Kibdelosporangium sp. and chitosan nanoparticles differentially mitigated mercury stress in two Phaseolus vulgaris L. cultivars: Improved photosynthetic efficiency, primary metabolism and redox homeostasis

Mercury contamination poses a significant risk to worldwide crop yields, negatively impacting their development and metabolic processes. Effective mitigation strategies can enhance plant resilience under such stress conditions. This study investigates the combined mitigation potential of Kibdelosporangium sp. and chitosan nanoparticles (CSNPs) in mitigating Hg toxicity in two Phaseolus vulgaris L. cultivars: the sensitive 'Nebraska' and the tolerant 'Giza 6'. Hg exposure reduced biomass and impaired photosynthesis, with fresh weight and photosynthetic rates declining by up to 57% and 65%, respectively, in Nebraska cultivar. However, the combined application of Kibdelosporangium sp. and CSNPs alleviated these stress effects, increasing fresh weight up to 84.7% and reducing Hg accumulation from 86.58 to 48.23 μg g -1 DW in Nebraska. The combined application restored photosynthesis and increasing the bioavailability of primary sugars. These served as essential carbon skeletons and energy precursors for the biosynthesis of organic, amino, and fatty acids. Consequently, the combined treatment reduced H 2 O 2 accumulation and lipid peroxidation by approximately 38–45% in Nebraska compared with the Hg-only treatment. Antioxidant defences (e.g., CAT, DHAR, SOD, TAC) were increased by 34–111% in Nebraska and 52–160% in Giza 6. under Hg stress, and combined treatments further increase this defence response. Hg significantly increased total soluble sugars in Nebraska (85%) and Giza 6 (100%), while proline rose by 33% in Nebraska. However, combined treatment mitigated these effects, normalizing sugar levels and balancing carbon allocation, especially in Giza 6. By stabilizing antioxidant systems and metabolic balance, the intervention effectively maintained high tocopherol levels. Particularly, the sensitive Nebraska cultivar showed greater recovery, pointing out higher responsiveness. This study demonstrates an innovative approach to enhancing legume resilience through microbial and nano-mitigants that optimize photosynthesis, metabolic accumulation, and redox homeostasis in heavy-metal-polluted environments.