Effect of Endophytic Bacteria Bacillus subtilis 26D and Bacillus velezensis M66 on Resistance of Potato Plants to the Causative Agent of Early Blight Alternaria solani

The effect of Bacillus velezensis M66 and Bacillus subtilis 26D bacteria on the resistance of potato plants to the necrotrophic fungus Alternaria solani (the causative agent of early blight) was studied. The accumulation of viable bacterial cells of these strains in the internal tissues of the potato stem, roots, and tubers over a long period of time was demonstrated for the first time. A significant reduction in the area affected by the early blight on leaves inoculated with plant endophytes, as well as inhibition of the pathogen growth under the action of bacterial strains, was detected, which can be explained by the synthesis of lipopeptide antibiotics (the genes responsible for the synthesis of which were found by the PCR method) and proteolytic enzymes (the activity of which was demonstrated in vitro). The formation of plant resistance under the influence of inoculation with B. subtilis 26D and B. velezensis M66 was accompanied by an accumulation of hydrogen peroxide in the first hours after plant infection with A. solani spores and by a decrease in this index at the late stages of pathogenesis due to an increase in the activity of catalase and peroxidases. Limitation of the fungus spread was accompanied by an increase in the activity of proteinase inhibitors in plants, which probably decreased the negative effect of proteolytic enzymes of the necrotrophic pathogen A. solani. It can be assumed that the inoculation of plants with bacterial cells of the B. velezensis M66 strain contributed to the formation of resistance of potato plants to the early blight by efficient priming of the phytoimmune potential, which is comparable to the B. subtilis 26D strain (successfully used in the field conditions), an active component of the Fitosporin-M biopreparation.