Endophytic plant-growth-promoting microorganisms can protect plants against pathogens, but they have rarely been investigated as potential biocontrol agents and triggers of induced systemic resistance (ISR), regulated by phytohormones, against viruses. We studied the role of endophytic strains Bacillus subtilis 26D and B. subtilis Ttl2, which secrete ribonucleases and phytohormones, in the induction of tomato plant resistance against potato virus X and potato virus Y in a greenhouse condition. The endophytes reduced the accumulation of viruses in plants, increased the activity of plant ribonucleases and recovered the fruit yield of infected tomato plants. Both the 26D and Ttl2 strains induced ISR by activating the transcription of genes related to salicylate- and jasmonate-dependent responses. The 26D and Ttl2 strains increased the content of cytokinins and decreased the level of indolacetic acid in plants infected with PVX or PVY. PVY led to an increase of the abscisic acid (ABA) content in tomato plants, and PVX had the opposite effect. Both strains reduced the ABA content in plants infected with PVY and induced ABA accumulation in plants infected with PVX, which led to an increase in the resistance of plants. This is the first report of the protection of tomato plants against viral diseases by foliar application of endophytes.

The use of biocontrol agents based on endophytic bacteria against phloem-feeding insects is limited by a lack of knowledge and understanding of the mechanism of action of the endophyte community that makes up the plant microbiome. In this work, the mechanisms of the additive action of endophytic strains B. subtilis 26D and B. subtilis 11VM on the resistance of bread spring wheat against greenbug aphid Schizaphis graminum, was studied. It was shown that B. subtilis 26D secreted lipopeptide surfactin and phytohormones cytokinins, and B. subtilis 11VM produced iturin and auxins into the cultivation medium. Both strains and their lipopeptide-rich fractions showed direct aphicidal activity against greenbug aphid. For the first time, it was shown that B. subtilis 26D and B. subtilis 11VM in the same manner, as well as their lipopeptide-rich fractions, activated the expression of salicylate- and ethylene-dependent PR genes, and influenced plant redox metabolism, which led to an increase in plant endurance against aphids. The composition of endophytic strains B. subtilis 26D + B. subtilis 11VM had an additive effect on plant resistance to aphids due to an increase in the number of endophytic bacterial cells, and, as well as due to the synergistic effect of their mixture of lipopeptides − surfactin + iturin, both on the aphid mortality and on the expression of PR1 and PR3 genes. All these factors can be the reason for the observed increase in the growth of plants affected by aphids under the influence of B. subtilis 26D and B. subtilis 11VM, individually and in composition. The study demonstrates the possibility of creating in the future an artificial composition to enhance plant microbiome with endophytic bacteria, which combines growth-promoting and plant immunity stimulating properties against phloem-feeding insects. This direction is one of the most promising approaches to green pesticide discovery in the future.

The role of salicylic acid (SA) in regulating wheat calli growth and peroxidase activity in the co-culture with bunt (Tilletia caries (D.C.) Tul.) and smut (Ustilago tritici Pers.) pathogens was studied in this work. We found that the influence of SA increased the number of globular high-density sites with meristema-like cells and reduced the number of low-structured sites with parenchyma-like cells in wheat calli. SA inhibited fungi spores germination, mycelium growth, and formation of its new spores during the co-cultivation of wheat calli with bunt or smut agent. Interaction of fungi mycelium with wheat calli treated by SA led to formation of zones with hypersensitive reaction in calli high-structured sites with meristema-like cells. The obtained data about the influence of SA on activation of wheat calli growth and the peroxidases with pI ~3.5 and ~9.8, which can bind to the mycelium of T. caries or U. tritici are discussed in this paper.

The effects of salicylic acid (SA) and jasmonic acid (JA) on plant defense responses were studied with aseptic potato (Solanum tuberosum L.) plantlets infected with Phytophthora infestans (Mont.) de Bary. Plant treatment with 10⁻⁶ M SA or 10⁻⁷ M JA induced plant resistance; the mixture of these acids was most efficient. After treatment with these compounds, phenolic compounds were accumulated and peroxidase was activated in the sites of pathogen localization, and this might be the reason of resistance enhancement. In addition, more H₂O₂ was accumulated in infected plants treated with JA or its mixture with SA but not in plants treated with SA alone. It might occur because of observed inhibition of catalase and activation of isoperoxidase with the isoelectric point (pI) of ∼9.3, which manifests an affinity for the pathogen cell wall. The data obtained allow us to recommend the application of these compounds for potato plant protection against late blight.

The effect of chitooligosaccharides (COSs) with a molecular weight of 5–10 kDa and a degree of acetylation (DA) of 65 and 13% at a concentration of 1.0 mg/L on the expression of the TC151917 gene, which encodes wheat anionic peroxidase, and the activity of “anionic” isoperoxidases in common wheat plants infected with Septoria nodorum Berk.—the agent of Septoria leaf blotch were studied. Treatment with COSs with a 65% DA and infection promoted the transcription of the anionic peroxidase gene and increased the enzymatic activity of the anionic peroxidase with an isoelectric point (pI) of 3.5 in soluble and ion-bound to cell walls protein fractions. Chitooligosaccharides with a 13% DA change these parameters to a lesser extent. These data suggest the importance of the degree of acetylation of COSs in the development of wheat defense response with the peroxidases.

The chitin-binding ability of isoperoxidases isolated from 23 plants of different species was studied. The activation of peroxidases in a protein extract in the presence of this polysaccharide was...