Cicer arietinum L. (chickpea) crops often suffer from attack by phytopathogens, which damage the crop and consequently limits crop yield. Fungicides are commonly used to enhance productivity by preventing phytopathogen-related damage (Shahid et al., 2020). However, massive and injudicious use of such chemicals can upset soil fertility and inhibit microbial communities (Walia et al., 2014) and enzymatic activities (Han et al., 2020). Apart from the cytotoxic and genotoxic effect of fungicides on soil microbiota, uptake and translocation of pesticides by different plant organs may severely damage important metabolic activities leading subsequently to death of plants (Eker et al., 2006). Exceptionally high concentrations of pesticides disrupt: (i) cellular organelles and membrane permeability (Shahzad et al., 2018); (ii) respiratory processes and carbohydrate metabolism (Kumar et al., 2012); (iii) physiologically active enzymes (Liu et al., 2006) and proteins (Yin et al., 2016); (iv) photosystems by blocking the effective quantum yield of PSII (ΦPSII) and quantum efficiency of PSII (Fv/Fm) (Niinemets and Kull, 2001); and (v) cause oxidative damage (Singh and Roy, 2017) and genetic makeup of cellular machinery (Gill and Tuteja, 2010). Zablotowicz and Reddy (2007) observed that the pesticide glyphosate considerably decreased nitrogenase activity of rhizobia. As a consequence, symbiotic events leading to nodule formation and root morphogenesis of plants were drastically diminished (Adami et al., 2017). In another study, the fungicide pyrimorph was found to strongly inhibit the electron transport (ET) reactions of chloroplasts and adversely affected the physiology of whole plants (Xiao et al., 2014).
To overcome these problems, certain physico-chemical approaches have been used to destroy pesticides in soil. However, these methods are expensive, and the remediation process often remains incomplete due to the transformation of the parent compound to metabolites which are sometimes more persistent and more toxic for non-target organisms than were the parent compounds. As a consequence, physico-chemical methods of pesticide removal have not been widely accepted. Alternative methods of pesticide degradation/detoxification are therefore necessary. Bioremediation offers some solutions to pesticide detoxification problems. This technique, often referred to as ‘microbial remediation’ relies on the identification of microorganisms to convert contaminants to simpler and harmless forms and hence, to mitigate pesticide pollution.
To this end, scientists have identified pesticide degrading/detoxifying microbes endowed with potential plant growth promoting activities. Chief among them belongs to genera Ensifer (Ma et al., 2017), Bradyrhizobium (Romdhane et al., 2016), Rhizobium (Hang et al., 2019), Alcaligenes (Silambarasan and Abraham, 2013), Actinobacteria (Lang et al., 2018) and Bacillus (Tang et al., 2018). Apart from degradation of toxic pollutants, plant growth promoting rhizobacteria (PGPR) have the ability to synthesize growth regulating substances. By expressing multifarious physiological activities, PGPR promotes the overall growth and yield of legumes (Matse et al., 2020) raised in soil contaminated during cultivation with pesticides. For instance, inoculation of Azotobacter (Gothandapani et al., 2017) and Bacillus (Lastochikna et al., 2017) had positive effects on pulses where they supplied N and growth stimulating substances including phytohormones, siderophores and EPS, and solubilized soil P. Some fungicide-tolerant and N2-fixing bacterial strains such as Rhizobium and Azotobacter and Gram-positive Bacillus sp. detoxified pesticides and enhanced legume production under adverse conditions (Alori et al., 2017). Likewise, pesticide-tolerant free-living PGPR such as Bacillus (Roy et al., 2018), Azotobacter (Gurikar et al., 2016) and Stenotrophomonas (Jaiswal et al., 2019) circumvented the toxicity of pesticides and concurrently improved growth of legumes.
Given the nutritive importance of C. arietinum in the diet, the negative impact of fungicides on crop productivity, the lack of adequate information on fungicidal response to C. arietinum and the bioremediation potential of PGPR, this study was formulated. The objectives were to: (i) assess the fungicidal toxicity to C. arietinum both under in vitro bioassays and pot-house conditions; (ii) assess the kitazin-induced distortion, oxidative damage and cell death in C. arietinum root cells; (iii) isolate and identify the symbiotic bacterium from C. arietinum root nodules; (iv) evaluate fungicidal tolerance by nodule bacteria; (v) determine the production of bioactive molecules under fungicide stress; (vi) evaluate the effects of M. ciceri on physiological and biochemical attributes of C. arietinum; (vii) determine the stressor molecules and antioxidant enzymes from C. arietinum foliage detached from fungicide-treated and bio-inoculated plants; and (viii) evaluate the rhizosphere/rhizoplane colonization potential of M. ciceri.