ReviewPseudomonas for biocontrol of phytopathogens: from functional genomics to commercial exploitation
Introduction
There is increasing public concern regarding the continued use of agrichemicals that are damaging to human health or the environment. Such concerns are driving the search for more environmentally friendly methods to control plant disease that will contribute to the goal of sustainability in agriculture. Biotechnology has the potential to contribute enormously to this goal. By the year 2005 it is expected that biotechnology activity in the European Union will be valued at approximately Euro 250 billion, with particular growth predicted within the agrifood sector.
Soil-borne, non-pathogenic bacteria with the ability to antagonise fungal phytopathogens and thus prevent plant disease represent a realistic alternative to chemical fungicides. Consequently, the scientific literature contains a vast body of research on many soil bacteria with biocontrol abilities. These bacteria are known by several generic names, including biological control agents (BCAs), plant growth promoting rhizobacteria (PGPR) and biopesticides. Because of their catabolic versatility, their excellent root-colonising abilities, and their capacity to produce a wide range of antifungal metabolites, the soil-borne fluorescent pseudomonads have received particular attention. In addition, some Pseudomonas BCAs have been shown to elicit a disease-resistance response in crop species, a phenomenon known as induced systemic resistance (ISR) (reviewed by van Loon et al. [1] and Pieterse and van Loon [2]). This dual activity of Pseudomonas BCAs (i.e. direct antagonism of phytopathogens and induction of disease resistance in the host plant) further highlights their potential as plant protection products (PPPs). In this review, we discuss the mechanisms by which Pseudomonas BCAs control plant disease, the strategies exploited to increase their biocontrol efficacy, and the industrial and regulatory issues that must be addressed before BCAs are developed for widespread use in agriculture.
Section snippets
Pseudomonas spp. and suppressive soils
Suppressive soils are soils in which phytopathogenic fungi are unable to persist or are present but fail to induce severe disease symptoms on susceptible crops. This phenomenon, although rare, has been well characterised and there is strong evidence that disease suppression is the result of the presence of certain rhizobacteria with antifungal activity. Several studies have demonstrated that Pseudomonas strains with the ability to produce the antifungal metabolite 2,4-diacetylphloroglucinol
Inoculant delivery systems
Although the vast body of research on Pseudomonas BCAs deals with their capacity to control soil-borne fungal pathogens, there has been limited success developing commercially viable products (see Fig. 2 for schematic diagram depicting requirements for commercial exploitation of Pseudomonas biocontrol agents). According to the records of the United States Department of Agriculture there are fewer than ten Pseudomonas inoculant products on the market for the control of fungal phytopathogens (//www.barc.usda.gov/psi/bpdl/bpdlprod/bioprod.html
Conclusions
It is now over 30 years since Pseudomonas spp. were first recognised as potential BCAs. Within this period, and particularly within the past five years, intense scientific research has given rise to several well-characterised Pseudomonas BCAs that have now become model strains for understanding regulatory mechanisms in Gram-negative bacteria. Although the understanding of such mechanisms is of considerable scientific interest in its own right, it is envisaged that the application of this
Acknowledgements
We thank colleagues at the BIOMERIT Research Centre for useful discussions. Research within the BIOMERIT Research Centre in this area has been supported by the European Union (BIO4-CT96-0027; BI04-CT98-0254; QLK3-CT-2000-31759) and Enterprise Ireland (SC/98/261).
References and recommended reading
Papers of particular interest, published within the annual period of review, have been highlighted as:
•of special interest
••of outstanding interest
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