Antioxidants, Vol. 11, Pages 2323: Mycosynthesis of Silica Nanoparticles Using Aspergillus niger: Control of Alternaria solani Causing Early Blight Disease, Induction of Innate Immunity and Reducing of Oxidative Stress in Eggplant

The risk of plant diseases is exacerbated by the increase in the severity of climate fluctuations, which increases the risks of plant production. The eggplant is one of the most important Egyptian vegetables that is exposed to many biological stresses such as fungal diseases, nematodes and bacteria [1,2]. Soil-born fungi including Fusarium and Alternaria caused highly noticeable destructive effects on plants in organic and conventional agriculture [3,4,5]. Early blight fungal disease in eggplant that caused by Alternaria solani is one of the most important vegetative diseases, which results in a weakness in the quantity and quality of production [6]. As the disease causes the leaves to fall and dry, the fruits become exposed to direct sunlight and the infection of the fruits leads to rotting, as well as to their unsuitability for marketing and storage [7,8]. Symptoms appear on adult leaves in the form of circular or polygonal spots defined with clear edges and within which are characteristic typical circles [9]. All plants possess a defense system capable of resisting pathogens. The plant’s synthetic or chemical immunity can be induced by bio stimulants [10,11,12]. Mineral nutrients such as silicon or its derivatives are considered one of the most important factors for inducing plant resistance to their direct entry in strengthening the internal structures of plant tissues [13,14]. Silica is important for plant health in general, and the use of silica can be applied at any stage of the plant life cycle [15]. Silica helps in strengthening plants by establishment cell walls from the inside and these results in stronger branches that are resistant to breakage during carry flowers and fruits [16]. Silica helps plants reduce plant shock during pathogen exposure and incomplete infection stages [17]. Silica induces plant immunity against pathogens and insects, with regular use silica accumulating in plant cell walls, thus making it difficult for the pathogen to penetrate the plant cells [18]. It also controls the rate of transpiration in plants. As the plant increases silicon levels, it increases nutrient uptake and distribution, and increases the concentration of chlorophyll and carboxylase in leaves, thus the sick plant recovers from disease [19]. Silicon also improves the efficiency of photosynthesis, and also increases the endurance of leaves and the xylem, the high rates of transpiration caused by the disease, by increasing the deposition of silicon other silicon-containing substances in the cell walls of roots, leaves, twigs, and the main stem [20]. In crops, after absorbing silicon, the hardness of the cell walls improved, the cortex layer increased, thus improving the defense capacity against insects, fungi, especially blight, rot, and bacterial diseases and reducing the incidence of red and white spiders [21,22,23]. The fabrication of nanomaterials is one of nanotechnology’s most challenging and rapidly expanding sectors [24]. Nanoparticle synthesis can be accomplished through physical, chemical, or biological methods [25,26,27,28,29]. The employment of biological mass, such as bacteria, fungus, yeast, plant extract or biomass, and algal extract or biomass, is an alternative to conventional methods for the ecofriendly, healthier, faster, and less expensive synthesis of nanoparticles [30,31,32,33]. Fungi also are able to solubilize silica by producing organic acids and different complexing agents which then helps in the release of soluble silica. (i) Production of Exopolysaccharides, Hydrogen Sulphide and Siderophores. Siderophores which are chelators with a small molecular weight having a high affinity for divalent and trivalent metals. Siderophores are also thought to have a role in silicate solubilization in acidic media [34,35]. (ii) Enzyme-based mechanism. Role of enzymes in silica extraction largely lacks experimental proof. However, there are few studies which indicate that enzymes are involved in this process. Piela and co-workers proposed enzyme-based mechanism for fungus-mediated bioleaching of silica nanoparticles. Under stressed circumstances, when fungal cells come in contact with the substrate, they produce specific proteins and enzymes which interact with the silica structure of biomass leading to the formation of an enzyme-silicic acid complex. This complex is further degraded by the hydrolytic enzymes secreted by fungal cells, leading to the release of siliceous groups present in the substrate in form of silica nanoparticles [36]. Generally, filamentous fungi have played a vital role in the industrial production of biological products and in the fermentation industry due to their ability to secrete proteins and enzymes, high growth rates, ease of handling in large-scale production, and low-cost requirements for production in comparison to other microorganisms [37]. The output of the filamentous fungi cultivation is a high-quality biomass (high protein and fat levels) that can be used as an alternative added to the main diet instead of more expensive sources such as soybean and fish [38]. SiO2-NPs have a variety of effects on plants and are excellent in reducing agricultural pests. Silica nanoparticles have additional uses as nanopesticides, nanoherbicides, and nanofertilizers. They might also be used in plants to transport molecules like proteins and nucleotides [39]. SiO2-NPs are gaining a lot of interest in the agriculture sector due to their enormous surface area and small size. Herein, this study aimed to biosynthesize silica nanoparticles using Aspergillus niger through an ecofriendly method. Furthermore, to apply SiO2-NPs for biocontrol of Alternaria solani causing early blight disease.

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