Auxins precursors

Inhibition of tomato and barley plants growing in soils infested with Centaurea repens (knapweed) was reported by Fletcher and Renney (38). A toxic component was isolated in highest concentration from the foliage of knapweed. The inhibitor was considered to be an indole alkaloid or auxin precursor because of its ultraviolet absorption spectrum and the positive reactions obtained with Salkowski and Ehrlich reagents. The presence of the inhibitor was considered to explain partially the rapid establishment of Centaura spp. in almost pure stands. 

Ms. Pat Hall and Drs. J. Nowacki and E. Epstein have provided our knowledge of the amounts and rate of metabolic turnover of the indolylic components of the kernels of lea mays (cf. 27, 28 Nowacki, unpublished Epstein, unpublished). This knowledge Fas enabled us, 1) to identify the "seed auxin precursor"—that is the compound which is transported from the seed to the growing shoot (39J, and 2) has provided a portion of the proof that IAA-myo-inositol and IAA are in reversible equilibrium in the shoot tissue. Proving that IAA and IAA esters are in reversible equilibrium in the tissue is essential if we wish to postulate hormonal homeostasis. 

Thus, the manner in which auxins were discovered led to confusion as to the site and mechanism of auxin synthesis. Was the hormone synthesized in the tip by some completely de novo route from simple non-aromatic precursors, or was it stored in the tip in an aromatic form such as tryptophan, or a related indolic compound that could readily be converted to lAA Alternatively, was an aromatic precursor transported from the seed to the tip of the seedling as it grew This latter possibility was suggested by Skoog [12], who called the upward transported form the seed auxin precursor , which he believed to be tryptamine. 

Radiolabeled lAA applied to the endosperm of germinating maize seedlings is transported to the shoot [113]. About 1% of the radioactivity in the shoot remained as free lAA, 2% was esterified, and 97% was metabolized to compounds not hydrolyzable to IA A. [ C]-labeled lAInos [131] was used to demonstrate that labeled lAInos applied to the endosperm could also be transported to the shoot and then hydrolyzed to yield free lAA [98,114]. lAInos-galactose, however, did not move out of the kernel and into the shoot until the galactose moiety was removed by hydrolysis. The resultant lAInos was then transported into the shoot [132]. From these results it was concluded that lAInos is a specific seed auxin precursor and that lAInos is the primary form in which LAA is transported from kernel to shoot. 

It has not previously been possible to determine the relationship between the rate of growth and the amount of the growth hormone, lA A. Early experiments involved exodiffusion of coleoptile tips, followed by a bioassay of the amount of lAA in the agar blocks that were used to collect the diffusing growth substance. Such experiments measured the capacity of the tips to supply lA A over a sustained time period and undoubtedly involved conversion of a seed auxin precursor [2, 23]. We now believe that the seed auxin precursor is indole-3-acetyl-wj< -inositol (lAInos), which upon hydrolysis yields free lAA [8,9,15,22]. Also, it is now possible to extract the tissue rapidly with aqueous acetone and to measure the free lAA by GC-MS. 

Nowacki J, Bandurski RS (1980) Myo-inositol esters of indole-3-acetic acid as seed auxin precursors of Zea mays L. Plant Physiol 65 422-427 Ohwaki Y, Tsurumi S, Nagao M (1974) Auxin transport in Vicia roots. In Plant growth substances 1973. Hirokawa, Tokyo, pp 1071-1078 Osborne DJ (1968) A theoretical model for polar auxin transport. In Vardar Y (ed) The transport of plant hormones. North-Holland, Amsterdam, pp 97-107 Osborne DJ (1974) Hormones and the shedding of leaves and bolls. Cotton Grow Rev 51 256-365... 

