Bioremediation Potential of SPSB2 in Alleviating NaCl, Cadmium, and Nickel Stress in Tomato Plants for Environmental Sustainability

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Salinity and heavy metals (HMs) pollution pose significant challenges to global agriculture, especially in arid and semi-arid regions. HMs mobility increases in saline soil, worsening soil pollution and adversely impacting plant growth, agricultural productivity and essential soil microbial communities. In the current study, we exposed tomato ( Solanum lycopersicum ) plants to individuals and combined stress conditions of 200 mmol·L -1 NaCl, 2 mmol·L -1 Cd, and 2 mmol·L -1 Ni to evaluate their impact on plant growth. Although many bacterial and fungal species have been reported to mitigate these stresses, our objective is to determine whether the selected bacterial strain, Nitratireductor aquimarinus SPSB2, can effectively alleviate the combined stress of salt and HMs. Our findings demonstrate that SPSB2, with its remarkable potential, establishes a symbiotic relationship with tomato plants and significantly influences plant growth, morphology, antioxidant systems, and gene expression patterns, particularly under stress conditions. Results revealed that salinity and HMs stresses, particularly in combination, significantly reduced plant growth parameters, including shoot and root lengths, biomass, and leaf area. However, SPSB2 inoculation mitigated these effects, enhancing shoot length and weight by 13.34% and 7.01% and root length and weight by 6.2% and 10.43%, respectively, under NaCl stress compared to their non-stressed counterparts as an example. SPSB2 also improved water relations, reduced water loss, maintained stomatal size, and alleviated electrolyte leakage. Moreover, SPSB2 inoculation enhanced photosynthetic pigments (chlorophyll-a by up to 8.37% and carotenoids by 104.75%), total protein content (by up to 3.72%), and modulated enzymatic antioxidants, as well as non-enzymatic antioxidants under the triple stress condition of NaCl+Cd+Ni stress. Additionally, SPSB2 inoculation promoted the accumulation of soluble sugars (37.84%) and starch (41.53%), while reducing ion accumulation and translocation and preserving cellular morphology. Notably, SPSB2 inoculation resulted in the downregulation of key stress-responsive genes, such as Solyc04g079730 , Solyc02g085730 and Solyc11g071600 , under stress conditions, indicating a potential regulatory role of SPSB2 in alleviating stress. These findings underscore the promising role of SPSB2 in promoting tomato plant growth, reducing salt and HMs toxicity, and pave the way for further research into the mechanisms underlying plant growth-promoting rhizobacteria-mediated stress alleviation in sustainable agriculture.