Viral diseases and their damage causing significant loss to economically important crops have increased by several folds during the last decade. All the conventional approaches are not able to eradicate the viral infection. Therefore, there is a need to look for efficient and eco-friendly viral disease-preventive measures. The genomic material of the majority of deleterious viruses of higher plants is RNA. One of the possible measures to control viruses is the use of ribonucleases (RNases), which can cleave RNA in the viral genome. Based on this, we investigated the RNase activity of endophytic
Bacillus
spp., which can enrich in 10
3
–10
5
colony-forming units per gram of wet mass of aboveground part of potato plants. A high level of RNase activity was observed in the culture medium of
Bacillus thuringiensis
B-6066,
Bacillus
sp. STL-7,
Bacillus
sp. TS2, and
Bacillus subtilis
26D.
B. thuringiensis
B-5351 had low RNase activity but high ability to colonize internal plant tissues,
Bacillus
sp. STL-7 with high RNase activity have relatively low number of cells in internal tissues of plants.
B. thuringiensis
B-6066,
B. subtilis
26D, and
Bacillus
sp. TS stimulate RNase activity in potato plants for a long time after application. Strains with high ability to colonize internal plant tissues combined with high RNase activity reduced severity of viral diseases symptoms on plants and reduced the incidence of potato viruses M, S, and Y. It is worth noting that
Bacillus
spp. under investigation reduced the number of
Leptinotarsa decemlineata
Say. egg clusters and larvae on treated plants and showed antifeedant activity. This results in increase of potato productivity mainly in the fraction of major tubers.
B. subtilis
26D and
Bacillus
sp. TS2 combining endophytic lifestyle, RNase, and antifeedant activity may become the basis for the development of biocontrol agents for plant protection.
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.
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