Ethylene auxin increase

Results are interpreted in the light of the suggested auxin effect (see introductory scheme) whereby lAA acting as a proton donor releases ethylene-forming protein or receptor by severing of electrostatic bridging. This hypothesis is furthermore supported by the observation (Fig. 1-right) that pressurization in the presence of auxin increased amount of Ca2+ release. Clearcut lAA-Ca interdependence has, moreover, been shown in rapid auxin action in maize roots. ... 

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. 

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]. 

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. 

Ethylene physiology of the plant can be manipulated in a variety of ways. In the past, the use of ethylene was limited to exposure of plants to the gas in containers thus, fleld applications were impractical. This limitation was removed by the discovery and commercial development of ethephon in which the liquid active ingredient, 2-chloroethyl phos-phonic acid, is converted to ethylene by the plant (59). Other means of modifying ethylene physiology have been recognized and discussed (4, 5). It is possible to stimulate ethylene synthesis with auxins (60, 61, 62, 63), abscisic acid (64), defoliants (65), ascorbic acid (66), cyclohexi-mide (66), and iron salts (66), among other compounds. A number of physical, environmental, microbial, and insect stresses increase ethylene synthesis, including moisture stress (67) and air pollutants (68). 

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... 

Subsequent work with lAA indicated that this auxin which was produced at the terminal bud stimulated cellular growth in the terminal bud but as it was transported down the shoot, somehow supressed growth of the lateral buds (22). There are indications that under the influence of auxin, the cells around lateral buds produce ethylene. It is the ethylene in turn which inhibits the growth of the lateral buds (23). When the terminal bud is removed and the auxin and ethylene are no longer present, the lateral buds grow, resulting in bushy plants. The number of flowers on the plant is also increased. 

Large amounts of auxin, however, by causing increased amounts of ethylene, encourage fruit drop, and are used in the thinning of fruit in the production of apples and olives (31). 

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). 

---