Beneficial microbes in plant salt stress tolerance: mechanisms, antioxidant defense, and multi-omics insights

Salinity stress represents a significant environmental challenge that affects plant growth, resulting in adverse impacts on crop productivity and overall yield. Salt accumulation in plants affects cellular homeostasis and provokes osmotic and ionic imbalance. It stimulates higher reactive oxygen species (ROS) production leading to oxidative stress damage and eventual cellular dysfunction. Beneficial microorganisms like plant growth-promoting rhizobacteria, arbuscular mycorrhizal fungi, endophytic fungi, and rhizosphere-associated microbes have recently emerged as resources for sustainable solutions to enhance plant resiliency against salinity stress. Their effects can involve osmotic adjustment, ion sequestration, oxidative-stress mitigation, phytohormone regulation, and rhizosphere-mediated stress buffering. The mechanisms to impart such benefits may include improved nutrients acquisition, assist in osmotic regulation, modulation of phytohormones signaling, and maintenance of cellular ion homeostasis. Often, beneficial microorganisms carry the potential to trigger both enzymatic and non-enzymatic antioxidant defense mechanisms. These responses contribute to oxidative-stress mitigation, yet, their interpretation requires contemplation of fine balance between oxidative damage and plant responses. For example, selected microbial inoculation studies have reported changes in the activity of crucial antioxidant enzymes, like superoxide dismutase, ascorbate peroxidase, catalase, and glutathione reductase. However, the magnitude and the direction of these responses are context dependent. Multi-omics approaches, including amplicon and shotgun metagenomics, metatranscriptomics, host transcriptomics, proteomics, and metabolomics, have expanded our understanding of the taxa, genes, transcripts, proteins, and metabolites associated with plant–microbe interactions under salinity. Therefore, here an effort has been made to present the evidential support for reported microbial effects and their proposed mechanisms of salt tolerance, with a focus on enzymatic and non-enzymatic defense mechanisms and the learnings of multi-omics studies. Frontier integrative mechanisms, including microbiome engineering, synthetic microbial consortia, and systems biology frameworks, are discussed for cultivating climate-resilient crops and sustainable agricultural systems in the salt-affected environments.