Metabolic interaction at the level of extracellular amino acids between plant growth-promoting rhizobacteria and plants of alfalfa (Medicago sativa L.)

We investigated the relationship between plant growth promoting rhizobacteria (PGPR) and plants at the level of metabolites in the rhizosphere. The study objects were rhizobacteria Bacillus sp. C-21N2 and root exudates of alfalfa (Medicago sativa L.). The strain C-21N2 showed high plant growth promoting (PGP) activity towards alfalfa. During field tests, it was found that the inoculation of seeds with bacteria increases germination by 30%. Moreover, the sowing density increases by 2.1 times and the yield of green mass by 25.2 c/ha compared with control. This study showed that in the rhizhosphere between rhizobacteria and plants there is an active interaction at the level of the exchange of extracellular amino acids. To prove the exchange of extracellular amino acids between PGPR and roots in the rhizosphere, alfalfa roots were immersed in the culture fluid of bacteria. After submerging the roots, of the six amino acids produced by the bacteria, only two remained (asparagine and leucine), the rest were assimilated by the roots. At the same time, three amino acids (valine, alanine and glutamic acid) secreted by the roots of alfalfa were found in the culture liquid of bacteria. That is, the plant roots absorbed the amino acids excreted by the bacteria, and bacteria adsorbed the amino acids excreted by the alfalfa roots. Our findings provide direct experimental evidence that in the rhizosphere between rhizobacteria Bacillus sp. C-21N2 and the alfalfa plants have a metabolic interaction at the level of extracellular amino acids. Also we concluded that the selected strain C-21N2 is a useful bioinoculant, and extracellular amino acids are one of the metabolites that stimulate the growth of alfalfa. The C-21N2 strain should be used in practice in agriculture to increase the productivity of alfalfa.