Biological control agents (BCAs) reduce disease of the target crop usually by one or more of the following mode of action: 1. Aantagonism 2. Hypovirulence 3. Induction of Host Resistance.
Mode # 1. Antagonism:
In biological control, antagonism is the central dogma which occurs routinely in nature. Antagonism is a “type of symbiosis (living together of two unlike organisms) in which one organism is harmed by the other either by the latter being parasitic or predatory on former, or through competition for food in short supply, or through secretion of certain toxic substances.”
In this way, as the definition itself explains, the antagonistic action can be broadly divided into three categories:
(i) Direct “parasitism” or “predation” of other organisms over pathogenic ones (exploitation),
(ii) Active demand of nutrient over supply, a situation which results primarily from quicker and greater utilization of available nutrients by saprophytic microorganisms with the result that pathogens face lysis or suppression due to starvation (competition), and
(iii) Suppression of pathogenic organisms due to secretion of toxic or inhibitory compounds by other microorganisms (antibiosis).
Antagonism has become an important part of the control measures of many diseases and, in recent years, plant pathologists have been trying to take advantage of it, and have been developing strategies by which biological control can now be used effectively against several plant diseases.
i. Exploitation (Parasitism/Predation):
Exploitation is an antagonistic condition wherein an organism directly harms another organism to get benefit out of the harm done to the organism. This phenomenon is operated through parasitism and predation; the two terms basically being same in their effect but differing in their mode of operation.
A parasite develops some sort of etiological relationship with its host and the latter is exploited slowly, whereas a predator physically eliminates its prey (host) by direct feeding on it without establishing any etiological relationship.
Most of plant diseases are due to fungi, and large numbers of fungi parasitizing fungi are known. When one fungus parasitizes another, the phenomenon is called mycoparasitism, hyperparasitism, direct parasitism, or interfungus parasitism. Almost all taxonomic groups of fungi are found to be involved in this phenomenon and often species within the same genus (e.g., Pythium) interact as host and parasite.
Some examples of mycoparasitism in reference to biological control of diseases are the following:
a. Trichoderma sp., mainly T. harzianum, is one of the most common mycoparasitic fungi. It has been found to parasitize mycelia of Rhizoctonia and Sclerotium. Several yeasts, for instance, Pichia guilliermondii, also parasitize and inhibit the growth of Botrytis, Penicillium, and other plant pathogenic fungi.
b. Fungal pathogen that develops sclerotia is difficult to control as these propagules persist for long periods in soil. Several fungi which invade sclerotia and act as mycoparasite have now been identified. One of the most interesting is Sporidesmium sclerotiorum, which obligately parasitizes sclerotia of five important pathogens, Sclerotinia sclerotiorum, S. minor, S. trifoliorum, Selerotium cepivorum and Botrytis cinerea. Spores of this mycoparasite added in sufficient quantities have been shown to give good control of diseases such as lettuce drop caused by Sclerotinia minor.
After infection, glucanose activity in the sclerotia increases, resulting in the production of glucose, which is readily assimilated by the mycoparasite and allows growth of germ tubes out of the selerotium and into the soil for a distance of up to 3 cm. Other sclerotia within this radius are infected. Exploitation of this property by disking in inoculum of spores of the fungus resulted in 53 per cent control of lettuce drop caused by S. minor. Another sclerotial parasite, Conyothirium minitans, has also demonstrated biological control potential, but only when added to soil at high inoculum rates.
c. Picard and co-workers (2000) state that Pythium oligandrum possesses bio-control properties that is of potential use against a wide range of plant pathogenic fungi. Moreover, the hyphae of P. oligandrum and P. nunn coil around and lyse hyphae of the P. ultimum, the damping off fungus.
Predation, which is an important mode of action in soil, may be exploited in controlling of insects pests but has yet not been employed to any successful extent in the suppression of soil pathogens. In contrast, however, the fungal spores are easily predated by large, motile amoebae, which penetrate even highly resistant spores and consume the spore contents. Such natural predation is no doubt important, but so far these antagonists have proved difficult to produce in culture.
ii. Competition:
An ability to compete successfully with a pathogen is an important property of biological control organisms. Often, successful competition occurs at the infection court, preventing the ingress of the pathogen, although in some instances, an ability of the bio-control agent to limit reproduction of the pathogen can also be important.
The fungus, Idriella bolleyi, controls take-all of wheat, caused by Gaeumannomyces graminis f. sp. tritici, by competition for both nutrients and infection sites. It does this by exploiting senescing cortical cells of the plant, which occur naturally early in its growth, and rapidly producing spores. These are carried down the root by water and continue its colonization.
Iron has an exceedingly low solubility in water and is therefore often limiting for both plants and microorganisms. Both plants and microorganisms capture iron by the production of iron-binding compounds known as siderophores. Siderophores are low molecular weight compounds, which possess a high affinity for iron and aid transport into cells. These chemicals efficiently act as scavengers of iron and may thus mop up all of the available supply of iron in the adjacent environment.
