Stress ethylene production

During drought stress, ABA regulates stomatal closure, whereas increased ethylene production has an inhibitory influence on ABA action. An inhibition of ethylene synthesis delays drought-associated chlorophyll loss, supporting the role of ethylene in drought-induced senescence. °°... 

When subjected to drought stress, excised wheat Triticum aestivum L.) leaves increase ethylene production as a result of an increased synthesis of ACC 71 and an increased activity of the ethylene-forming enzyme (EFE) which catalyzes the conversion of ACC 71 to ethylene. Rehydratation to relieve water stress reduces EFE activity to levels similar to those in non-stressed tissue. Pretreatment of the leaves with N-benzyladenine (BA) 75 or indole-3-acetic acid lAA 76 prior to drought stress caused further increase in ethylene production. Conversely, pretreatment of wheat leaves with abscisic acid ABA 77 reduced ethylene production to levels of non-stressed leaves, accompanied by a decrease in ACC 71 content, Eq. (29). 

Plants under water stress are known to produce increased amounts of ethylene, show a rise in ABA and a decline in endogenous cytokinins (52,53). Other plant hormones are also probably involved in the response to water stress and other stress and wounding actions. The surge of ethylene production upon stress may therefore represent a response to a disturbance of the hormonal balance in tissues. The dampened oscillation curve for wound ethylene production may reflect the dynamic return of the disturbed hormonal system to a proper hormonal balance under the new tissue conditions, and thus may also reflect a healing phenomenon. 

Ethylene is produced in measurable amounts in a number of fruits, leaves, and shoots under normal conditions (7, 46, 59, 72) and in large amounts after treatment with certain chemicals 14, 43, 44, 46), mechanical stresses (72), and adverse environmental factors (73). Thus, acceleration of abscission by many agents seems to occur via ethylene production—a fact that is being used to assay chemicals as potential accelerating agents for abscission 74). Apparently, this is the basis for the induction of abscission by placement of abscission chemicals on the surface of an organ such as an orange 41). 

Defoliation. Interest in defoliation has been low in recent years. One relatively new development is the "wiltant which is applied only shortly before harvest (51). As an outgrowth of some basic studies, several auxin transport inhibitors, TIBA, DPX-1840, Alanap (N-l-naph-thylphthalamate), and morphactins (2-chloro-9-hydroxyfluorene-9-car-boxylic acid), were shown to promote ethylene- and ethephon-mediated leaf abscission (52, 53). Subsequently, CA3 was found to be even more active in promotion of ethylene-induced abscission (54). It now appears that the CA3 counteracts the inhibitory effect of auxin on ethylene-induced leaf abscission (55) thus, CA3 might improve the performance of any defoliant that achieves part of its action by stimulating stress-induced ethylene production and lowering the natural auxin content of the dam-... 

In some cases it might be desirable to inhibit ethylene synthesis chemically to prevent responses mediated by naturally produced ethylene or stress-produced ethylene. Although some substances do inhibit ethylene production modestly—e.g. TIBA (69)—no outstanding regulator of this nature has been discovered. Another possibility is to promote or inhibit ethylene action. Promotion can be accomplished by auxin transport inhibitors and GA in cases where auxins and ethylene have opposite effects 52, 53, 54, 55). Recently, silver ion was found to be a potent inhibitor of ethylene action (70). Ethylene action also can be inhibited by lowering the temperature and O2 level or increasing the CO2 level... 

Brassinosteroids are reported to stimulate overall plant growth and development, especially under stress conditions, to enhance auxin-induced growth as well as auxin-induced ethylene production (5, 6). Brassinosteroids interact with most of the phytohormones, such as cytokinins and gibberellins, and in particular with auxin. 

Garty, J., Kloog, N., Wolfson, R., Cohen, Y., Kamieli, A., Avni, A., 1997d. The influence of air pollution on the concentration of mineral elements, on the spectral reflectance response and on the production of stress-ethylene in the lichen Ramalina duriaei. New Phytol. 137, 587-597. 

The effect of chemical elements on the production of stress-ethylene in lichens... 

