MODE OF ACTION OF BIO CONTROL AGENTS
ANTIBIOSIS
Antibiosis can be defined as an antagonism interceded by definite or non- defmite metabolites of microbial origin, by lytic agents, enzymes, volatile compounds, or other toxic substances (Shanna et al., 2013). It plays an imperative role in biological control, and it displays a condition where the metabolites are discharged by belowground components of plants, soil microorganisms, plant residues, etc. (Shanna et al., 2013). It occurs when the pathogen is inhibited or killed by metabolic products of the antagonists (Junaid et al., 2013). The products include lytic agents, enzymes, volatile compounds, and other toxic substances (Shanna et al., 2013). Bio agents are known to produce three types of antibiotics viz., nonpolar/volatile, polar/ non-volatile, and v'ater-soluble (Junaid et al., 2013). Among all of these, the volatile antibiotics are extra efficient as they can act at the sites away from the site of production (Junaid et al., 2013).
HYPERPARASITISM/MYCOPARASITISM
Hyper-parasitism or mycoparasitism can be defined as the measured and the most direct outw'ard appearance of “antagonism” (Pal et al., 2006).
This phenomenon happens when the opponent attacks the “pathogens” by secreting various types of enzymes (Pal et al., 2006). It is the phenomenon of one fungus being parasitic on another fungus and takes place beneath the control of biocatalysts (Hannan, 2000). It has also been accounted that the attachment of “chitinase” and “p-1, 3 glucanase” in the Trichoderma arbitrate biological control (Hannan, 2000).
COMPETITION
Competition is an important process involved in the mechanism of “biocontrol” operation. From the microbial viewpoint, soil and plants are recurrently “nutrient” inadequate environs. Therefore, to settle on the “phytosphere,” microbes should successfully contend for the offered “nutrients” (Pal et al., 2006). Microorganism contends for “breathing space,” raw materials, and organic “nutrients” to propagate and endures in their innate environment. Bio-control agents struggle for the infrequent but crucial “micronutrients,” such as Fe and Mn principally in extremely in v’ell-aerated soil environs (Pal et al., 2006). Competition for micronutrients exists because bio-control agents are more capable of utilizing uptake systems for extracting vital constituents than the “pathogens” (Nelson, 1990). This practice of contest is considered to be an oblique dealing involving the “pathogen” and the “biocontrol agent” (Pal et al., 2006) whereby the “pathogens” are expelled by the diminution of food and place (Loritoet al., 1994). Interaction has been recommended to play a role in the “bio-control” of Fusarium and Pythium by some strains of fluorescent pseudomonas (Nelson, 1990).
PRODUCTION OF LYTIC ENZYMES
Production of “lytic enzymes” is an eco-friendly approach to manacle the proliferation of harmful pests. Lysis is the incomplete or entire obliteration of a cell by enzymes. Many microorganisms produce “lytic enzymes” that can “hydrolyze” an extensive range of “polymeric compounds” (including cliitin, proteins, cellulose, hemicellulose, and DNA). Appearance and release of enzymes by diverse “microbes” can seldom consequence in the repression of plant “pathogen” actions (Clay et al., 2009; Muller et al., 2010). Many rhizobacteria produce HCN that will help in bio-control of “plant-pathogen” and escalating the produce. Capricious compounds such as NH3 produced by “Enterobacter cloacae” can result in the inhibition of Pythium ultimum-induced damping-off of cotton (Howell et al., 1988).
INDUCED SYSTEMIC RESISTANCE (ISR)
ISR is another potent mode of action as far as biocontrol agents are concerned. Advantageous rhizospheric microbes can perk up plant wellbeing by leading the complete plant to enhance the defense in opposition to different pathogens and herbivore pests by the process of ISR (Pieterse et al., 2014). The induced resistance is a generic term (Kuc, 1968) for the induced status of resistance in plants triggered by biological or chemical inducers, which protects non exposed plant parts against future attack by pathogenic microbes and herbivorous insects (Kuc, 1968). ISR is a considerable method by which particular “plant growth-promoting bacteria” and fungi in the plant root environment lead the enhanced protection in opposition to a broad assortment of “pathogens” and “herbivore insects” (Mewis et al., 2006; Kim et al., 2008). An extensive diversity of PGPR and PGPF (including Pseudomonas, Bacillus, Trichoderma, and mycorrhiza species) incite resistance in plants against herbivores and plant disease (Campos et al., 2014; Pieterse et al., 2014). Generally, ISR bestows an improved stage of safety against a wide continuum of attackers (Walters et al., 2013). Furthermore, it is synchronized by a complex of integr ated signaling routes in which “phytohonnones” play a crucial role (Pieterse et al., 2012; Mejia et al., 2014).
