Antioxidant status of medicinal and aromatic plants under the influence of growth-promoting rhizobacteria and osmotic stress

Environmental stress represents a major limitation for increasing crop productivity and the use of natural resources. Most plants are exposed to various types of stress to different degrees and have an inherent ability to adapt to seasonal variations, but when they are subjected to drought and salinity stress, a series of morphological, physiological, biochemical and molecular changes occur. Moreover, both drought and salinity result in osmotic stress, which inhibits growth and causes disturbances at metabolic level. An effect of osmotic stress is the production of high levels of reactive oxygen species (ROS). At low concentrations, ROS are essential participants in cell signaling, but an excess generation of ROS results in toxicity, damaging macromolecules leading to cell death. To avoid the deleterious effects of ROS and adjust this imbalance, plants have evolved antioxidant systems that can be classified as enzymatic and non-enzymatic, and together, these preserve homeostasis in all cell compartments. In addition, oxidative stress can be measured indirectly following the formation of oxidative byproducts of lipids, proteins, or nucleic acids, with malondialdehyde (MDA) being one of the most widely used markers. It has been observed that the negative effects on plant development caused by water stress can be mitigated by the use of PGPR (Plant Growth Promoting Rhizobacteria) microorganisms, which is an alternative technology for improving the capacity of tolerance to abiotic stress in plants. In the present review, by considering enzymatic and non-enzymatic responses, we elaborate on the role of PGPR in helping medicinal and aromatic plants to cope with osmotic stress through antioxidant defenses. This review paper also emphasizes a future research requirement involving investigating the combined utilization of osmotic stress and PGPR in order to enhance the content of secondary metabolites. In addition, this present review examines the antioxidant responses in MAPs subjected to osmotic stress and inoculated with PGPR, which have not been extensively reviewed before.