Auxin effects

No matter how unsatisfactory it seems from the standpoint of working with the simplest hypothesis, one must consider the possibility that there is no master, auxin-controlled reaction rather several reactions may separately or in concert account for the various auxin effects. In this view it is not surprising that the type of response evoked by auxin is very dependent upon the tissue in question. 

Pilet, P. E. Braun, R. Riboneclease activity and auxin effects in the Lens root. Physiol. Plant, 1970, 23,... 

There is a close relationship between the effect of various substances as auxins and their herbicidal effects, with some easily explained exceptions. 4-Phenoxybutanoic acids (e.g., MCPB) have no auxin effect, yet they are efficient herbicides in many plants, providing they are metabolically transformed to the corresponding acetic acid. Although MCPA is selective with toxicity mainly against broadleaves, MCPB has an even more narrow selectivity because broadleaved plants, which do not metabolize it to MCPA, are not killed. 

TBA is readily absorbed and translocated by plants. It moves both acropetally and basipetally and causes typical auxin effects in sensitive plants. It strongly inhibits apical growth and leaf formation (Zimmermann and Hitchcock, 1951). It does not cause epinasty in the MCPA-tolerant weeds but stops their growth so that they cannot compete with the crop at the later growth stages. 

Obviously, many of these effects are indirect or secondary, as they become measurable only later, after the manifestation of the rapid auxin effect (Evans and Ray, 1969). 

The latter investigations also demonstrate another type of biochemical mode of action of herbicides with auxin effect. 

Triclopyr is a nonpersistent herbicide with auxin effect (Chang and Foy, 1978) rapidly broken down in the soil by microorganisms. 

Some biological roles of auxins. The involvement of other hormones is indicated as (+) if the hormone has the same effect as auxin and (-) if it inhibits the auxin effect. Speed of response rapid (R), occurs in less than 1 hr intermediate (I), 1-24 hours slow (S), > 1 day. 

More recently, auxin effects on ion transport have been characterized in protoplasts from Vida faba guard cells and tobacco protoplasts. In both systems, patch clamp techniques have been used to demonstrate auxin regulation of anion channels [14,15]. Alternatively, electrode impalement of protoplasts from faba guard cells has demonstrated that low concentrations of auxin promote inward movement of K, while higher concentrations inhibit this same activity. These effects were correlated with opening and closing of the stomata, respectively [16]. 

Indoleacetic acid, see also under Auxins effect of combined application of thiamine and, on roots, VI, 259ff. as growth factor for callus tissue, VI, 239... 

Other studies of the fate of indoleacetic acid in plant tissues have concentrated on non-oxidized products. It has been shown that indoleacetic acid can readily be esterified to its ethyl ester (XXIII) by plant enzymes, and there is some evidence this that is a naturally occurring substance. Similarly, when indoleacetic acid is applied to plant tissue it is rapidly conjugated into indoleacetamide (XVIII) and indoleacetylaspartic acid (XXII). The details of this process have been extensively studied by Andreae and his coworkersi, but the nature of the enzymes involved remains unclarified. AU three of these compounds can produce auxin effects when applied to tissue, hence they can act as indoleacetic acid precursors. However, the kinetics of the reactions which form them appear to favor the conjugated substances and they are more correctly considered as products rather than precursors of... 

Seth A, Wareing PF (1964) Interaction between auxins, gibberellins and kinins in hormone-directed transport. Life Sci 3 1483-1486 Seth AK, Davies CR, Wareing PF (1966) Auxin effects on the mobility of kinetin in the plant. Science 151 587-588... 

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