Phenylacetic acids, auxins

Auxins are defined as organic substances that promote cell elongation when applied in low concentrations to plant tissue segments in a bioassay. By this definition, there are several other native auxins that have been reported to occur in plants in addition to the most often studied auxin, IAA. These include the halogen-substituted 4-C1-IAA,23 as well as phenylacetic acid and indole-3-butyric acid.24 All native auxins are found in planta as both free acids and conjugated forms through ester or amide linkages. IAA, the auxin most extensively studied, will be the focus of this chapter. 

In 1926 Frits Went, a student in the Netherlands, detected that the tips of wheat seedlings contained a substance that caused the seedlings to bend toward the light. The identity of the substance, which was given the name auxin from the Greek auxein (meaning "to increase"), was unknown. A few years later it was shown that auxin was indole-3-acetic acid (I A A). Today, four natural auxins are known. Besides IAA there are indole-3-butyric acid, phenylacetic acid, and 4-chloroindole-3-acetic acid. (It is quite interesting to find a chlorinated aromatic substance as a natural substance. It is synthesized... 

The natural auxins indole-3-acetic acid, 4-chloroindole-3-acetic acid, phenylacetic acid, and indole-3-butyric acid... 

The major natural auxin is indole-3-acetic acid (lAA) [42]. A number of related compounds exist in plants, including indolebutyric acid and indoleacetonitrile (Fig. la). These related compounds are active primarily when first converted to lAA [42]. In addition, there are a series of LAA conjugates with sugars and amino acids [43]. Some of these may be detoxification products, but others may be reservoirs of releasable lAA, especially in seeds. Phenylacetic acid (Fig. la) has auxin activity, and exists in sizable amounts in a few plants such as tobacco [42], but it is unclear that this compound actually moves from one part of a plant to another. In addition to the natural auxins, a whole host of synthetic auxins are known. The most widely used are a-naphthaleneacetic acid (NAA) and 2,4-dichlorophenoxyacetic acid (2,4-D) (Fig. la). 

Fig. la. Structures of plant hormones (a) Auxins Indole-3-acetic acid (lAA) Indoleacetonitrile (IAN) Phenylacetic acid (PAA) 2,4-dichlorophenoxyacetic acid (2,4-D) a-naphthalene acetic acid (NAA). 

Auxin antagonists inhibitors of the effects of auxins on growth and development. The action of A. a. can be at least partly reversed by auxins. The term is used independently of the mechanism of inhibition competitive inhibitors are called antiauxins. The A. a. include many structurally different kinds of compounds, including some synthetic auxins, e.g. phenylacetic acid and phenylbutyric acid. 

Some lines of evidence point to other, non-indolic, natural products that may also act as auxins in plant tissues. Some are as yet unidentified chemically, others (like phenylacetic acid) act as auxins in bioassays, occur in plant tissues, but have not been shown to play a role in natural growth regulation. Also some other hormones, such as the gibberellins, have at least weak auxin activity. Interpretation of the importance of substances such as these will have to await the development of specific chemical methods of analysis which are at least as sensitive as the relatively non-specific bioassays now employed out of necessity. 

It was interesting in this connection to find that -isopropyl-PAA (VII e) is an auxin antagonists. This holds also for the w-butyl-, tert-h xiy - and M-amyl derivative (racemates, VII f,g,h). Apparently when a certain size of the ix-substituent is exceeded, normal interaction at the active site is prevented cf. also the discussion in Section 12). That rather subtle factors play a part here is evident if one compares the effects of the n- and isopropyl derivatives, and this is more apparent still when analyzing the activities of alkylidenephenylacetic acids. Whereas the activity of double bond is co-planar with the benzene nucleus (U.V.-spectra), one is inclined to make a comparison with the active 2-substituted i-naphthoic acids (Section 7). The picture is not complete, however, as the ethylidene- and M-propyUdene-phenylacetic acids (VIII b,c) were found to be practically inactive, for which no explanation is at hand so far. 

Moreover D-configuration could also be determined for the auxins (-l-)- -allyl-phenylacetic acid, (—)-indane carboxylic acid and (-)-i,2,3,4-tetrahydro-i-naphthoic acid (LVIII, LIX, LX). 

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