ACYL-HOMOSERINE LACTONES FOR CROP PRODUCTION AND STRESS TOLERANCE OF AGRICULTURAL PLANTS (review)
Acyl homoserine lactones (AHL) are a class of mediator molecules coordinating cell activity in the gram-negative bacteria population. AHLs synchronize individual genomes due to which bacterial populations function as a multicellular organism. AHLs provide a remote signaling between bacteria colonizing the phytosphere that enables the bacterial population to respond to external influences and establish symbiotic or antagonistic relationships with the host plant (A.R. Stacy et al., 2018; A. Shrestha et al., 2020). Autoreception of quantitative parameters of the bacterial population is called "quorum sensing" (QS) (R.G. Abisado et al., 2018). QS systems form autoinducer signaling molecules that easily penetrate from cells into the environment and back into the cell (M.B. Miller et al., 2001; B. Bassler, 2002). QS systems play a key role in the regulation of metabolic and physiological processes in a bacterial cell (M. Frederix et al., 2011; M. Whiteley et al., 2017). Bacterial signaling is perceived by eukaryotes, which form a symbiosis with microbial communities (A. Schenk et al., 2015; L.M. Babenko et al., 2016, 2017). Plant growth and development, nutrients assimilation, and stress resistance are largely determined by the pattern of this interaction (H.P. Bais et al., 2006; R. Ortíz-Castro et al., 2009; S. Basu et al., 2017). In the plant, bacterial signaling is controlled by the quorum quenching (QQ) system (N. Calatrava-Morales et al., 2018), whose mechanism of action is to suppress AHL synthesis by plant metabolites, compete with AHL for binding to receptor proteins, and repression of QS-controlled genes (H. Zhu et al., 2008; R. Sarkar et al., 2015). However, to date, the molecular mechanisms by which plants respond to bacterial signaling are not fully understood. Individual metabolites of AHL signaling have been characterized, but their role in the chemical interaction of partners in most cases requires further study. It has been shown that the QS phenomenon and its participants are involved in the regulation of prokaryotic-eukaryotic interactions, including the formation of biofilms, the synthesis of phytohormones, the transfer of plasmids, the production of virulence factors, bioluminescence, sporulation, and the formation of nodules (L.M. Babenko et al., 2017). Differences in the structure of molecules ensure that bacteria recognize their own AHL and separate foreign ones. The transfer of AHL from a bacterium to a host plant is carried out by means of membrane vesicles (M. Toyofuku, 2019). In recent years, there has been an active study of genetics, genomics, biochemistry, and signaling diversity of QS molecules. The regulation of the functions of the rhizosphere, the most dynamic site of interaction between the plant and the associated microflora with the participation of AHL, is of particular importance in the development of new biotechnological approaches aimed at increasing the yield and stress resistance of agricultural crops. One of the effective technologies for increasing resistance to biotic and abiotic stresses is pre-sowing treatment (priming) of seeds (A. Shrestha et al., 2020). Both direct (on plants) and indirect (on rhizosphere microflora) effects of AHL priming was established (O.V. Moshynets et al., 2019). AHL induce an increase of growth, of photosynthetic pigments content, as well as cause changes in the ratio of phytohormones in organs and tissues, affect the formation of defense mechanisms, which increases the productivity of agricultural crops (A. Schikora, S.T. Schenk, 2016; A. Shrestha et al., 2020). AHL meet the requirements of intensive organic farming, they are considered as promising ecological phytostimulants and phytomodulators capable of safely increasing the quantity and quality of agricultural products.