Genotypic and phenotypic diversity in populations of plant-probiotic Pseudomonas spp. colonizing roots.

[accepted on June 25, 2007] Abstract -- Several soil micro-organisms colonizing roots are known to naturally promote the health of plants by controlling a range of plant pathogens, including bacteria, fungi, and nematodes. The use of theses antagonistic microorganisms, recently named plant-probiotics, to control plant-pathogenic fungi is receiving increasing attention, since they may represent a sustainable alternative to chemical pesticides. Many years of research on plant-probiotic rhizobacteria have indicated that Plant-probiotic Fluorescent Pseudomonads (PFP) are largely involved in the suppression of the most widespread soilborne pathogens. Phenotype and genotype analysis of PFP have shown considerable genetic variation among these types of strains. Such variability plays an important role in the rhizosphere competence and the biocontrol ability of PFP strains. Understanding the mechanisms by which genotypic and phenotypic diversity occurs in natural populations of PFP could be exploited to choose those agricultural practices which best exploit the indigenous PFP populations, or to isolate new plant-probiotic strains to use as inoculants. A number of different methods have been used to study diversity within PFP populations. Because different resolutions of the existing microbial diversity can be revealed depending on the approach used, this review first describes the most important methods used for the assessment of fluorescent Pseudomonas diversity. Then, we focus on recent data relating how differences in genotypic and phenotypic diversity within PFP communities can be attributed to geographic location, climate, soil type, soil management regime, interactions with other soil micro-organisms and host plants. It becomes evident that plant-related parameters exert the strongest influence on the genotypic and phenotypic variations in PFP populations.