N-ethylmaleimide (NemA) reductase mediated hexavalent chromium transformation by Plant growth promoting Bacillus tropicus V1 facilitates post stimulatory response in tomato (Solanum lycopersicum) cultivated in chromium contaminated soil

In this study, a Cr ^VI -tolerant Bacillus tropicus V1, was isolated from soil and evaluated for its plant growth promoting and bioremediation potential. The strain produced IAA (46.91 ± 5.69 µg/ml), gibberellic acid (32.50 ± 5.72 µg/ml), ammonia (2.29 ± 0.46 µg/ml) and solubilized phosphate (42.74 ± 5.23 µg/ml), along with ACC deaminase activity (0.693 ± 0.06 µmol/min/mg protein). B. tropicus V1 exhibited a Cr ^VI reduction efficiency of 63.81 ± 0.75%. The NemA gene showed 1.8-fold upregulation under Cr ^VI stress, indicating an inducible detoxification response. Molecular docking revealed strong binding affinity of the NemA:FMN complex (-6.1 kcal/mol), stabilized by FMNH ₂ , Cr ^VI , and key active site residues. This study highlights a flavin-dependent redox mechanism where NemA catalyses the enzymatic reduction of Cr ^VI to the less toxic Cr ^III via electron transfer. Field trials on Solanum lycopersicum under Cr ^VI stress revealed significant growth enhancement in B. tropicus -induced plants: suggesting shoot width increased by 31%, shoot length by 22% and root length by 33% relative to uninoculated controls. Cr ^VI reduced chlorophyll content by 15% whereas B. tropicus treatment elevated it by 47% over control and 23% under stress. Carotenoids declined 1.51-fold with co-treatment. Antioxidant enzymes (APX, POD, and SOD) were upregulated in treated plants. Total Cr accumulation in controls followed an apoplastic barrier pattern, with highest accumulation in roots (12.79 ppm), followed by shoots (4.33 ppm) and leaves (1.30 ppm). In contrast, B. tropicus treatment significantly reduced Cr levels: roots (9.76 ppm), shoots (2.74 ppm) and leaves (0.841 ppm), established its potential for Cr ^VI bioremediation and phyto-protection.