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Nature Reviews Microbiology
3,
307-319
(2005);
doi:10.1038/nrmicro1129
BIOLOGICAL CONTROL OF SOIL-BORNE PATHOGENS BY FLUORESCENT PSEUDOMONADS
Dieter Haas & Geneviève Défago
about the authors
Abstract
Particular bacterial strains in certain natural environments prevent infectious diseases of plant roots. How these bacteria achieve this protection from pathogenic fungi has been analysed in detail in biocontrol strains of fluorescent pseudomonads. During root colonization, these bacteria produce antifungal antibiotics, elicit induced systemic resistance in the host plant or interfere specifically with fungal pathogenicity factors. Before engaging in these activities, biocontrol bacteria go through several regulatory processes at the transcriptional and post-transcriptional levels.
Summary
- In suppressive soils, crop plants are protected from soil-borne root pathogens (mostly fungi). Disease suppression depends on the prevailing environmental conditions in the soil and, moreover, has a strong biological component, which consists of root-colonizing plant-beneficial bacteria and fungi. Among these plant-beneficial 'probiotic' microorganisms, selected strains of fluorescent pseudomonads have been studied extensively for biocontrol mechanisms using both biochemical and molecular genetic approaches.
- Many biocontrol strains of fluorescent pseudomonads produce extracellular secondary metabolites that inhibit the growth of fungal pathogens and account for part of the disease-suppressive activity. Siderophores (iron chelators) are occasionally involved in biocontrol but do not seem to have the crucial role that was once attributed to them.
- Induced systemic resistance in the host plant is a second important mechanism that is involved in the biocontrol of root pathogens. Although it remains unclear which bacterial signals are most apt to elicit resistance, the jasmonate- and ethylene-responsive defence pathways seem to be most important in the host plant, as judged from experiments with mutants of Arabidopsis thaliana.
- Subtle interactions between a pathogen and a biocontrol agent can tip the balance in favour of plant disease or health. The evidence for such direct and specific interactions is still fragmentary, and is limited to few well-studied cases.
- Progress has been made recently in the understanding of the molecular mechanisms that regulate the expression of biocontrol factors in fluorescent pseudomonads. At the transcriptional level, several secondary metabolites positively regulate the expression of their own biosynthetic genes. At a post-transcriptional level, small noncoding RNAs that are controlled by the GacS/GacA two-component system can relieve the repressive action that RNA-binding proteins exert on the expression of target mRNAs. These small RNAs therefore determine the expression of biocontrol factors.
- The soil, the plant, the pathogen and the biocontrol agent interact with each other through both biotic and abiotic signals, many of which remain unknown. A better understanding of these signals and their mode of action might facilitate the practical application of biocontrol microorganisms in the future.
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