Auxins antagonism

Some chemically related compounds antagonize the action of auxins, for example, relieving the inhibition of root growth caused by 2,4-D. In contrast, 2,3,5-triiodbenzoic acid synergizes the action. 

Ethylene as a stimulator of growth and development. The most observed actions of ethylene on growing plants involves growth inhibition, or acceleration of senescence. These actions are especially evident in the antagonism or opposition of ethylene to auxins, gibberellins and cytokinins (27), as already outlined above. Actually ethylene stimulates growth in many types of cells, especially in water plants (Table II). When ethylene acts to stimulate cell elongation, as in water plants, auxins and CC>2 enhance the ethylene effect (38,39). This interaction is the reverse of that observed on land plants wherein ethylene opposes the effects of auxin, GA3 and cytokinins. 

There also have been clear indications that interactions of ethylene with auxins, cytokinins, gibberellins and ABA are involved in both ethylene production and action. Generally the effects of ethylene tend to antagonize those of auxins, cytokinins and gibberellins, and tend to reinforce those of ABA, depending, however, on tissue systems involved. Reinforcement of ethylene by ABA and vice versa occurs more frequently in senescence. 

Abscisic acid is a negative regulator in that it primarily antagonizes the action of cytokinins, auxins, and in particular, gibberellins. Abscisic acid decreased the activity of polymerase in radishes (52), peas (53), maize coleoptiles (54), and pear embryos (55). More detailed studies are needed before the question of ABA-induced "modification" of RNA polymerase (54) or "alterations" in the number of sites for template activity (56) can be answered. In barley aleurone cells, ABA-induced suppression of GA-induced <-amylase formation was presumed to involve the continuous synthesis of a short-lived RNA (57). 

Of possible relevance in this respect are the observations that BS and auxin appear to produce opposite effects in root systems whereas they usually act in a similar (but not identical) way on the shoot and commonly show important interactive effects. The most strikingly different effects of these two compounds in roots relate to adventitious root formation in cuttings which is inhibited by BS (24, 30, 31) and promoted by auxin (24), and proton secretion and membrane potential in root segments which are enhanced and hyper-polarized respectively by BS and inhibited and depolarized respectively by auxin (39, 40). These data could suggest either that BS act independently of auxin in roots or that they antagonize... 

Diclofop-methyl is a postemergence grass herbicide whose action is not related to photosynthetic electron transport. Its herbicidal effect, still not well known, probably involves an antagonism of auxin-mediated processes and an increase in membrane permeability (1,2). Its physiological effects have not been described. 

X 10" M sharply decreased auxin levels. In seedlings of Amaranthus caudatus the inhibition of light-induced beta-cyanin synthesis by abscisic acid was antagonized by several phenols including coumarin (maximal effect at 10 M). ... 

Wickson M, Thimann KV (1958) The antagonism of auxin and kinetin in apical dominance. Physiol Plant 11 62-74... 

Auxin inhibitor herbicides include the so-called wild oat herbicides shown in Figure 5.22. Their classification as auxin inhibitors is based on their inhibition of auxin-induced responses in auxin bioassays and their antagonism of auxin herbicides. " This anti-auxin activity of diclofop-methyl is almost certainly secondary in importance to its inhibition of acetyl-CoA carboxylase (see Chapter 3). 

D, the activity of 2,4-D on broadleaf weeds is not affected by DM. Other auxinlike herbicides such as dicamba and MCPA also antagonize the action of DM on wild oats. " The timing of the application of DM and auxin herbicides appears to be critical for their antagonistic interaction on wild oats. In oat coleoptiles, 2,4-D reverses the action of DM when applied within 2 h after exposure to DM, " whereas in whole plants, antagonism occurs even when auxin herbicides are applied within 8 h following the application of DM. " ... 

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