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Introduction to Nanobiofungicides

Frank Abimbola Ogundolie1* and Michael O. Okpara2

1Department of Biotechnology, Baze University, Abuja, Nigeria

2Department of Biochemistry, Federal University of Technology, Akure, Ondo State, Nigeria

Abstract

Economic losses resulting from the plant pathogenic fungi have raised the concerns all over the world due to the wide loss it causes to yield and quality of agricultural products. This also affects the health of man. Attempts over the years in the management of this plant pathogens through use of synthetic fungicides has been observed to raise some serious health concerns ranging from environmental pollution to health challenges and recently, increased pest resistance been observed worldwide. Technology advancement in the area of plant protection today has resulted in a safer way of managing plant pathogenic fungi through the use of biological organisms to monitor their activities. Today, the use of nanoparticles based biofungicides to prevent plant pathogenic fungi is fast yielding positive and desirable results. In this chapter, we look into some nanoparticle based materials used in biofungicides.

Keywords: Pathogens, nanobiofungicides, fungi, nanoparticles, diseases, mode of action

1.1 Introduction

Plant fungal infections are among the major causes of economic loss in food production by crop plants of economic importance. Plant fungal pathogens are reported to be responsible for the destruction of about one-third of crop produce amounting to USD 60 billion annually [1, 2]. Therefore, inhibiting the growth/survival of plant fungal pathogens will be critical to achieving global food security, safety, and sustainability. However, measures to control the destructive activities of plant fungal pathogens are quite limited; and with the steady rise in world population, food scarcity may become a global challenge in the future. Consequently, tackling the food-destroying impact of plant fungal pathogens will require an innovative approach or method [3, 4].

Nanobiofungicides are biological organisms or products of biological organisms - with a size below 100 nm - that exhibit fungicidal activity against plant pathogenic fungi. These biological organisms can include animals, plants, bacteria, or fungi provided the biological organism or its product can inhibit the growth/survival of pathogenic fungi or their spores. The production and utilization of nanobiofungicides for controlling plant pathogenic fungi is an emerging field that has proved to be more advantageous for food security than the use of artificially synthesized fungicides. Unlike the synthetic fungicides which the world has largely depended on over the years, nanobiofungicides are mostly bacteria and fungi that are ubiquitous in the soil. Moreso, nano-based bio fungicides are more precise in targeting plant pathogenic fungi compared to synthetic fungicides. As described by Kookana et al., [5], nanobiofungicides are generally ecofriendly while being toxic to plant pathogenic fungi.

The mode of action of nanobiofungicides vary from one organism to another and could be any of the following: (1) secretion of chemicals/antibiotics or metabolites or enzymes that are harmful to plant pathogens, (2) outcompeting plant pathogens for available soil nutrients, (3) activation of the plant's immune response against fungal diseases, (4) secretion of fungicidal nanoparticles, or (5) mycoparasitism.

1.2 Mechanism of Action of Nanobiofungicides

Nanobiofungicides, like other biocides, are critical for the control and/or destruction of unwanted fungi and their spores. The mechanism of action of nano bio fungicides varies and depends on factors like the antagonistic association between the fungicide-producing organism and the parasitic fungi, the concentration of nano bio fungicide applied, stimulation of metabolic change in the parasitic microbes and so on. To inhibit the destructive activities of plant fungal pathogens, nanobiofungicides must be able to secrete antibiotics, act as an antagonizing organism to the pathogen, secrete nanoparticles with fungicidal properties, or stimulate disease resistance mechanism in the plant. And with the advances made in nanotechnology so far, the delivery of bio fungicides to the sites of action is even now more precise.

Bacteria species such as Bacillus subtilis, Bacillus amyloliquefaciens, Bacillus velezensis, and Bacillus pumilus [6, 7] and fungi species such as Trichoderma harzianum [8, 9], Trichoderma viride, and Trichoderma virens [10] are some examples of micro-organisms that possess biofungicidal properties and can be applied to plants to control and/or destroy parasitic pathogens. Some other microbes belonging to this class are pathogenic [9, 11].

