA source, since free IAA applied to tobacco leaves remains in very low concentrations (4,18), due to its metabolic removal via decarboxylation and conjugation (4), as has been also reported for pea epicotyls (5).…”