Many pathogens require iron as an essential mineral nutrient for growth, and in some cases iron is required for virulence. Hence, production of a siderophore by a biological control agent may reduce the growth of a pathogen, or its ability to attack the host. A strain, GL20, of Pseudomonas fluorescens was isolated by Lim and Kin in 1997 from the rhizosphere of ginseng.
This strain produced a hydroxamate siderophore in an iron deficient medium, which inhibited spore germination and genu tube elongation of Fusarium solani. In pot trials of the strain GL20, using kidney bean as the test plant, incidence of disease caused by F. solani was reduced by 68% and plant growth was increased nearly 1.6 fold.
iii. Antibiosis:
Antibiosis is that antagonistic condition in which there is suppression of pathogenic microorganisms due to secretion of toxic or inhibitory compounds (antibiotics) by other microorganisms.
Such compounds range from hydrogen cyanide (HCN) to enzymes and the microorganisms involved are often species of Trichoderma and Gliocladium among fungi, and Bacillus and Pseudomonas among bacteria. Some of the pioneer work on the isolation and identification of antibiotic compounds from BCAs concerned those synthesized by fungal antagonists.
For instance, while comparing the ability of isolates of Triclioderma harzianum to reduce take-all disease of wheat (Gaeumannomyces graminis f. sp, tritici), Ghisalberti and coworkers (1990) isolated two pyrone antibiotics that suppressed the disease.
Inhibition in the growth of plant pathogenic fungi (Rhizoctonia solani, Phoma betae, and Pythium ultimum) by Laetisaria arvalis, a basidiomycete, has been observed in both laboratory and field trials; the inhibitory substance has been a long chain fatty acid called laetisaric acid.
Bacteria are the microbial BCAs that are known to produce most diverse range of antimicrobial compounds. Bacillus subtilis effectively controls Rhizoctonia solani in many crops by producing bacilysin and fengymycin. Bacilysin inhibits yeasts and bacteria and fengymycin inhibits filamentous fungi. In 1994, however, zwittermicin A antibiotic has been obtained from Bacillus cereus strain UW 85, another successful biological control agent of damping off and root rot of soybean (Phytophthora sojae).
The first commercial BCA was probably strain K84 of Agrobacterium, which has been used successfully to control crown gall disease by Agrobacterium tumefaciens by agrocin 84 antibiotic. Strains of Pseudomonas produce several toxic inhibitory compounds including phenazine-1-caboxylic acid, phenazine-1-carboxamide, anthranilic acid, diacetyl phloroglucinol, pyoluteorin, pyrrolnitrin, and viscosinamide.
Mode # 2. Hypovirulence:
Hypovirulence is the phenomenon of reduced virulence of a pathogen strain than normal ones developed as a result of its infection by double-stranded RNA (dsRNA). When a hypovirulent strain was co-inoculated with highly virulent strain of a fungus, the latter became hypovirulent normally by hyphal contact (anastomosis).
Some transmissible factor moved from the hypovirulent strain into the more aggressive one. The agent(s) responsible were shown to be cytoplasmic and were subsequently identified as double-stranded RNA (ds RNA) molecules.
The phenomenon of hypovirulence is well-established in a number of fungal pathogens. Cryphonectria parasitica and Ceratocystis ulmi, the pathogens of chestnut blight and dutch elm disease, respectively, both harbour dsRNA because several different-sized dsRNAs have been isolated from hypovirulent strains of these fungi.
Studies of the organization and expression of the dsRNA from C. parasitica as well as its replication suggest that it is viral in origin and therefore it has been referred to as a hypovirus. Since the hypovirulence is conferred on fungal pathogens by mycoviruses, considerable interest has developed for their use as biological control agents.
Mode # 3. Induction of Host Resistance:
Disease suppression through the induction of resistance in host in an alternative, and quite different, mode of action of biological control agents. It has been found during recent years that rhizosphere bacteria (rhizobacteria) applied to seeds or roots induce systemic resistance response expressed against pathogens infecting aerial tissues.
For instance, when Pseudomonas fluorescens was applied to roots of carnation and the stems were inoculated one week later with Fusarium oxysporum f.sp. dianthi, the vascular wilt causing fungus, the incidence of disease was reduced as a result of increase in resistance of the host.
Similarly, the resistance was induced to leaf pathogens such as Colletotrichum orbiculare and Pseudomonas syringae in cucumber, and bacterial blight (Pseudomonas syringae pv. phaseolicola) in bean with the inoculation of biological control agents.
This induction of resistance, as appeared, was presumably due to the production of a signal molecule(s) by the colonizing BCA, which activates systemic acquired resistance (SAR) pathway resulting in release of pathogenesis-related (PR) proteins. More recent evidences suggest that different bio-control bacteria may operate through different pathways distinct from the typical systemic acquired resistance response.
When a bioactive strain of Pseudomonas fluorescens was applied to the roots of Arabidopsis plants, resistance to both a vascular wilt fungus and a foliar bacterial pathogen was subsequently increased, but there was no accumulation of pathogenesis-related (PR) proteins or salicylic acid in disease reaction.
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