Nano-bio priming of ZnO and Sinorhizobium meliloti mitigates salt stress in alfalfa via improved mineral uptake, antioxidant gene expression, and cellular protection

Soil salinity constitutes a significant challenge for global agriculture, affecting over 20 % of irrigated agricultural land output by disturbing plant water relations, ionic balance, and cellular metabolism. Nanoparticle-based growth stimulants provide eco-friendly and innovative solutions to address such issues. This study investigated the effects of various doses of fluorescein isothiocyanate (FITC) -tagged zinc oxide nanoparticles (nZnO) on alfalfa seedlings growing in association with the micro-symbiont partner Sinorhizobium meliloti (Rhz) . Adding nZnO significantly reduced the negative effects of salt and enhanced the morphometric traits that are favoured by bacterial adhesion to root hairs. The effects were demonstrated by increased biomass (root biomass, shoot biomass), gaseous exchange traits, and chlorophyll fluorescence components, as well as greater accumulation of potassium, nitrogen, nitrate and other minerals and a decrease in the contents of sodium, and ammonium in salt-stressed alfalfa plants compared with the control group. Confocal laser microscopy, transmission electron microscopy, and scanning electron microscopy confirmed the presence of FITC-labelled nZnO and Rhz colonization in roots jointly well-established an environment that substantially alleviated salinity-induced damage to leaves, the root epidermal surface, and cellular organelles. Co-application of 100 mg · L −1 nZnO dose and Rhz up-regulated the qRT-PCR expressions of antioxidant enzymes and ion transporters genes that ameliorated mineral uptake, protected against salt-stress-mediated membrane damage and lipid peroxidation, as revealed by the visuals of diaminobenzidine and nitroblue-tetrazolium staining. Overall, integrating Rhz with nZnO was an effective green remediation strategy to enhance alfalfa tolerance to salinity while reducing the risk of transmitting them to environmental compartments. • Nanoparticles and microbe-ameliorated morphometric traits of alfalfa under salinity. • Ultracellular visual showed shielded root epidermal and leaf chloroplast structure. • Histochemical markers showed reduced membrane damage by higher antioxidant, enzymes. • Microscopic visuals showed fine stomatal behavior, and chlorophyll florescence traits.