Methionine Metabolism and Ethylene Biosynthesis in Senescent Flower Tissue of Morning-Glory

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Rib segments excised from flower buds ofIpomoea tricolor Cav Labelng experiments using 33P as marker showed that the rate at which radioactivity was lost from phospholipids during aging was parallel to the rate at which the level of total phospholipids declined. Exogenously applied ethylene accelerated the loss of phospholipid and the senescence of rib segments while benzyladenine retarded both of these processes.Ag+, which counteracts the effect of ethylene in many plants, inhibited rolling up of the rib segments but did not affect either spontaneous and ethylene-induced ethylene generation, or phospholpid loss.In contrast, Co2+, a purported inhibitor of ethylene synthesis, reduced ethylene production, rolling up, and phosphoUpid loss. The inhibition of ethylene-induced rolling up by Co2+ could not be overcome with exogenous ethylene, however.Our results indicate that phospholipid loss is a marker for membrane degradation in rib segments. Changes in membrane integrity and in cellular compartmentation may be the basis for ethylene synthesis during aging of flower tissue.

Section: Discussionmentioning

confidence: 85%

Membrane Lipids in Senescing Flower Tissue of Ipomoea tricolor

Beutelmann

Kende²

1977

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“…1). The inhibition indicates that methionine is the principal precursor of ethylene biosynthesis in leaves (3,4,21) as in fruits (21) and flowers (18). Effects of exogenous ethylene and AVG on rates of respiration and Chl loss are shown in Figures 2 and 3, respectively.…”

Section: Resultsmentioning

confidence: 99%

Ethylene as a Regulator of Senescence in Tobacco Leaf Discs

Aharoni

Lieberman²

1979

118

The regulatory role of ethylene in leaf senescence was studied with excised tobacco leaf discs which were aflowed to senesce in darkness.Exogenous ethylene, applied during the first 24 hours of senescence, enhanced chlorophyil loks without accelerating the cHimacteric-like pattern of rise in both ethylne and CO, which occurred in the advanced stage of leaf senescence. Rates of both ethylene and C02 evolution increased in the ethylene-treated leaf discs, especially during the first 3 days of senescence.The rhizobitoxine analog, amnnoethoxy vinyl glycine, markedly inhibited ethylene production and reduced respiration and chlorophyll loss. Pretreat At appropriate time intervals gas samples were withdrawn with a hypodermic syringe for determination of ethylene and CO2 as previously described (6). After sampling, flasks were flushed with sterile fresh air, and when required, ethylene or CO2 was reintroduced.Chl was extracted from leaf discs with dimethylformamide (6), determined spectrophotometrically at 665 nm, and expressed in O.D. units.Number of replicates, leaf discs per replicate, and repeat experiments were as detailed previously (6). RESULTS AND DISCUSSIONThe rapid Chl breakdown phase in tobacco leaf discs senescing in darkness was accompanied by a rise and then a decline in both respiration and ethylene production (6). In fruit these phenomena characterize the climacteric stage and represent the onset of ripen- AHARONI AND LIEBERMAN ing (13, 28). Tests with leaf discs led us to conclude that these phenomena do not indicate the start of senescence in leaves but are associated with a stage rather late in the process. The aging process in morning glory flowers also commenced before ethylene production began (19). Applying exogenous ethylene to precimacteric fruit (27) or flowers (25) hastened the rise in ethylene production. However, treatment of tobacco leaf discs with 10 ,ul/ 1 ethylene for the first 24 h did not significantly hasten the climacteric-like rise in ethylene production (Fig. 1). Ethylene evolution was somewhat greater in ethylene-treated than in control leaf discs, and could represent either increased synthesis and/or release of the exogenous absorbed ethylene. Continuous treatment with AVG markedly inhibited ethylene synthesis during senescence (Fig. 1). The inhibition indicates that methionine is the principal precursor of ethylene biosynthesis in leaves (3, 4, 21) as in fruits (21) Further evidence of a regulatory role for ethylene in leaf senescence was obtained with Ag+, which has been reported to oppose effects of ethylene on growth, senescence, and abscission (9-11). Increasing concentrations of Ag+ from 0 to 20 mg/l completely nullified the ability of ethylene to enhance both Chl loss (Fig. 4) and rate of respiration (Fig. 5). The level of Ag+ that prevented Chl loss in ethylene-treated or ethylene and AVGtreated leaf discs was 20 mg/l or 10 mg/l, respectively. The senescence-retarding effect of AVG was not completely nullified by exogenous ethylene (Fig. 4), probably because tre...

