Auxin synthesis

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. 

PCIB can therefore be either due to, a) a disruption of endogenous auxin transport, b) an inhibition of auxin synthesis and or accumulation, and c) competition for auxin-binding sites. Based on data available at present, these alternatives cannot be distinguished. 

Finally, it must be mentioned that a large number of alkaloids in plants contain the indole structure. Most of these are of much higher molecular weight than the substances mentioned here, and only one, gramine (XIX), has been proven to be metabolically linked to well known indoles. But theoretical considerations ensure that connections do exist. Many of these indole alkaloids have potent effects on animal tissues, but virtually nothing is known of their physiological role, if any, in the plant. Potentially, they could be produced from simpler indoles and thus serve to divert materials from auxin synthesis, but this hardly seems an adequate explanation for molecules of such complexity. Their structures and chemistry are amply monographed elsewhere . 

Davies FS, Mau SC, Nooden LD (1975) Auxin synthesis in crown gall tumor tissue A comparison of three putative precursors. Physiol Plant 33 39-41 Davison RM, Young H (1973) Abscisic acid content of xylem sap. Planta 109 95-98 Dostal R (1967) On integration in plants. Harvard Univ Press, Cambridge Mass Esau K (1965) Plant anatomy, 2nd edn. Wiley and Sons, New York Evans LS, Tramontano WA (1981) Is trigonelline a plant hormone Am J Bot 68 1282-1289... 

Andreae WA, Venis MA, Jursic E, Dumas T (1968) Does ethylene mediate root growth inhibition by indole-3-acetic acid Plant Physiol 43 1375-1379 Anker L (1973) The auxin production of the physiological tip of the Avena coleoptile and the repression of tip regeneration by indoleacetic acid (not by naphthylacetic acid and 2,4-dichlorophenoxyacetic acid). Acta Bot Neerl 22 221-227 Anker L (1975) Auxin-synthesis inhibition by abscisic acid, and its reversal by gibberellic acid. Acta Bot Neerl 24 339-347... 

Aflatoxins, Aspergillus flavus and A. parasiticus metabolites, can be carcinogenic, mutagenic, teratogenic and toxic. Zinc is required for auxin synthesis and therefore, both plant growth and development. Cultivars (cvs). of Zea mays seedlings vary in aflatoxin (AFBi)... 

It has been evidenced that auxin and ACC stimulate ethylene production in some lower plants, for example, in the moss Funaria hygrometrica and in the ferns Pteridium aquilinum and Matteuccia struthiopteris Moreover, ethylene synthesis via the ACC-dependent pathway and in the presence of endogenous ACC and its conjugate in marine unicellular Acetabularia algae was reported. Probably during the evolution of land plants, a relatively primitive pathway of ethylene production was replaced by the ACC-dependent synthesis pathway that now predominates. 

Orlova I, Marshall-Colon A, Schnepp J, Wood B, Varbanova M, Fridman E, Blakeslee JJ, Peer WA, Murphy AS, Rhodes D, Pichersky E, Dudareva N (2006) Reduction of benzoid synthesis in petunia flowers reveals multiple pathways to benzoic acid and enhancement in auxin transport. Plant Cell 18 3458-3475... 

The induction of PAL activity by the two growth factors can be separated in time so that they may act at different sites within the cell to bring about the response (40). Auxin added at the time of subculture of the tissue changes the pattern of protein synthesis of the cells by changing the transcription pattern of the mRNA after two hours (43). Kinetin does not have this effect (44). 

Phenolic compounds naturally occurring in plants have induced many physiological responses that duplicate those reported for ozone and/or peroxyacetylnitrate (PAN). Chlorogenic acid is a competitive inhibitor of lAA-oxidase (35) and plant growth is adversely affected by increased concentrations of auxins (36). Concentrations of chlorogenic acid are increased in tobacco tissue exposed to ozone ( ) Phenols inhibit ATP synthesis (37), oxidative phosphorylation ( ) and SH enzyme activity (27) they increase respiration (38), reduce CO2 fixation (22), modify both membrane permeability (40) and oxidation rate of reduced NADH... 

Further studies are required to fully elucidate the role of flavonoids in auxin regulation in vivo to determine, for example, whether changes in the synthesis or deposition of specific flavonoids within the cell act to change the rate or direction of auxin transport. There is the question of how such different organs or developmental outcomes as nodules, lateral roots,... 

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