Agricultural microbiology and symbiogenetics: synthesis of classical ideas and construction of highly productive agrocenoses (review)

Agricultural microbiology (AM) is presented as a discipline addressing the prokaryotic and eukaryotic microorganisms that influence operation of the major components of agrocenosis - plants, animals and soils. Development of AM is based on the synthesis of ideas and methods of microbiology, plant physiology, soil science and genetics. This synthesis is aimed to study the organization and evolution of biosystems in which symbiotic microorganisms perform adaptively important functions in cooperation with each other and with host organisms. Upon migration from environment into the endosymbiotic niches of plants and animals, microorganisms form with them multicomponent complexes - holobionts (E. Rosenberg, I. Zilber-Rosenberg, 2018). They possess own systems of heredity, symbiogenomes and hologenomes, which have become the subjects of a new discipline, symbiogenetics (I.A. Tikhonovich, N.A. Provorov, 2012). Microorganisms forming symbioses with plants perform the important adaptive functions - nutritional (N2 fixation, absorption of soil nutrients, firstly phosphates), defensive (biocontrol of pathogens and phytophagans) and regulatory (synthesis of phytohormones that optimize plant development and improve their resistance to adverse environment) (I.A. Tikhonovich, N.A. Provorov, 2009). The broadly studied and practically important plant symbionts include: a) nodule bacteria or rhizobia ( Rhizobiales ) - N2-fixing symbionts of legumes; b) arbuscular mycorrhizal fungi ( Glomeromycota ) - phosphate-mobilizing symbionts of a wide range (more than 80 % species) of plants (A. Berruti et al., 2016); c) rhizospheric and endophytic bacteria (e.g., Azospirillum , Bacillus , Pseudomonas ) which stimulate the development of plants and determine their resistance to antagonists (pathogens, pests) and stresses (drought, salinity of soils, their contamination with xenobiotics or heavy metals) (M.A. Hassani et al., 2018). In animals, trophic symbionts determine the assimilation of plant biomass (intestinal or rumen microbiota), synthesis of essential amino acids and cofactors (intestinal and intracellular symbionts), and N2 fixation (symbionts of some herbivorous animals) (E. Rinninella et al., 2019). The study of microbial effects on plants and animals makes it possible to create microbial preparations that improve the nutrition of hosts, their resistance to biotic and abiotic stresses, and increase the soil fertility. In crop production, preparations of N2-fixing and growth-stimulating bacteria are widely used, which ensure a drastic reduction in application of environmentally hazardous nitrogen and phosphorus fertilizers. Preparations of microorganisms that are antagonists of phytopathogens - Pseudomonas , Bacillus (B.J. Lugtenberg et al., 2001; V.K. Chebotar et al., 2009), rodents - Salmonella enteritidis , Serratia plymuthica (A. Soenens, J. Imperial, 2019) or phytophagous insects - Bacillus thuringiensis, Beauveria bassiana (A.V. McGuire, T.D. Northfield, 2020) are used broadly for their biocontrol to significantly reduce the pesticide load on agrocenoses. By studying the integrative functions of agronomically valuable microorganisms, AM invests a significant contribution to the fundamental biological research, including the genetic and molecular interactions of prokaryotes and eukaryotes, evolution of cell and of its genome, and formation of supraorganismal genetic systems (I.A. Tikhonovich, N.A. Provorov, 2012). Based on these studies, methods of symbiotic engineering are being developed aimed at constructing the highly productive biosystems, including the cereal and vegetable cultivars capable of symbiosis with rhizobia, as well as N2-fixing plants.