“…Since investigations with intact flowers and buds are restricted by slow uptake and uneven distribution of metabolites and inhibitors, we turned to excised flower and bud rib segments for further experiments on permeability changes (5), methionine metabolism (6), and the role of ethylene evolution during aging. Using such segments, all compounds tested were readily absorbed and production of "wound ethylene" posedno problems as it had subsided by the following morning.…”

Section: Discussionmentioning

confidence: 99%

Relationship between Ethylene Evolution and Senescence in Morning-Glory Flower Tissue

Kende

Hanson²

1976

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125

An excised tissue system consisting of coroUla rib segments was developed to study the relationship between senescence and ethylene production in morning-glory flowers (Ipomoea tricolor). Such segments, isolated 1 or 2 days (day -1 or day -2) before flower opening (day 0) passed through the same developmental phases as did the corresponding tissues of the intact organ. When excised on day -1 and incubated overnight, the rib segments turned from purple to blue and changed from a slightly curled to a flat configuration. On day 0, these segments rolled up during the afternoon and turned purple again, as did the ribs of an intact corolla; the roling up coincided with an increased rate of ethylene production. Premature rolling up and associated ethylene evolution were induced by ethylene or propylene treatment. When segments were excised on day -2 and incubated overnight, there were no changes in color or shape; during day -1, no spontaneous rolling up and Uttle ethylene evolution occurred. Application of ethylene or propylene to these immature segments elicited rolling up but did not stimulate endogenous ethylene production.Overnight treatment of segments cut on day -1 with 10-6 M benzyladenine markedly retarded spontaneous rolling up and ethylene evolution, although the response to applied ethylene was only slightly slowed. Overnight treatment of segments cut on day -1 with the ethoxy analog of rhizobitoxine (10-5 to 10-4 M) resulted in almost complete (>99%) inhibition of both spontaneous and propylene-induced ethylene evolution. Although spontaneous rolling up was delayed, it was not abolished, and ethylene-induced rolling up was almost unaffected.These data indicate that an ethylene-generating system develops as an integral part of the aging process in flower tissue. Ethylene hastens aging of the flower, but may not play an obligatory role in flower senescence.Intact flowers of the morning-glory (Ipomoea tricolor) open at 5:00 to 6:00 AM and begin to fade at about 1:00 PM of the same day. During the fading process, the corolla rolls inwards driven by curling up of the ribs, and a sharp increase in the rate of ethylene evolution occurs; the rolling up and ethylene evolution can be induced prematurely by the application of exogenous ethylene (7). Rolling up of the corolla is accompanied by increases in the activities of several hydrolases and by a decrease in vacuolar pH that causes a color change from blue to purple (9). Since the above changes all take place within a single day, the senescence of morning-glory flowers provides a useful model for investigation of the role of ethylene in plant senescence, especially the phenomenon of ethylene-induced ethylene synthesis that is common to both flower senescence (3,7,11) (5) in an environmental chamber with a 16-hr photoperiod (from 5:00 AM to 9:00 PM). Buds were harvested between 4:00 PM and 7:00 PM 1 or 2 days before flower opening. On the day before flower opening, buds were approximately 5 cm long, were loosely coiled, and had lost all Chl from the coroll...