Carbohydrates Stimulate Ethylene Production in Tobacco Leaf Discs

Abstract

Various natrlly occurrg carbohydrates, applied at a concentration range of I to 100 m, lated ethyene producto for severl days in indoleacetic acid (IAA)-treated or untreated tobacco (Nicotiana tabacam L cv 'Xanthi') laf discs. The lag period for this sugr-stimulated ethylene prodution was 8 to 12 hours after excision in the untreated leaf discs, but less than 2 hours in the IAA-treated ones. Among the tested carbohydrates, 12 were foud to increase synergistically ethylene production, with D-galatose, sucrose, and lactose being the most active; annitol and L-ghose had o effec. The extent and duration of the increased ethylene production was dependent upon the type of sur applied, the tissu's age, and the existence of both exogeonos IAA and sar in the medium. Sucrose appeared to elicit a continuous IAA effect for 48 hours, as expressed by increased ethylene production, even when IAA was removed from the medium after a 4-hour pulse. Sucrose stimulted both the uptake and decarboxylation of [1-'4CIAA, as well as the hydrolysis of the esteric and amide IAA conjuptes formed in the tissue after appication of free IAA. This gradual hydrolysis was accompanied by a further accumulation of a third IAA metabolite. Moreover, synthetic indole-3-acetyl-L-alanine increased ethylene production mainly with sucrose, and this effect was accompanied by its increased decarboxylation and turnover pattern suggesting that release of free IAA was involved. An esteric IAA conjugate, tentatively identified by GC retention time was found to be the major component (84%) of the naturally occwuing IAA conjugates in tobacco leaves. Accordingly the sucrosestimuted ethylene production in tobacco leaves can be ascribed maily to the sucrose-stimulated hydrolysis of the esteric IAA conjugate.carbohydrates tested could elicit a stimulating effect in either of those systems, nor in pea epicotyls (8), and it was therefore suggested that the increased ethylene production in these tissues might be related to the toxicity of galactose (I 1) or stress effect of mannitol (21).Our recent study (3) established that sucrose could stimulate IAA-induced ethylene production in a synergistic action. This occurred only in tissues which were responsive to IAA with respect to ethylene production.A possible involvement of IAA in the sugar-stimulated ethylene production could be related to ACC2 formation from SAM, which is the rate-limiting step in ethylene biosynthesis and was found to be stimulated both by IAA (26) Intact leaves as well as leafdiscs of tobacco, incubated in plain water or buffer, produced very small amounts of ethylene (2). When 2% sucrose was added to tobacco leaf discs as a medium constituent, a remarkable increase in ethylene production, which lasted for several days, was observed. This occurred both in IAAtreated and untreated leaf discs, with the increase being much more pronounced in the former (3). A carbohydrate-stimulated ethylene production has been described so far in mung bean hypocotyls treated with D-galactose (I 1) and recent...

Galactose Inhibits the Conversion of 1-Aminocyclopropane-1-Carboxylic Acid to Ethylene in Aged Tobacco Leaf Discs

“…2A), whereas the Glc-and Sucstimulated ethylene production increased gradually throughout the course of incubation (12,15). Consequently, Table I shows that at the end of 103 h of incubation, Suc-and Glc-treated discs, as compared with Gal-treated ones, accumulated higher levels of ethylene and much lower levels of ACC.…”

“…Where indicated, 50 mM of the sugars Gal, Glc, or Suc, 1 mm AVG plus 10 mm ACC were included in the medium. Ethylene and CO2 evolved were absorbed, respectively, by 0.25 M Hg(C104)2 and 10% KOH solutions held in two plastic center wells as detailed previously (12). In one experiment the KOH solution was omitted and a 10% CO2 atmosphere was provided by injecting pure CO2 into the sealed flasks.…”

“…Tobacco (Nicotiana tabacum L. cv Xanthi) plants were grown in a greenhouse under 18 h of light at temperatures between 20 and 30C. Fully expanded mature leaves were washed, surface sterilized with NaOCl (0.5%), and further treated under sterile conditions as outlined previously (2,12,15). Samples of eight tobacco discs, weighing about 0.1 g, were incubated on filter paper in 50-ml flasks containing 2 ml of 50 mm Na-phosphate buffer (pH 6.1) and 50 mg/L chloramphenicol as described previously (15).…”

“…The exact cause of this toxicity is still in doubt, whether it is exerted by the monosaccharide itself or by accumulation of its metabolites (1 1, 17). Since Gal stimulated ethylene production in vegetative tissues (4,12,15) as well as in tomato fruit (6), and there was a similarity between the Gal-induced phenomenon and the ethylene effects, it has been suggested that the effect of Gal might be ethylene-mediated (4). Based on the drastic Gal-stimulated ethylene production, which lasted for a relatively short period (12,15), it was suggested, that unlike other sugars, which exert a 'Supported by grant 1- 3 Abbreviations: ACC, 1-aminocyclopropane-l-carboxylic acid; AVG, aminoethoxyvinylglycine; GAI, o-galactose; Glc, D-glucose; MACC: 1-(malonylamino)cyclopropane--carboxylic acid; Suc, sucrose.…”

“…Since Gal stimulated ethylene production in vegetative tissues (4, 12, 15) as well as in tomato fruit (6), and there was a similarity between the Gal-induced phenomenon and the ethylene effects, it has been suggested that the effect of Gal might be ethylene-mediated (4). Based on the drastic Gal-stimulated ethylene production, which lasted for a relatively short period (12,15) (2,12,15). Samples of eight tobacco discs, weighing about 0.1 g, were incubated on filter paper in 50-ml flasks containing 2 ml of 50 mm Na-phosphate buffer (pH 6.1) and 50 mg/L chloramphenicol as described previously (15).…”