Antioxidant System Activity in Roots and Shoots of Bean Cultivars in Response to Seed Treatment with Auxin as a Potential Model of Interaction with Endophytic Bacteria

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 (<i>cvs</i>) of <i>Phaseolus vulgaris</i> 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 <i>cv</i> 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 <i>cv</i>. 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 <i>cv</i> response to exogenic IAA in contrast to the shown earlier response to inoculation with endophytes. The growth of only the main root of Elsa <i>cv</i> 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 <i>cv</i> were neutral or negatively responded to all concentration IAA, but the number of roots increased by 2–4 times. For Zolotistaya <i>cv</i>, 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.