The capacity of rhizoshere bacteria to influence plant hormonal status, by bacterial production or metabolism of hormones, is considered an important mechanism by which they promote plant growth, and productivity. Nevertheless, inoculating these bacteria into the plant rhizosphere may produce beneficial or detrimental results depending on bacterial effects on hormone composition and quantity in planta, and the environmental conditions under which the plants are growing. This review considers some effects of bacterial hormone production or metabolism on root growth and development and shoot physiological processes. We analyze how these changes in root and shoot growth and function help plants adapt to their growth conditions, especially as these change from optimal to stressful. Consistent effects are addressed, along with plant responses to specific environmental stresses: drought, salinity, and soil contamination (with petroleum in particular).

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The phytoremediation of soil contaminated with petroleum oil products relies on co-operation between plants and rhizosphere bacteria, including the plant growth-promoting effect of the bacteria. We studied the capacity of strains of Pseudomonas, selected as oil degraders, to produce plant hormones and promote plant growth. Strains with intermediate auxin production were the most effective in stimulating the seedling growth of seven plant species under normal conditions. Bacterial seed treatment resulted in about a 1.6-fold increase in the weight of barley seedlings, with the increment being much lower in other plant species. The strains P. plecoglossicida 2.4-D and P. hunanensis IB C7, characterized by highly efficient oil degradation (about 70%) and stable intermediate in vitro auxin production in the presence of oil, were selected for further study with barley. These strains increased the seed germination percentage approximately two-fold under 5% oil concentration in the soil, while a positive effect on further seedling growth was significant when the oil concentration was raised to 8%. This resulted in a 1.3–1.7-fold increase in the seedling mass after 7 days of growth, depending on the bacterial strain. Thus, strains of oil-degrading bacteria selected for their intermediate and stable production of auxin were found to be effective ameliorators of plant growth inhibition resulting from petroleum stress.

A bacterial strain IB-4, antagonistic to plant pathogenic fungi of the genera Fusarium, Bipolaris, and Alternaria, was isolated from arable soils of the Mechetlinskii district, Bashkortostan. Physiological, biochemical, and culture morphological properties of strain IB-4 supported its classification within the genus Pseudomonas. In spite of some discrepancies in the results of phenotypic and chemotaxonomic research, analysis of the 16S rRNA gene sequence, DNA-DNA hybridization, GC-content, and MALDI mass spectral data provide considerable evidence supporting its identification as a Pseudomonas koreensis strain. P. koreensis strain IB-4 was shown to possess the valuable features characteristic of PGPR microorganisms: antifungal and nitrogenase activities and ability to synthesize indole-3-acetic acid (IAA) and cytokinin-like compounds. Field test, in which potato plants were treated with the culture liquid of P. koreensis strain IB-4 revealed a positive effect on potato yield and resistance to plant pathogens.

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