Trichoderma spp. are among the most well-investigated and most promising bio fungicides for controlling the parasitic activities of plant pathogenic fungi. Trichoderma harzianum, Trichoderma virens and Trichoderma viride are three species of Trichoderma that are widely applied and commercially available bio fungicides [12].

Trichoderma harzianum has been applied as a bio fungicide against plant pathogenic fungi like Rhizoctonia solani in pepper plants [13], cotton, tobacco plants [14], peanuts [15, 16] and rice [17]. It has also been applied against Macrophomina phaseolina in eggplant [18], Fusarium oxysporum in melon plants [19] and Phytophthora erythroseptica in tomato and potato [20]. Trichoderma virens has been shown to possess biofungicidal activity against plant pathogenic fungi like Sclerotium rolfsii [21], Phytophthora erythroseptica [20] and Rhizoctonia solani [21] in tomato plants. Trichoderma viride has been reported to show biofungicidal activity against plant pathogenic fungi like Rhizoctonia solani in tomato plants and Sclerotinia sclerotiorum in cowpea plants [21]. Herein, Trichoderma spp. will be used to describe the general modes of action of nanobiofungicides.

One of the modes of action of bio fungicides like Trichoderma sp. is to grow around the plant's root, possess the rhizosphere and prevent the growth or survival of parasitic fungal pathogens without affecting the activities or survival of the plant's symbionts. In their study, Wojtkowiak-Gebarowska and Pietr [22] demonstrated that Trichoderma harzianum and Trichoderma viride efficiently colonized the root systems of cucumber and inhibited the growth of plant pathogenic fungi Sclerotinia sclerotiorum and Fusarium culmorum [22, 23].

Furthermore, the occupation of the plant's root by Trichoderma spp. stimulates the plant's immune response against pathogens through the production of different defense-related chemicals like alkaloids, phenolics and terpenoids. These chemicals are produced in different locations on the plant and as such, the plant's defense mechanism stimulated by Trichoderma spp. in the root can also have a protective effect on other plant parts. For instance, Trichoderma asperellum has been demonstrated to exhibit a protective effect on tomato plants against Botrytis cinerea [24]. Botrytis cinerea is an airborne necrotrophic fungus found on the leaves of a wide range of economically important oil, fiber, protein, vegetable, and horticultural plants [25-28].

Trichoderma spp. are also very strong parasites against many plant pathogenic fungi and this property are being exploited by agriculturalists in applying Trichoderma spp. as bio fungicides. Upon their application to the soil as bio fungicides, Trichoderma spp. locate and get attached to plant pathogenic fungi. Then they disrupt the parasite's cell wall to gain entry by secreting cell wall degrading enzymes like cellulases, proteases, chitinases, and glucanases [29, 30]. However, the mode of cell wall degradation differs from one Trichoderma sp. to another [31-33]. While inside the plant pathogenic fungi, Trichoderma spp. act as mycoparasites by absorbing the nutrients of the plant pathogenic fungi and eventually causing their death [34].

Trichoderma spp. also secretes volatile and non-volatile compounds that are toxic to parasitic fungi and inhibit fungal mycelia growth. Trichoderma spp. has been reported to produce metabolites and/or antibiotics - such as alamethicins, tricholin, massoilactone, harzianic acid, 6-pentyl-a-pyrone, polyketides, terpenoids, heptelidic acid, and gliovirin among others which are toxic to plant pathogens [35, 36]. However, the kind of toxic compounds produced by Trichoderma spp. differ from one species to another.

Unlike many plant pathogenic fungi, Trichoderma spp. can obtain energy from complex cell wall components of fungi (chitin) and plants (cellulose) through the secretion of chitinase and other cell wall degrading enzymes [31, 32]. Thus, while most plant pathogenic fungi die from starvation, Trichoderma spp. can survive for a longer time after outcompeting the plant pathogenic fungi for available soil nutrients. This trait is also exploited by agriculturists in the application of Trichoderma spp. as a bio...