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Role of metabolites in biological control
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Some of the metabolites implicated in biocontrol appear to be broad-ranging in their inhibitory action. For example, phloroglucinols and phenazines have been shown to inhibit a wide range of fungal pathogens in the laboratory. Siderophores exhibit both fungistatic and bacteriostatic
| Table 1. Examples of specific microbial metabolites implicated in the control of crop diseases | |||
| Disease | Pathogen | Effective metabolite | Refs |
| Take-all of wheat | Gaeumannomyces graminis var. tritici (Ggt) | Phenazines c-Acetyl phloroglucinols | 10, |
| Tan spot of wheat | Pyrenophora triticirepentis | Pyrrolnitrin | |
| Pre-emergent damping-off of: cotton; sugarbeet | Pythium spp. Pythium ultimum | Oomycin A Pyoluteorin 2, 4-Diacetylphloroglucinol | |
| Black root-rot of tobacco | Thielabiopsis basicola | Hydrogen cyanide 2, 4-Diacetylphloroglucinol | |
| Crown gall of fruit trees | Agrobacterium tumefaciens | Agrocin 84 | 17-19 |
| Flax wilt | Fusarium oxysporum | Pseudobactin B10 | |
| Damping-off | Pythium spp. | Ammonia |
effects in the laboratory under conditions of low iron. In the field, these iron-chelating compounds are thought to deprive the pathogen of iron, a limiting essential nutrient.
Other metabolites are known to have very specific effects and to target particular pathogens; for example, agrocin 84, produced by Agrobacterium radiobacter, is specific for virulent strains of Agrobacterium tumefaciens. At the molecular level, agrocin 84 (a di-substituted nucleotide) is thought to act by chain termination of DNA synthesis. However, the precise mode of actions of many other metabolites is poorly understood.
Effects such as direct killing or pathogen inhibition exhibited by a particular metabolite in the laboratory cannot always be translated directly into biological control in the field, as the conditions experienced in the field are much more complex. However, there are a number of studies demonstrating a direct effect of particular antimicrobial metabolites on pathogen numbers in soil. One such example is the suppression of root pathogens by plant-growth-promoting rhizobacteria (PGPR) (see Glossary). In another case, the population size of antibiotic-producing Pseudomonas fluorescens 2-79 was shown to correlate positively with a reduction in the number of lesions on wheat roots caused by Ggt.
Evidence is accumulating to support the theory that metabolite production has beneficial effects on the ecological competence of the producer strain. Production of these compounds is thought to provide the producing strain with a selective advantage in the highly competitive environment of the plant rhizosphere. This idea has been substantiated by a recent report, which demonstrated that phenazine (Phz) antibiotics contributed to the persistence of the producer strains (P. fluorescens 2-79 and P. aureofaciens 30-84) compared with Phz-deficient mutants in a simulated wheat-rotation microcosm.
| Glossary |
| Biological control –The use of living organisms to modify the agricultural ecosystem to control a crop disease or prevent the establishment of a pest. Rhizosphere –The root surface and that part of the soil directly influenced by the root system. Plant-growth-promoting rhizobacteria (PGPR) –Bacteria isolated from the rhizosphere, which have been shown to improve plant health or increase yield. They may exert their effects directly through the production of metabolites that can act as plant hormones and/or indirectly by the suppression of plant pathogens (biological control). Induced systemic acquired resistance –The ability of an agent, for example, a bacterium, chemical, virus, nematode, etc., to induce plant-defence mechanisms that lead to systemic resistance to a number of pathogens. Siderophore –Low-molecular-weight compound that is produced as a specific iron chelator by a microorganism. |
Persistence may lead to improved competition and colonization of the producer strain on the plant surface, leading to niche-exclusion of the pathogen. The role of siderophores and other metabolite(s) in biocontrol may not be simply a direct antagonism of the pathogenic fungus, but may also involve more subtle ecological effects.
Alternative, more indirect, modes of action could involve stimulation of the plant’s own defence mechanisms by induced systemic acquired resistance or, possibly, by direct uptake and translocation of the metabolite within the plant.
D. N. Dowling and F. O’Gara are at the Department of
Microbiology, University College Cork, Ireland.
| TIBTECH APRIL 1994 (VOL 12) |
© 1994, Elsevier Science Ltd 0167 – 7799/94/S7.00
ª ASSIGNMENTS
1. Read the scientific text above and present it taking into account the branch of knowledge it belongs to.
2. Analyze the text in terms of the category of informativity. Comment on how paralinguistic means of conveying information, used in the text alongside the verbal ones, add on to the factual information of the scientific text in question.
3. Comment on the lexical peculiarities of the present scientific text distinguishing the three lexical layers in its vocabulary.
4. Analyze the text in terms of the category of cohesion focusing on the lexical means conducive to the logico-semantic integrity of the scientific writing under consideration.
5. Compose and write a coherent essay summing up your observations on the linguostylistic and textlinguistic peculiarities of the scientific text “Metabolites of Pseudomonas involved in the biocontrol of plant disease”.
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[1] ) Mind that the Russian word ‘науки’ may refer both to science subjects such as chemistry, biology, physics, etc. and to humanities subjects such as history, literature, philosophy, linguistics, etc., while in English there are two different words: sciences vs humanities.