Auxin ethylene biosynthesis

The pathway of ethylene biosynthesis in higher plants is from l-methionine4 (Figure 5.9). Methionine is an intermediate in other metabolic processes and the control of ethylene biosynthesis via the interference of methionine production is not realistic. The ACC synthase step from S-adenosyl methionine to ACC appears more susceptible to chemical modification auxin promotes ethylene production by increasing the activity of ACC synthase. Subsequent steps from ACC are less controlled and ethylene is readily produced from the conversion of ACC in most tissues. 

The stimulatory effect of auxin on ethylene biosynthesis is the basis for the use of synthetic auxins (Figure 5.11) to promote the abscission of... 

Fig. 1. Ethylene biosynthesis. The numbered enzymes are (1) methionine adenosyltransferase, (2) ACC (l-aminocyclopropane-l-carboxylic acid) synthase, (3) ethylene forming enzyme (EFE), (4) 5 -methylthio-adenosine nucleosidase, (5) 5 -methylthioribose kinase. Regulation of the synthesis of ACC synthase and EFE are important steps in the control of ethylene production. ACC synthase requires pyridoxal phosphate and is inhibited by aminoethoxy vinyl glycine EFE requires 02 and is inhibited under anaerobic conditions. Synthesis of both ACC synthase and EFE is stimulated during ripening, senescence, abscission, following mechanical wounding, and treatment with auxins.

The effect of homoBR on flowering tissues was to produce bisexual and pistillate flowers on a staminate inflorescence. Also, sepals were deformed (24), and one would suspect some of these effects were due to induced ethylene biosynthesis, as the dosage of the brassinosteroid used was very high. Excess hormone levels are known to induce the biosynthesis of ethylene BR can also do this, and it interacts with auxin and cytokinin in the induction (43). BR can also affect endogenous auxin and abscisic acid levels in treated tissue (21,44,45). Thus BR does have multiple and modulatory effects. 

Thus, cell enlargement, for instance, depends upon auxin and involves the uptake of water, extension of the cell membrane and protein synthesis. The auxin dose-response curve consists of two peirts promotion by low concentrations and inhibition by higher concentrations via the formation of ethylene. Cytokinins and abscisic acid may possibly induce also, under special conditions, the production of ethylene. Many publications deal with effects of these plant hormones, especially of auxin, on ethylene biosynthesis in plants which occurs after a lag phase of 30 - 60 minutes and is specifically blocked by rhizobitoxin as well as by inhibitors of ENA and protein synthesis indicating that a continuous synthesis of protein is required for high rate of ethylene production (Eef. 20). 

TAAl was also isolated from a genetic screen for weak ethylene-insensitive mutants, wei mutants, in Arabidopsis. The taalAveiS mutants have elongated roots in the presences of ACC (1-aminocyclopropane-l-carboxylate), an ethylene biosynthesis precursor, whereas in wild-type root elongation is inhibited under the same conditions. Moreover, simultaneous inactivation of TAAl and of two of its close aminotransferase related homologs, TARl and TAR2, led to developmental defects similar to those in two well-known auxin mutants [171],... 

Fig. 2. Pathway and regulation of ethylene biosynthesis. Dotted arrow represents the rate-limiting reaction solid heavy arrow indicates induction by auxin of synthesis of the enzyme hollow arrow indicates inhibition by AVG of the conversion (from Yu et al., 1979).

In earlier studies of ethylene biosynthesis [16], we obtained evidence inconsistent with the view that N. glutinosa compensates for mutant tms genes by increased auxin production. We showed that levels of the ethylene precursor, ACC, were about 50-fold higher in A6-transformed cells than in cells transformed by the tms-muiani A66, a result in accord with the well-known induction of ACC synthesis by auxin. Results of ACC analysis were quantitatively similar for N. glutinosa and the non-compensating N. tabacum and Lycopersicon esculentum, indicating that A66-transformed N. glutinosa did not accumulate auxin. 

Auxin sensitivity, on the other hand, appears to be mediated by ACC, but not by ethylene. Thus, ACC may play a role in plant development independent from its role in ethylene biosynthesis. The sensitivity of cell lines to ACC was related to... 

D (Auxin) Stimulates the cambial activity and RDs formation, and promotes the expression of ACC synthase gene (enzyme involved in ethylene biosynthesis) P. elliottii, P. palustris [38. 73. 85. 97. 112, 113]... 

Acton GJ, Murray PB (1974) The roles of auxin and gibberellin in reversing radiation inhibition of hypocotyl lengthening. Planta 117 219-226 Adams DO, Yang SF (1979) Ethylene biosynthesis identification of 1-amino-cyclopropane-1-carboxylic acid as an intermediate in the conversion of methionine to ethylene. Proc Natl Acad Sci USA 76 170-174... 

Mulkey TJ, Evans ML, Kuzmanoff KM (1982 a) Promotion of growth and shift in the auxin dose/response relationship in maize roots treated with the ethylene biosynthesis inhibitors, aminoethoxyvinylglycine and cobalt. Plant Sci Lett 25 43-48 Mulkey TJ, Kuzmanoff KM, Evans ML (1982 b) Promotion of growth and hydrogen ion efflux by auxin in roots of maize pretreated with ethylene biosynthesis inhibitors. Plant Physiol 70 186-188... 

Yang SF (1980) Regulation of ethylene biosynthesis. Hortscience 15 238-243 Yeoman MM, Brown R (1971) Effects of mechanical stress on the plane of cell division in developing callus cultures. Ann Bot 35 1101-1112 Yu Y-B, Adams DO, Yang SF (1979) Regulation of auxin-induced ethylene production in mung bean hypocotyls role of 1-aminocyclopropane-l-carboxylic acid. Plant Physiol 63 589-590... 

Furthermore, SAM not only plays a pivotal role in PA and ethylene biosynthesis, but also in the formation of putrescine-derived alkaloids. Thus, SAM acts as a methyl donor in the conversion of putrescine into A-methylputresdne, an intermediate in the biosynthesis of nicotine (Mizusaki et al., 197 lb). Therefore, it would be interesting to explore the auxin-ethylene-PAs interaction on PA conjugates and putrescine-derived alkaloids metabolism in the above-mentioned tobacco callus tissue system. Indeed, it has been reported that inhibition of SAM decarboxylase by MGBG resulted in an increase of nicotine production in tobacco root cell suspension cultures (Blume, 1985). Unfortunately, nothing about PA conjugates was indicated in this report. 

The formation of plant cellulases has been found to be closely regulated by different growth hormones, particularly auxin (6,11), steroids (12), or ethylene gas (13). The hormones act in different tissues under different circumstances, and they seldom lead to such high cellulase activity that there is a net decline in total cellulose. Indeed, cellulose biosynthesis usually continues even while partial hydrolysis occurs, and net cellulose deposition often keeps pace with growth under all of these conditions (14). 

I04"C) is often considered as a plant growth substance because it acts like a fruit-ripening hormone. The biosynthesis of ethylene in the plant from 1 -ami-nocyclopropanecarboxylic acid is stimulated by auxins abscisic acid and cytokinins can - depending on the type of plant - have stimulating or inhibiting effects, and tissue injuries in plants lead to the formation of the so-called wound ethylene . For information on the numerous roles of ethylene, see Other... 

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