Підручник для студентів старших курсів

Role of metabolites in biological control
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	9 10, 11
Tan spot of wheat	Pyrenophora triticirepentis	Pyrrolnitrin	12
Pre-emergent damping-off of: cotton; sugarbeet	Pythium spp. Pythium ultimum	Oomycin A Pyoluteorin 2,4-Diacetylphloroglucinol	13 14 15
Black root-rot of tobacco	Thielabiopsis basicola	Hydrogen cyanide 2,4-Diacetylphloroglucinol	16 10
Crown gall of fruit trees	Agrobacterium tumefaciens	Agrocin 84	17-19
Flax wilt	Fusarium oxysporum	Pseudobactin B10	20
Damping-off	Pythium spp.	Ammonia	21

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.