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Nanomaterials 2020, 10, 1146 14 of 24 stress, chromosomal abnormalities decreased growth, deviation in transcriptional profile of several genes, raising susceptibility to natural toxins (e.g., Arsenic) [170]. 7. Toxicity of Metallic Nanoparticles on Beneficial Microbes Associated with Plants Microbes are epiphytically and endophytically associated with plants, in rhizosphere soil, and bulk soil near the plant root system, and may promote plant growth through the production of phytohormone (e.g., auxin) and siderophore, nitrogen fixation, and phosphate solubilization [187]. NPs applied to plants and soils may be toxic to these beneficial microbes in the same manner as they are to the plant pathogens. The effects on soil microbial community can be assessed by the observation of respiration and enzymatic activities in soils [188]. For example, CuONPs and TiO2NPs reduced biomass, total phospholipid fatty acids and activities of enzymes, such as urease, phosphatase and dehydrogenase, of soil microbiota in a flooded rice field. The principle component analysis of phospholipid fatty acids and diversity indices indicated that CuONPs affected the microbial composition and diversity [189]. Similarly, CeO2, Fe3O4, TiO2 and ZnONPs had effects on bacterial community and activities of enzymes (catalase, invertase, phosphatase, and urease) in saline alkali and black soils [190]. ZnONPs and CeO2NPs reduced thermogenic metabolism, numbers of Azotobacter, phosphate solubilizing and potassium solubilizing bacteria, and enzyme activities in soil; TiO2NPs reduced the enzymatic activities and abundance of soil bacteria [191]. NPs can affect plant-associated and soil microbes beneficial to plants at the time for phytopathogen control. Chavan and Nadanathangam evaluated the effects of AgNPs and ZnONPs on nitrogen fixers (Rhizobium leguminosarum MTCC 10096, Sinorhizobium meliloti clsxc_S_SNF, Azotobacter chroococcum clsxc_A_FLNF), phosphate solubilizers (Arthrobacter sp. MTCC 8160, Bacillus sp. clsxc_ NPS, Serratia marcescens MTCC 7642, Pantoea dispersaclsxc_PSD) and biofilm formers (Bacillus subtilis MTCC 441, Bacillus sp. clsxc_TYA, Pseudomonas aeruginosa MTCC 7763, Klebsiellapneumonia clsxc_AZ2). AgNPs showed bactericidal effects at lower concentrations of 2–22 μg/mL whereas ZnONPs showed a bacteriostatic effects up to concentrations of 3000 μg/mL in some cases. Moreover, AgNPs significantly changed the bacterial community in soil [192]. Another study assessed the toxicity of ZnONPs and CeO2NPs against Sinorhizobium meliloti, a symbiotic alfalfa inhabiting bacterium using advanced microscopic and spectroscopic techniques. ZnONPs were more toxic than CeO2NPs according to the viable cell counts and inhibition of growth dynamics [193]. Microbial community analysis using culture-dependent and independent methods showed that ZnONPs altered the microbial community in soil [194]. 8. Conclusions and Perspectives Recent studies show that the biogenic or green-synthesized metallic NPs using microbes and plants without hazardous chemicals are promising in control plant pathogens including bacteria, fungi, oomycetes, and viruses. On the other hand, the biogenic metallic NPs have toxic potentials to plants and plant-associated beneficial microbes and ultimately to human beings. Therefore, the positive and negative effects of these NPs on agriculture and environments should be ascertained before their commercial use in plant disease management in the field. To achieve the green control of plant diseases with the green-synthesized NPs, we should pay attention to the following aspects in the near future. (1) AgNPs is the major metallic NPs tested on plant pathogens and shows toxicities to plants and plant-associated microbes. Less phytotoxic metals, such as Zn, Fe, Mg, and Mn, should be tested. (2) Minimum inhibitory concentration of metallic NPs on pathogens should be determined in vitro and in vivo. (3) Metallic NP effects on plant growth and development at the working concentration should be determined. (4) Metallic NPs at the working concentration in vivo should be less toxic to other plant-associated microbes than the target pathogens; Metallic NP effects on plant microbiota should be determined. (5) The dynamic of the metallic NPs and their effects on soil microbiota should be determined. Few studies on plant disease control in fields byPDF Image | Plant and Microbe-Based Synthesis of Metallic Nanoparticles
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