Plant hormones indoleacetate

Besides being fundamental constituents of proteins they are the parent substances from which powerful hormones are derived, for example, adrenaline (epinephrine), noradrenaline (norepinephrine), thyroxine and related substances, 5-hydroxytryptamine (enteramine, serotonin), and the plant hormone indoleacetic acid. Tryptophan is also the precursor of the B vitamin nicotinic acid and hence of part of the important pyridine nucleotides. All three aromatic amino acids are potential precursors of other substances having powerful physiological activity, for example, many of the alkaloids. Errors in the metabolism of the aromatic amino acids in man can give rise to sometimes serious, but fortunately comparatively rare, disorders such as alkaptonuria and phenylketonuria. The numerous metabolic pathways involved in aromatic amino acid metabolism therefore make an important as well as an interesting study. 

Tryptophan also gives rise to the important plant hormone, indoleacetic acid, and microorganisms and especially plants metabolize the aromatic amino acids to a wide range of natural products, for example, certain antibiotics, alkaloids (e.g., diagrams 25-28), flavonoids, and possibly lignin. These are briefly considered. 

The plant hormone auxin has been shown to be radiosensitive. The product of the irradiation of auxin (p-indoleacetic acid) is a polymer similar to that obtained in the radiolysis of indole. 

The work of many investigators soon showed it to be extremely widespread in plants, and it became clear that it was a fundamental plant hormone (e.g., reviews 530, 655a). Excessive production of indoleacetic acid by parasites is responsible for certain types of plant tumor (e.g., 948). The synthetic plant growth regulators, which are in general substituted phenoxyacetic acids, probably function as indoleacetic acid analogues. 

Plant hormone analysis tends to be a very difficult task due in part to the low levels that occur naturally. Over the past several years Weiler and his coworkers and a number of other groups have developed extremely sensitive immunoassays for many of the natural plant hormones ( ). These assays have greatly reduced the time and effort required to analyze indoleacetic acid, cytokinins, gibberellins, and abscisic acid. 

In animals hormones are produced either in special glands (cf. Table 64) or are formed in tissues in which the production of hormones is a biochemical side activity only (so-called tissue hormones). In plants and microorganisms, special hormone-producing glands are absent, but also in plants hormone synthesis is unequal in the different types of cells. Cytokinins, for instance, are produced predominantly in roots and 3-indoleacetic acid in apical meristems. 

The demonstration that the growth of plants is under hormonal control has profoundly influenced the direction of botanical research. Among the well-established plant hormones the most studied belong to the group known as auxins. At present, all conclusively identified natural auxins are compounds chemically related to indoleacetic acid (X). 

The relationships of all known simple indoles in plants to the growth hormone indoleacetic acid is indicated in Fig. i. Although the enz3unes for many steps indicated have yet to be demonstrated, the present arrangement is in reasonable accord with such evidence as is available. 

Konings H (1968) Auxin in root geotropism. In Vardar Y (ed) The transport of plant hormones. North-Holland, Amsterdam, pp 237-250 Konings H (1969) The influence of acropetally transported indoleacetic acid on the geotropism of intact pea roots and its modification by 2,3,5-triiodobenzoic acid. Acta Bot Neerl 18 528-537... 

Botanists have been interested in the plant hormone heteroauxin which is indoleacetic acid. It occurs in the urine and has been shown to be a tryptophan metabolite in plants (see 7J). Another focus of general biological interest of tryptophan is that it is the precursor of the brown eye pigments of insects, ommochrome (875). 

Such indolealkylamines in plants may represent mere metabolic end products without any physiological role. However, the function of indoleacetic acid (lAA) as one of the growth factors (auxins and heteroauxins) is well documented. Trypt-amine and its derivatives could serve as precursors for several plant hormones and pigments. 

Microorganisms (including intestinal bacteria) break down the side chains of tryptophan. In the process there may be formed indolepyruvate, indoleacetate, skatole (methylindole), and indole itself. Indoleacetate is a plant hormone auxin] Chapt. XX-15). 

