<i>Bacillus mojavensis</i>, a Metal-Tolerant Plant Growth-Promoting Bacterium, Improves Growth, Photosynthetic Attributes, Gas Exchange Parameters, and Alkalo-Polyphenol Contents in Silver Nanoparticle (Ag-NP)-Treated <i>Withania somnifera</i> L. (Ashwagandha)

Abstract

Discharge of nanoparticles (NPs) into aquatic and terrestrial ecosystems during manufacturing processes and from various commercial goods has become a significant ecotoxicological concern. After reaching soil systems, NPs cause deleterious effects on soil fertility, microbial activity, and crop productivity. Taking into consideration the medicinal importance of <i>Withania somnifera</i> (L.) (ashwagandha), the present study assessed the potential hazards of silver nanoparticles (Ag-NPs) and the toxicity amelioration by a metal-tolerant plant growth-promoting rhizobacterium (PGPR). <i>Bacillus mojavensis</i> BZ-13 (NCBI accession number MZ950923) recovered from metal-polluted rhizosphere soil, tolerated an exceptionally high level of Ag-NPs. The growth-regulating substances synthesized by <i>B. mojavensis</i> were increased with increasing concentrations (0-1000 μg mL^-1) of Ag-NPs. Also, strain BZ-13 had the ability to form biofilm, produce alginate and exopolysaccharides (EPSs), as well maintain swimming and swarming motilities in the presence of Ag-NPs. Soil application of varying concentrations of Ag-NPs resulted in a dose-related reduction in growth and biochemical features of ashwagandha. In contrast, following soil inoculation, <i>B. mojavensis</i> relieved the Ag-NPs-induced phytotoxicity and improved plant productivity. Root, shoot length, dry biomass, and leaf area increased by 13, 17, 37, 25%, respectively, when <i>B. mojavensis</i> was applied with 25 mg/kg Ag-NPs when compared to noninoculated controls. Furthermore, the soil plant analysis development (SPAD) index, photosystem efficiency (Fv/Fm), PS II quantum yield (FPS II), photochemical quenching (qP), non-photochemical quenching (NpQ), and total chlorophyll and carotenoid content of BZ-13-inoculated plants in the presence of 25 mg Ag-NPs/kg increased by 33, 29, 41, 47, 35, 26, and 25%, respectively, when compared to noninoculated controls that were exposed to the same amounts of NPs. In addition, a significant (<i>p</i> ≤ 0.05) increase in 48, 18, 21, and 19% in withaferin-A (alkaloids), flavonoids, phenols, and tannin content, respectively, was recorded when plants were detached from bacterized and Ag-NP-treated plants. Leaf gas exchange parameters were also modulated in the case of inoculated plants. Furthermore, bacterial inoculation significantly decreased proline, lipid peroxidation, antioxidant enzymes, and Ag-NP's absorption and build-up in phyto-organs. In conclusion, soil inoculation with <i>B. mojavensis</i> may possibly be used as an alternative to protect <i>W. somnifera</i> plants in soil contaminated with nanoparticles. Therefore, phytohormone and other biomolecule-synthesizing and NP-tolerant PGPR strains like <i>B. mojavensis</i> might serve as an agronomically significant and cost-effective remediation agent for augmenting the yield and productivity of medicinally important plants like ashwagandha raised in soil contaminated with nanoparticles in general and Ag-NPs in particular.