Plant growth promoting rhizobacteria improve the antioxidant status in Mentha piperita grown under drought stress leading to an enhancement of plant growth and total phenolic content

An experiment was conducted to investigate the effects of plant growth promoting rhizobacteria (PGPR) on Mentha piperita grown under drought stress. We performed root inoculation with strains of two PGPR species (Pseudomonas fluorescens WCS417 r and Bacillus amyloliquefaciens GB03) on peppermint plants and subjected them to moderate (MS) or severe drought stress (SS). To determine the growth-promoting potential and ability of PGPR to increase the drought tolerance in peppermint, different plant growth parameters were measured, along with the activities of antioxidant enzymes such as peroxidase (PX) and superoxide dismutase (SOD), as well as the non-enzymatic antioxidant proline and total phenolic content (TPC). In addition, to determine whether drought stress induces oxidative damage in peppermint, membrane lipid peroxidation was analyzed. An increment in the level of drought stress produced a reduction in plant growth, fresh weight, leaf number and leaf area. However, these negative effects of drought were mitigated in plants exposed to PGPR inoculation, resulting in significantly less reduction in the above growth traits related to plants not treated with PGPR, regardless of the severity of the drought treatment. In addition, drought-stressed plants treated with PGPR had a significantly higher total phenolic content than water-stressed plants without PGPR. Higher enzymatic activities were also observed in drought-stressed plants inoculated with PGPR. The proline content did not change in stressed plants, but inoculation reduced the amount of proline in the different stressed conditions tested. Membrane lipid peroxidation was also decreased in inoculated plants grown under drought conditions. These results are important as they illustrate the potential of PGPR to mitigate the adverse consequences of drought stress, and offer a way of increasing the tolerance of peppermint plants grown and TPC under water deficit conditions.