Phenylalanine, tyrosine, and tryptophan are converted to a variety of important compounds in plants. The rigid polymer lignin, derived from phenylalanine and tyrosine, is second only to cellulose in abundance in plant tissues. The structure of the lignin polymer is complex and not well understood. Tryptophan is also the precursor of the plant growth hormone indole-3-acetate, or auxin (Fig. 22-28a), which has been implicated in the regulation of a wide range of biological processes in plants. 

The shikimate/arogenate pathway leads to the formation of three aromatic amino acids L-phenylalanine, L-tyrosine, and L-tryptophane. This amino acids are precursors of certain homones (auxins) and of several secondary compounds, including phenolics [6,7]. One shikimate/arogenate is thought to be located in chloroplasts in which the aromatic amino acids are produced mainly for protein biosynthesis, whereas the second is probably membrane associated in the cytosol, in which L-phenylalanine is also produced for the formation of the phenylpropanoids [7]. Once L-phenylalanine has been synthesized, the pathway called phenylalanine/hydroxycinnamate begins, this being defined as "general phenylpropanoid metabolism" [7]. 

If the pea cellulases are related as an inactive intracellular precursor (BS) to an active wall-bound enzyme (BI), a delay would be expected between the appearance of BS after auxin induction and its secretion as BI. Such a delay has been observed repeatedly in tests of the relative rates of development of the two enzymes following auxin treatment of decapitated or intact tissue (Figure 3). During the first day after treatment, BS cellulase activity increases up to 18-fold over controls, while BI cellulase activity barely changes. By 2 days, although the BS cellulase activity level has increased further, BI cellulase activity surpasses it. The ascendancy of BI continues for several days, i.e., until effects of the hormone subside and levels of both cellulases begin to decline. Similar... 

Hall et al. (1998) reported that an ALS-resistant biotype of false cleavers was cross-resistant to a broad range of ALS inhibitors, as well as to an auxin-type herbicide, quinclorac, which had never before been applied to these fields. A similar case of quinclorac multiple resistance in smooth crabgrass has been reported in California when plants were previously treated with ACCase herbicides. Data suggest a target site-based mechanism of resistance involving the accumulation of cyanide derived from stimulated ACC synthesis, which is a precursor of ethylene (Abdallah et al., 2004). 

The formation of IAA conjugates is widely believed to be a means for removal of excess IAA produced during certain times of plant development and also in mutant plants where indolic precursors and IAA metabolites accumulate.32 In all higher and many lower plants, applied IAA is rapidly conjugated to form IAA—aspartate.33 The ability of plant tissues to make aspartate conjugates of a variety of active auxins is induced by pretreatment with auxin,34 and this induction was shown to be blocked by inhibitors of RNA and protein synthesis. After almost 50 years of study, an in vitro system from plants was described that accounts for the formation of IAA amide conjugates35 via a mechanism where the acidic auxin is adenylated followed by acyl transfer to the amino acid. The gene for this reaction had been discovered almost 20 years before, when GH3 from soybean was shown... 

Precursor of Indole-3-acetic acid (IAA, auxin) hallucinogen Dimethyltryptamine... 

The saxl (hypersensitive to abscisic acid and auxin) mutant probably has a defect in the oxidation and isomerization of 3p-hydroxy-A precursors to 3-oxo-A steroids (Ephritikhine et ah, 1999). AtHSDl (At5g50600) encodes a protein with homology to animal ll- 3-hydroxysteroid dehydrogenase (Li et ah, 2007) however, no obvious sequence similarities exist with the 3P-HSD involved in cardenolide bios)mthesis (see Section 6.3.1.2). 

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). 

D, 2,4-DP, 2,4,5-T and 2,4,5-TP, applying them alone and simultaneously with the parent acid. Of the 21 acid amides investigated 13 exhibited an antiauxin action, antagonising the auxin activity of the respective phenoxyalcanoic acid. At the same time, the compounds also had their own auxin action, showing on the one hand that amides too have an auxin action, and on the other hand that they are metabolised in plants by the enzymatic pathway to the respective acids and are thus the precursors of the acids. 

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