Characterization of a free-living maize-rhizosphere population of Burkholderia cepacia: effect of seed treatment on disease suppression and growth promotion of maize

Abstract

A <it>Burkholderia cepacia</it> population naturally occurring in the rhizosphere of <it>Zea mays</it> was evaluated by metabolic and molecular profiling and for some traits associated with biocontrol and plant growth promoting (PGP) activity. The purpose was to investigate the potential of this bacterial species closely associated with maize to act as a PGP inoculant. The bacterial strains, isolated on semiselective PCAT medium, were assigned to the species <it>B. cepacia</it> by an analysis of the restriction patterns produced by amplified DNA encoding 16S rRNA (16S rDNA) (ARDRA) with the enzyme <it>Alu</it>I. Biodiversity among the 14 <it>B. cepacia</it> isolates was analyzed by the Biolog GN system and by the random amplified polymorphic DNA (RAPD) technique with two 10-mer primers. The analysis of the Biolog data revealed that all rhizosphere <it>B. cepacia</it> strains formed a tight phenetic cluster which includes <it>B. cepacia</it> LMG 11351, a reference strain isolated from the maize rhizosphere, and allowed us to distinguish single isolates from one another. The analysis of RAPD patterns allowed us to identify two principal groups in this bacterial population. Other tests included in vitro inhibition of the maize pathogens <it>Fusarium</it> spp., analysis of siderophore production, bioassay using maize seeds coated with <it>B. cepacia</it> in soil artificially infested with the maize pathogens <it>Fusarium</it> spp., and greenhouse-based plant growth promotion experiments with maize. The data obtained demonstrated that all <it>B. cepacia</it> strains displayed a wide antibiosis against the phytopathogenic fungi studied and produced, under low iron conditions, hydroxamate-like and thiazo-like siderophores. Moreover, the bioassay allowed us to select six and eight <it>B. cepacia</it> strains with a potential for the biological control of <it>F. proliferatum</it> ITEM-381 and <it>F. moniliforme</it> ITEM-504, respectively. Growth promotion experiments showed that the effect of seed bacterization with <it>B. cepacia</it> isolates on maize growth depended on the potting medium used. When a sand-peat/manure mixture was used, almost all <it>B. cepacia</it> isolates promoted maize growth; whereas, when the soil collected from the field of bacterial isolations was used, only four strains exerted a positive effect on maize growth.