Mechanistic insights of bioremediation potential of Bacillus altitudinis and its impact on rice grown under contaminated soil.

Chromium (Cr) and Cadmium (Cd) contamination pose significant threats to the environment, plant and human health. Among green remediation strategies, plant growth-promoting rhizobacteria (PGPR)-mediated bioremediation offers an eco-friendly alternative to conventional chemical and physical methods. Though the bioremediation potential of Bacillus species is well known, but this study explores the mechanistic basis of bioremediation potential of the PGPR strain Bacillus altitudinis (IHBT-705) against Cr and Cd toxicity. Results showed that the strain tolerated 30 mM Cr, and10 mM Cd and retained key plant growth-promoting traits. Time-course experiments over 12-72 h showed maximal bioaccumulation efficiencies of 96% for 15 mM Cr, and 95% for 5 mM Cd within 48 h. Transmission electron microscopy revealed Cr and Cd accumulation as electron-dense granules within the cytoplasm. Expression studies confirmed the role of heavy metal transporters and siderophores in the uptake of Cr and Cd. The bioremediation efficiency of IHBT-705 was further illustrated through a pot experiment using rice seedlings. IHBT-705 inoculation reduced heavy metal accumulation and improved growth, with shoot length, root length and total root number increased by 57%, 45% and 76%, respectively, under Cr stress, and by 86%, 65%, and 57% under Cd stress. Importantly, IHBT-705 also enhanced chlorophyll content, antioxidant enzyme activities, and nutrient content- including potassium, iron, magnesium, manganese, calcium and zinc under both Cr and Cd stress. Overall, this study rationally explores the heavy metal bioremediation potential of IHBT-705 and substantiates its functional role through integrated molecular analyses, highlighting it as a promising candidate for the sustainable remediation of Cr and Cd contaminated agroecosystems.