The extensive literature relating to the production of ethylene by fungi refers to the presence of chemicals as indicative of alterations in the production of ethylene. Thus, we were led to test the probability of ethylene-production by lichens and investigate the impact of chemicals, either applied under controlled conditions or accumulated in the field. Changes in the mode of ethylene production would be indicative of the presence of stress-factors. 

The production of stress-ethylene was measured in thalli of both sites and found to be relatively high in the polluted site. In an additional study, transplanted lichens in an industrial region in Israel (Haifa-Bay, NW Israel) were found to produce greater amounts of ethylene than control lichens left in the unpolluted site (HaZorea) (Garty et al., 1993b). However, as no measurements were made of the content of airborne ehemical elements in the entire thallus in the above-mentioned studies, we were unable at this stage to explain this alteration. 

Garty, J., Kauppi, M., Kauppi, A., 1995b. Differential responses of certain lichen species to sulfur-containing solutions under acidic conditions as expressed by the production of stress-ethylene. Environ. Res. 69,132-143. 

Ethylene is unique in that it is found only in the gaseous form. It induces ripening, causes leaves to droop (epinasty) and drop (abscission), and promotes senescence. Plants often increase ethylene production in response to stress, and ethylene often is found in high concentrations within cells at the end of a plant s life. The increased ethylene in leaf tissue in the fall is part of the reason leaves fall off trees. Ethylene also is used to ripen fruits (e.g., green bananas). 

An important characteristic of ACC synthase in plant tissues is its lability. It has been well recognized that when ethylene production in plant tissues is induced by lAA treatment or by various stresses [33], ethylene production declines rapidly following induction, and this decline is accompanied by a corresponding decline in ACC content. Two mechanisms are responsible for this rapid decline in ethylene production one is the conjugation of ACC into MACC catalyzed by ACC malonyltransferase resulting in a reduced ACC level in the tissue, and the other is the inactivation of ACC synthase. The apparent half-life of ACC synthase in wounded green tomato pericarp and in lAA-treated mungbean hypocotyls has been estimated to be 30-50 min, based on the decay kinetics of the enzyme activity extracted from induced tissues in the presence of cycloheximide which blocks the new synthesis of the enzyme [1, 36]. 

Aside from its conversion to ethylene, the other metabolic fate of ACC in plant tissues is its conjugation into MACC, a biologically inactive end product of ACC. Because endogenous levels of ACC can increase duiing development or in response to stress, it seems logical that the plant would require some means to sequester ACC to prevent overproduction of ethylene. The N-malonylation of ACC serves this purpose by reducing the tissue level of ACC and, consequently, ethylene production. 

Wounding is assumed to exert its effect at the step where SAM (=S-adenosylmethionine) is converted into ACC (1-aminocyclopropane-l-carboxylic acid), the direct precursor of ethylene. Normally this step, regulated by the enzyme ACC synthase, is rate-limiting in the cascade of events leading to ethylene production (Yang and Hoffman 1984). By means of stress the rate can be increased... 

Stress effects Species Ethylene production (E.P.) References... 

Thuring JWJF, Harren FJM, Nefkens GHL, Reuss J, Titulaer GTM, de Vries HSM, Zwanenburg B (1994) Ethylene production by seeds of Striga hermonthica induced by germination stimulants. Proc 3rd Int Worksh on Orobanche (8-12 Nov), Amsterdam Voesenek LACJ, van der Veen R (1994) The role of phytohormones in plant stress too much or too little water. Acta Bot Neerl 43 91-127... 

Effects on Ethylene. Direct desiccation of detached leaves causes a rapid increase in ethylene production. Since the response is larger in leaves with an abscission zone, this stress-induced ethylene production may be related to stress-induced abscission (Aharoni 1977). 

Aharoni N (1977) Enhancement of ethylene production in water-stressed leaves. Plant Physiol 59 Suppl, p 39... 

Hiraki Y, Ota Y (1975) Relationship between growth-inhibition and ethylene production by mechanical stimulation in Lilium longiflorum. Plant Cell Physiol 16 185-189 Hiron RWP, Wright STC (1973) The role of endogenous abscisic acid in the response of plants to stress. J Exp Bot 24 769-781... 

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