Other Indolic Plant Growth Hormones

An auxin in apple endosperm (565) not identical with indoleacetic acid was identified as ethyl indoleacetate (863, cf. 697), but this may be an artifact due to esterification during isolation (377). 

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). 

All these indolic plant hormones or hormone precursors are likely to arise from tryptophan. Other reactions of trytophan, for example, in alkaloid l)iogenesis, are discussed in the next section. 

---

Tryptamine (1) appears to be a precursor of the plant growth hormone lAA (indole acetic acid) (2) (Fig. 28.4). This compound also is produced in the metathoracic glands of the leafcutter ant, Atta sexdens, and is a factor in the ant s culture of fungi on fresh leaf material. Gall-forming wasps and a number of other insects also synthesize lAA (Schild-knecht, 1976). 

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). 

Tryptophan is metabolized by several different pathways (Fig. 1) each yielding biologically important substances such as tryptamine and in particular serotonin (5-hydroxytryptamine), which seems to be involved in certain mental disorders. Indole-3-acetic acid is a plant growth hormone its precursor is tryptamine or indole-3-pyruvic acid. In humans, the microorganisms of the large intestine can further degrade indole-3-acetic acid to yield indole, skatole (3-methyl-indole) and other substances. 

Auxins are one of the five major classes of plant-produced hormones that affect plant growth including bud formation and root initiation (see http //en.wikipedia. org/wiki/Plant hormone Auxins), lndole-3-acetic acid is the most common auxin found in plants. Although the small amounts produced internally have the desired effects, auxins are toxic to plants in larger amounts. The nefarious weed-control products 2,4-D and 2,4,5-T target the auxin receptor but bear little resemblance to the natural ligand. Other man-made auxins such as 1-naphthaleneacetic acid and indole-3-butyric acid are used, not to kill weeds, but to stimulate root production in cuttings taken from the parent plant (Scheme 6). 

Hundreds of applications have been mentioned in the Zweig (1968) review acids, alkaloids, amino acids, antibiotics, antioxidants, food and feed additives, bases and amines, bile acids, carbonyls, dyes, enzymes, lipids, hydrocarbons, hormones, indoles, natural products, peptides, proteins, pesticides, plant growth regulators, pharmaceutical products, phenols, pigments (chlorophylls, xanthophylls, porphyrins, melanin, pterins, pteridines, anthocyanins, ilavonoids, etc.), polymers, purine and pyrimidine derivatives, quinones, RNA, DNA, organic sulfur compounds, steroids, sugars, toxins, vitamins, inorganic ions, and others. 

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. 

---