Psychrophilic endophytic pseudomonas as potential agents in biocontrol of phytopathogenic and putrefactive microorganisms during potato storage

Abundant pathogens which attack seed and food potato tubers are a serious problem of the modern potato growing that causes significant losses during storage. Regular use of chemical fungicides and agrochemicals has led to emergence of resistance and an increased aggressiveness of plant pathogenic microorganisms. New fungal and bacterial strains and races are also appearing which spread rapidly and cause great damage to agricultural production. In this regard, the biologicals based on biocontrol microorganisms, instead of chemicals, are considered particularly relevant to protect seed and food potatoes from infectious diseases. A genus Pseudomonas belonging to the group of plant-growth promoting rhizobacteria (PGPR) includes species most effectively colonizing higher plants and used as active agents of biological products. Scientific novelty of our work lies in the fact that this study is the first to report the effect of psychrophilic strain Pseudomonas spp. RF13H on the storage of potato tubers. The cold resistant commercial varieties recommended for cultivation in the Leningrad region has been involved. For the first time we estimated the efficiency of tubers protection from a number of pathogenic microorganisms under refrigerated storage, assessed the population dynamics of introduced strain, and visualized the pattern of bacteria distribution and localization on the surface, using fluorescent in situ hybridization and confocal laser scanning microscopy. Using molecular genetic analysis, we have clarified the taxonomic position of several Pseudomonas strains, including psychrophilic strain Pseudomonas spp. RF13H. This strain possessed fungicidal and bactericidal activity against saprogenic and pathogenic microorganisms and was agronomically and physiologically tested. Its growth at low temperatures and production of auxin-like phytohormones were studied. Pseudomonas spp. RF13H influenced positively preservation and biochemical processes in potato tubers under refrigerated storage. Its microcolonies were often localized in different cracks, grooves and recesses on the stored tuber surface that indicated beneficial plant-microbial relations and explained a strategy for tuber colonization at room and low temperature during storage. The occurrence of surface phytopathogenic microorganisms decreased in the presence of Pseudomonas spp. RF13H, and the average number of all type-infected tubers was about 50 % lower among those treated with Pseudomonas spp. RF13H comparing to untreated ones. This trend continued for 3 month storage, i.e. the amount of infected untreated tubers reached 30 % and reduced up to 10-13 %, when treatment with biocontrol strain. At a temperature of 4 °С, the bacterial counts was quite stable for 5 months, and then significantly decreased (up to trace quantities) to the end of storage. The bacteria concentrated at the boundaries between the individual cells of tuber periderm and in the places of exudation of biochemical substances that serves as nutrients for bacteria. Introduction of Pseudomonas spp. RF13H was shown to contribute to redox processes (activation of antioxidant protection system enzymes - catalase, peroxidase, superoxide dismutase) thus increasing plant resistance to pathogens. Additionally, this strain has a positive effect on the microbiome composition in potato epidermis. Induction of own immunity and creation of a barrier preventing pathogen penetration helped to reduce infection in the potatoes tuber.