INTRODUCTION
The rapid economic development unintentionally added a challenge for mankind to manage and clean up the environmental pollution piling up as a severe threat to environmental sustainability. One of the major environmental contaminants are the heavy metals including arsenic (As), cadmium (Cd), Lead (Pb), Nickel (Ni), Chromium (Cr), Cobalt (Co), Copper (Cu) and Zinc (Zn). Among this As in the last few decades has been found to be frequently discharged in groundwater possibly due to geogenic activity in Bay of Bengal. Studies suggests that a very minute quantity of As can cause serious health issues in human, animal and even in plants system. Arsenic is a group 1 carcinogen and thus chronic exposure may lead to neurodegenerative diseases, type 2 diabetes mellitus and even cancer (Tam et al. 2020). World Health Organization (WHO) has set 10µg L-1 as the permissible limit of As in potable water and up to 20 mg Kg-1 in agricultural soils. Recently Shaji et al. (2021) concluded that >100 countries are affected with arsenic contamination, among which Bangladesh and India from Asia are highly exposed to this metal (Chakrobarty et al. 2018; Bhowmick et al. 2018). Surprisingly, in India around 20 states along with four union territories are listed as arsenic affected area. A recent comprehensive study stated the threat of As toxicity to around 94 to 220 million population of which almost 94% is only from Asia (Podgorski and Berg, 2020). The methods of management of arsenic pollution in environment are only limited. The physico-chemical procedures that included soil washing with nitric acid, sulfuric acid or adsorption by specific media, coagulation, precipitation, immobilization etc. are expensive and generate secondary pollutants. Among the biological methods, the phytoremediation was used either by free floating plants or rooted plants (Yan et al. 2020). Microorganisms with their inherent capabilities to tolerate heavy metals are the alternative simple, viable, cost effective and sustainable remediation tools for these contaminants. Biosorption by the bacterial cells is well known and simplest mechanisms in which heavy metals are adsorbed on the cell surface through different functional moieties, and accumulated at the cell membrane (Giri et al. 2013; Titah et al. 2018). This kind of interaction has been recognized through Fourier Transform Infra-Red Spectroscopy (FTIR) in various studies where amide, halide, nitro and hydroxyl chemical functional groups play an active role in metal sorption (Tian et al. 2012; Prasad et al. 2013; Singh et al. 2013; Altowayti et al. 2022). Molecular mechanisms pertaining to As tolerance have been considered as arsenate reduction (ars ), arsenate respiratory reductase (arr ), arsenite extrusion genes (acr ), arsenite oxidation (aio ) and methylation (Renu et al. 2020). Studies indicated that the bacteria followed above-mentioned strategies to combat arsenic toxicity. Dey et al. (2016) reported arsenite oxidation in Bacillus sp. and Aneurinibacillus aneurinilyticus isolated from As contaminated region in Burdwan district of West Bengal. Similarly, arsenic extraction efficiency of lower plants has also been increased by the inoculation of arsenic tolerant bacteria (Lampis et al. 2015). On the other hand, the importance of arsenate reductase gene (arsC ) has been reported in majority of bacteria possessing arsenic tolerance (Sun et al. 2004; Suhadolnik et al. 2017). In addition to this, arsenic removal in few studies suggested mediation of arsenite extrusion genes (acr ) (Gu et al. 2019) and arsenic respiratory reductase (Mirza et al. 2017). It has been established that the redox cycling of arsenic by microbes plays a crucial role in arsenic ion mobility in the environment (Stolz, 2006).
In the present study, we have reported As-tolerant bacterial strainBacillus mycoides NR5 isolated from presumptive As contaminated site for its bioremediation potential and evaluated PGP ability in spinach. The study focusses on mitigation of As toxicity in spinach plants in presence of the selected bacterial strain. The study reports in-depth mechanism of As tolerance in the bacterium and its role in alleviation of As toxicity in spinach.