Ways of Auxin Biosynthesis in Microorganisms

Abstract

Among plant hormones, auxins, in particular indole-3-acetic acid (IAA), are the most studied and researched. Almost all groups of soil microorganisms, both plant-associated and non-plant-associated bacteria, fungi, and phytopathogenic microorganisms are capable of producing auxins. The development of preparations for crop production is directly related to the production of bacterial strains with high auxin-synthesizing potential, which is possible only with a full understanding of the ways of regulation and synthesis of auxins in bacteria. The synthesis of auxins in microorganisms can take place in two ways: by the gradual conversion of tryptophan to IAA (tryptophan-dependent pathway) or by the use of other intermediates (tryptophan-independent pathway). The latter is poorly clarified, and in the literature available today, there is only a small amount of information on the functioning of this pathway in microorganisms. The review presents literature data on the ways of auxin biosynthesis in different groups of microorganisms, as well as approaches to the intensification of indole-3-acetic acid synthesis. The formation of IAA from tryptophan can be carried out in the following ways: through indole-3-pyruvate, through indole-3-acetamide, and through indole-3-acetonitrile. The vast majority of available publications are related to the assimilation of tryptophan through the formation of indole-3-pyruvate as this pathway is the most common among microorganisms. Thus, it functions in rhizospheric, symbiotic, endophytic, and free-living bacteria. The concentration of synthesized IAA among natural strains is in the range from 260 to 1130 μg/mL. Microorganisms in which the indole-3-acetamide pathway functions are characterized by lower auxin-synthesizing ability compared to those that assimilate tryptophan through indole-3-pyruvate. These include bacteria of the genera Streptomyces, Pseudomonas, and Bradyrhizobium and fungi of the genus Fusarium. The level of synthesis of IAA in such microorganisms is from 1.17×10−4 to 255.6 μg/mL. To date, only two strains that assimilate tryptophan via the indole-3-acetonitrile pathway and form up to 31.5 μg/mL IAA have been described in the available literature. To intensify the synthesis of indole-3-acetic acid, researchers use two main approaches: the first consists in introducing into the culture medium of exogenous precursors of biosynthesis (usually tryptophan, less often indole-3-pyruvate, indole-3-acetamide, and indole-3-acetonitrile); the second — in increasing the expression of the corresponding genes and creating recomindolebinant strains-supersynthetics of IAA. The largest number of publications is devoted to increasing the synthesis of IAA in the presence of biosynthesis precursors. Depending on the type of bacteria, the composition of the nutrient medium, and the amount of exogenously introduced precursor, the synthesis of the final product was increased by 1.2—27 times compared to that before the intensifi cation. Thus, in the presence of 11 g/L tryptophan, Enterobacter sp. DMKU-RP206 synthesized 5.56 g/L, while in a medium without the precursor, it yielded only 0.45 g/L IAA. Recombinant strains Corynebacterium glutamicum ATCC 13032 and Escherichia coli MG165 formed 7.1 and 7.3 g/L IAA, respectively, when tryptophan (10 g/L) was added to the culture medium. The level of auxin synthesis in microorganisms may be increased under stress conditions (temperature, pH, biotic and abiotic stress factors), but in this case, the IAA concentration does not exceed 100 mg/L, and therefore this method of intensification cannot compete with the others above.