Biological control of plant diseases by bacteria
1996
Haas, D. (Lausanne Univ. (Switzerland). Lab. de Biologie Microbienne)
The use of micro-organisms to control plant diseases has attracted considerable scientific interest in recent years, however, few bacteria (and fungi) have given consistent plant-beneficial effects upon introduction into the environment. Bacillus thuringiensis (Bt), usually applied as a mixture of endotoxin, spores and lysed cells, is a biological pesticide which works dead or alive, and Bt-based products are at present the only commercially important biocontrol agents. Whenever introduced biocontrol bacteria have to be alive, present in sufficient numbers and metabolically active in order to be effective, their performance in the field tends to be inconsistent and their commercial success has remained modest. Yet, in natural suppressive soils, plant diseases are consistently suppressed and specific plant beneficial micro-organisms (especially Pseudomonas and Bacillus species) have been shown to be involved in this type of disease suppression. We have chosen Pseudomonas fluorescens CHA0, a root isolate from a suppressive soil in Switzerland, as a model bacterium to investigate which properties of this strain account for its disease-suppressive effects on several root-pathogenic fungi. Extracellular secondary metabolites (antibiotic compounds) were found to be important. CHA0 derivatives engineered to overproduce two antibiotics have improved biocontrol activities in some plant-pathogen systems. However, the regulation of secondary metabolism in pseudomonads is highly complex involving e.g. the sensor Lema and response regulator CHA0, and bacterial traits other than secondary metabolism, e.g. aggressive root colonisation, are also crucial for biocontrol. Hence, the search for potent biocontrol bacteria is still based on empirical screening rather than on a rational approach.
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