According to four field experiments, after the inoculation of Phaseolus vulgaris L. cultivar Ufimskaya with the commercial strain Bacillus subtilis 26D and the promising strain B. subtilis 10–4, it was found that inoculation with B. subtilis 10–4 improved seed productivity (SP) by 31–41% per plant, but only in dry years. In contrast, all 4 years of inoculation with B. subtilis 26D were ineffective or neutral. It was intended to determine the growing and biochemical characteristics of inoculated 7-day-old plants, which correlate with the field SP of bacterial preparations. The SP of inoculated plants (average of 4 years) correlated with root length (0.83), MDA content (−0.98), and catalase (CAT) activity in roots (−0.96) of week-old seedlings. High correlation coefficients between the H₂O₂ content in the roots and SP (0.89 and 0.77), as well as between the H₂O₂ content in shoots and SP (0.98 and 0.56), were observed only in two dry years, when the influence of bacteria was detected. These physiological indicators were identified as potential markers for predicting the effectiveness of the endophytic symbiosis between bean plants and B. subtilis strains. The findings may be used to develop effective microbial-based, eco-friendly technologies for bean production.

The effectiveness of crop inoculation using growth-stimulating bacteria can depend on the plant variety and environmental conditions. The reactions of three-week-old plants of the Ufimskaya and Zolotistaya green bean varieties to inoculation with strains 26D and 10-4 in normal conditions and under stress (1% NaCl 48 h) were analysed. Although the Zolotistaya variety formed less biomass and leaf area, it surpassed the Ufimskaya variety in terms of chlorophyll and root water content, as well as reacting to stress by reducing the content of photosynthetic pigments and root water content. Inoculation contributed to the preservation of these indicators at the level of non–stressed plants: the positive effect of strain 26D was expressed in the effect on the composition of pigments, while that of strain 10-4 was expressed in the hydration of roots. Under stress, the Ufinskaya variety improved the water status of the roots, on which inoculation had a weak or negative effect; however, the decrease in the level of synthetic pigments under stress was compensated by inoculation with both strains. According to the biomass of three-week-old plants, the Zolotistaya variety was shown to respond positively to inoculation with both strains both normally and under stress, while the Ufimskaya variety tended to react negatively to inoculation with strain 26D; under stress, the effect of inoculation with both strains was positive. Compared with the stress-induced control, the malondialdehyde content in the roots of inoculated plants of both varieties decreased in comparison with that of the uninoculated control both in normal conditions and under stress. The revealed differences in plant reactions to inoculation serve as a basis for further analysis of the effectiveness of variety-strain combinations of symbiotic partners.

Plant growth-promoting endophytic bacteria (PGPEB), producing auxins, are offered for a promising eco-friendly crop production. Precise bacterial strain selection is essential to ensure consistent and effective plant growth and resilience. Creating a model for the optimal dose-dependent interactions between PGPEB and hosts is necessary for understanding the mechanisms of high-precision selection of the inoculant composition to enhance bacterial preparations’ efficacy. This study investigated the impact of pre-sowing treatment with exogenous auxin indole-3-acetic acid (IAA) at various concentrations (0, 10, 1, 0.1, 0.01, 0.001, 0.0001, 0.00001 mg L⁻¹) on the growth and antioxidant responses of three cultivars (cvs) of Phaseolus vulgaris L. (bean): Ufimskaya, Elsa, and Zolotistaya. The findings showed dose-dependent and cultivar-specific responses of 7-day-old bean seedlings to exogenous IAA. Ufimskaya cv exhibited significant increases in shoot, main root, and total root lengths at 0.001 mg L⁻¹ IAA, while higher and lower concentrations inhibited growth. The reduced catalase (CAT) activity in roots and the elevated CAT activity in shoots correlated with shoot length and total root length of Ufimskaya cv. Importantly, the growth parameters exhibited weak or no correlations with malondialdehyde (MDA) and H₂O₂ content in roots and shoots, which is a peculiarity of the Ufimskaya cv response to exogenic IAA in contrast to the shown earlier response to inoculation with endophytes. The growth of only the main root of Elsa cv peaked at 0.1 mg L⁻¹ IAA, and there were neutral or inhibitory effects with other concentrations. The positive correlation between CAT activity in shoots and the main root length and total root length as well as positive correlation between MDA content in roots and the total root length of Elsa cultivar were revealed. The shoot length and total root length of Zolotistaya cv were neutral or negatively responded to all concentration IAA, but the number of roots increased by 2–4 times. For Zolotistaya cv, positive correlations were observed between CAT activity in roots and the length of the main root and the total root length. Overall, these cultivar-specific antioxidant responses to exogenous IAA may help create models for optimal dose-dependent interactions between auxin-producing PGPEB and plants, enhancing the effectiveness of microbial preparations for consistent bean growth promotion.

For successful application of bacterial preparations, it is important to determine the stimulating and inhibiting doses of the exogenous phytohormone affecting the growth of the host plant. Bean seeds were treated with different doses of 6-benzylaminopurine (BAP) 0.00001–10 mg/l. For the Ufimskaya variety, the stimulating doses were 0.01–0.1 mg/l, for the Elza variety – only 0.001 mg/l BAP, for the Zolotistaya variety – 0.001 and 0.1 mg/l BAP. In this case, the Ufimskaya and Zolotistaya varieties increased the development of adventitious roots, while the Elza variety increased the total root length. Individual varietal reactions of the antioxidant system in response to seed treatment with cytokinin were determined. The identified characteristics of the varieties highlight the importance of understanding the complex balance between phytohormone signaling and antioxidant defense mechanisms in plant-microbe interactions and necessitate the use of precision doses of growth regulators and cytokinin-producing bacteria to specific varieties to stimulate growth.