Plant growth promoting rhizobacteria (PGPR) are rhizosphere bacteria that have a direct or indirect positive effect on plants. The direct mechanisms of their action include the fixation of atmospheric nitrogen and the increase in the availability of less mobile mineral nutrients such as phosphorus and iron. The mobilization of these nutrients is caused by the release of bacterial ectoenzymes (e.g. phosphatases), organic acid anions, and siderophores. Phytohormones (auxin, cytokinin and gibberellin), formed by PGPRs, directly affect the physiological and developmental processes as well as growth of the plant. The synthesis of 1-amylocyclopropane 1-carboxylate deaminase, however, reduces the synthesis of ethylene, which is synthesized by plants under the influence of stress biotic and abiotic factors. Indirect effects of PGPRs include a reduction in the activity of phytopathogens that are affected by PGPR through antagonistic compounds such as antibiotics and lytic enzymes. The release of PGPR siderophores reduces the availability of nutrients for pathogens, since siderophores are species-specific. By activating induced systemic resistance in plant, the PGPR indirectly affect protection against pathogens in plants. The beneficial effects of PGPR can be exploited in crop production. In practice, the use of preparations containing plant-beneficial microorganisms has proven successful, while the demand for them is growing yearly. They have proven to be suitable for organic production, while in integrated production they offer a good complementary technology to growth control and yields increase. In production of preparations with PGPR, an emphasis should be given to the efficiency, which depends on the choice of the microorganism strain and the formulation of the preparation, while adequate stability should also be ensured.
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