Principles in biological control of soil-borne diseases: Colonization, antagonism, plant growth promotion and induced resistance

摘要

Biological control of soil-borne pathogens comprises the decrease of inoculum or of the diseaseproducing activity of a pathogen through one or more mechanisms. Interest in biological control of soilborne plant pathogens has increased considerably in the last few decades, because it may provide controlof diseases that can not or only partly be managed by other control strategies. Recent advances in microbial and molecular techniques have significantly contributed to new insights inunderlying mechanisms by which introduced bacteria function. Colonization of plant roots is an essentialstep for both soil-borne pathogenic and beneficial rhizobacteria. Visualization technologies using laserscanning microscopy in combination with marked strains have increased our knowledge on rhizospheremicrobiology. Colonization patterns showed that rhizobacteria act as biocontrol agents or as growthpromoting bacteria form microcolonies or biofilms at preferred sites of root exudation. Suchmicrocolonies are sites for bacteria to communicate with each other (quorum sensing) and to act in acoordinated manner. Mutants defective in root colonization traits were impaired in their ability to controlroot diseases. Biocontrol mechanisms by which rhizosphere bacteria may protect plants against soil-borne pathogensare multifarious. The bacteria may compete for space and nutrients in the rhizosphere. Evidence has beenaccumulated that antibiotics produced by rhizobacteria constitute an important factor in suppression ofroot diseases. The degree of disease control by rhizobacteria producing antibiotics depends on thesensitivity of the pathogen population to the antibiotics and the quantities produced. Fluorescentpseudomonads produce siderophores that sequester iron, thereby depriving the pathogen from thisessential element during its deleterious activities in the rhizosphere. Mechanisms by which plant growthpromoting rhizobacteria (PGPR) may stimulate plant growth are either by suppressing deleteriousmicroorganisms or pathogens or by producing plant growth regulators (such as auxins, cytokinins andgibberellins) as well as by lowering ethylene levels in plants. In addition, selected strains of rhizobacteria reduce diseases through activating resistance mechanisms inplants. Rhizobacteria-mediated resistance is expressed toward a broad spectrum of plant pathogens.Certain strains of rhizobacteria produce salicylic acid (SA) at the root surface and trigger the SAdependent systemic acquired resistance (SAR)-pathway. Other rhizobacteria trigger a different signallingpathway named induced systemic resistance (ISR). ISR is dependent on the plant hormones jasmonicacid (JA) and ethylene. Simultaneous activation of both the JA/ethylene-dependent ISR pathway and theSA-dependent SAR pathway may result in enhanced levels of protection.