Scheme 1. Molecular structure of the plant growth hormone auxin (indoleacetic acid, IAA). Extremely small amounts (nanomolar) can be detected by the auxin standard test 5 mm long segments of pea shoots elongate faster in the presence of exogenous auxin, which can be taken as a sensitive assay...

Important indole derivatives (see Scheme 2) include (i) indigo, a vat dye known and widely used since antiquity, and originally obtained from indican, a (3-glucoside of indoxyl which occurs in some plants. Indigo is now prepared synthetically. Tyrian purple, a natural dye used since classical times, is 6,6 -dibromoindigo (ii) the numerous indole alkaloids, with complex derivatives such as yohimbine and strychnine (iii) tryptophan, an essential amino acid found in most proteins. Its metabolites include skatole and tryptamine and (iv) 3-indoleacetic acid, which is important as a plant growth hormone. 

The indole ring system 14 is particularly important and occurs widely in nature. Tryptophan 23 is one of the essential amino acids and is found in most proteins. Its metabolites include tryptamine 24. 3-Indoleacetic acid 25 is an important plant growth hormone. The indole alkaloids, exemplified by yohimbine 26, are an important family of natural products. 

Other indolic auxins besides indoleacetic acid occur in plants (e.g., 396). Both indoleacetaldehyde (51, 107) and indoleacetonitrile (51, 377, 452) have been claimed to be plant growth hormones. Indoleacetonitrile has been isolated from plant sources (377) and shown to be of wide distribution, but whether it is a hormone per se or only acts as a precursor of indoleacetic acid is not yet clear (cf. 838, 865). 

FIGURE 9-27 Metabolites produced in Agrobacterium-mierAeA plant cells. Auxins and cytokinins are plant growth hormones. The most common auxin, indoleacetate, is derived from tryptophan. Cytokinins... 

Indoleacetic acid (lAA) (17) is involved in many aspects of plant growth and development (Bonner and Varner, 1976 Kosuge and Sanger, 1986). This hormonal substance is derived in most plants by conversion of tryptophan to indole 3-pyruvic acid (15) (tryptophan amino transferase), decarboxylation to the indole 3-acetaldehyde (16) (indole pyruvate decarboxylase), and oxidation to indole 3-acetic acid (17) (indole acetaldehyde oxidase) (Fig. 7.6) (Goodwin and Mercer, 1983). 

As previously mentioned, tryptamine (1) serves as a precursor for indoleacetic acid, a plant growth hormone. Indole (10) is a synomone (i.e., the compounds benefits... 

Other well-known indoles that have various natural sources are skatole (3-methylindole) (2), serotonin (3), L-tryptophan (4), tryptamine (5), the plant growth hormones 3-indoleacetic acid (6) and 4-chloro-3-indoleacetic acid (7) [19], the mushroom hallucinogen psilocin (8), and the indole-derived ancient dyes indigo (9) [20] and Tyrian Purple (10) [19] (Scheme 1). 

Serotonin S-hydroxytryptamine, M, 176.2, a plant and animal hormone. It is produced by hydroxylation of L-tryptophan to 5-hydroxytryptophan, followed by decarboxylation. The synthesis occurs in the central nervous system, lung, spleen and argentaffine light cells of the intestinal mucosa. S. is stored in thrombocytes and mast cells of the blood. It acts as a Neuro-transmitter (see), stimulates peristalsis of the intestine, and causes a dose-dependent constriction of smooth muscle. It stimulates the release from arterial endothelium of a dilator substance which counteracts its primary constricting effect [T.M. Cocks X A. Angus Nature 305 (1983) 627-630]. S. is a precursor of the hormone Melatonin (see). It is inactivated and degraded by monoamine oxidases and aldehyde oxidases to 5-hydroxy-indoleacetic acid. 

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