Hormonal interactions with ethylene

The simplest unsaturated carbon compound, ethylene, exerts a major influence on many if not all aspects of plant growth and development. Although ethylene is a gas at physiological temperatures and pressures, it is now recognized as a plant hormone because it is a natural product of metabolism, acts in trace amounts and is neither a substrate nor cofactor in reactions which are associated with major developmental plant processes. Whether or not ethylene meets all the standard criteria established for hormones, there is no question that this gas is a powerful natural regulating substance in plant metabolism, and that it acts and interacts with other recognized plant hormones. With the advent of gas chromatography, ethylene has become the simplest plant hormone to assay since it is evolved from the tissues and requires no extraction or purification prior to analysis. 

Much like the phenolic acids, early work with scopoletin showed it inhibited oxidation of IAA and thus could affect growth in this manner. Inhibition of several other enzymes by scopoletin and coumarin has been shown. Coumarin was reported to induce ethylene synthesis.47 Also, it is one of several phenolic compounds that antagonize abscisic acid-induced inhibition of growth and stomatal closure.52 Undoubtedly, these and possibly other interactions with hormones are part of the physiological action of the coumarins. 

Plant hormones rarely act alone hormones interact to produce a final effect. According to Gaspar et al., Some responses of plants to auxins may be caused by increased ethylene synthesis in response to auxin treatment. At high ethylene concentrations, microtubule and microfibril orientation are altered, which results in decreased cell elongation and increased cell expansion. The role of ethylene is hard to understand because it effects vary with developmental stage and because low concentrations can promote (or sometimes inhibit) a process, whereas higher levels have the opposite effect [22]. 

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. 

Another important factor of hormone-hormone interaction is the hormonal control of hormonal movement and polarity. Thus, various cytokinins have been reported to increase the polar movement of indole acetic acid and vice versa. Gibberellin treatment was shown also under cer- bain circumstances to increase the basipetal auxin movement. On the other hand, abscisic acid decreases the auxin movement and ethylene, according to some evidence, the gibberellin movement, perhaps through a promotion of conjugation of the auxins with aspartate and of the gibberellins with glucose (cf. Eef. 27). 

A major factor leading to abscission is the weakening of the middle lamella, cell walls, or cells in a separation zone across the petiole, pedicel, or stem. Although any of the known plant hormones can alter the progress of abscission, ethylene remains unique as the principal stimulus of the increased activity of wall-degrading enzymes in abscission, whereas auxin can be given a central role in the retardation of abscission. With the present level of understanding, it would seem that abscission control involves interactions between auxin and ethylene. 

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