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Modern concepts of auxin’s action. 1. History of discovery, metabolism, transport

Auxin (indolyl-3-acetic acid, IAA) is one of the key classical phytohormones with a very wide range of physiological effects. The first part of the scientific lecture describes the main stages of discovery of the hormone. The main pathways of auxin synthesis in plant tissues, which is carried out in two different ways: tryptophan-dependent and tryptophan-independent, are considered in detail. At the same time, multiple pathways of the auxin formation from tryptophan have been found in plant tissues. Among them, the mechanisms that occur with the formation of such intermediate metabolites as indole-3-acetaldoxime, indole-3-pyruvate and indole-3-acetamide are considered. The indole-3-pyruvate pathway is currently considered the main mechanism of hormone synthesis. Experimental evidence has also been obtained for the functioning of the tryptophan-independent pathway of auxin synthesis, the key enzyme of which is cytoplasmic indole synthase. It is assumed that the precursor of auxin in the tryptophan-independent pathway may be some intermediate metabolite between anthranilic acid and tryptophan. The article also describes the routes of auxin inactivation through the formation of conjugated forms and oxidation. A brief characterization of IAA dioxygenases, belonging to the 2-oxoglutarate-Fe (II)-oxygenases family, which are currently considered the main catalytic systems for auxin oxidation, is presented. The mechanisms and significance of polar and lateral transport of auxin are discussed. The characteristics of transmembrane auxin transporters belonging to the families PIN/PIL, ABCB/PGP and AUX/LAX are given.