Characterization of native fluorescent Pseudomonas isolates associated with alfalfa roots in Uruguayan agroecosystems

Abstract

Establishment of alfalfa crops is continuously threatened by seedling diseases caused by soilborne pathogens. The use of plant beneficial bacteria as inoculants is a feasible and environmentally friendly means to control soil pathogens. Identifying effective plant growth-promoting strains to use on local crops under local environmental conditions requires the screening of large collections of native isolates. A collection of 738 rhizospheric fluorescent Pseudomonas isolates was obtained from alfalfa plants from three agroecological regions representative of Uruguayan agricultural systems. The isolates were evaluated for in vitro pathogen inhibition, biosurfactant production, phosphate solubilization and the presence of genes involved in antibiotic synthesis. Isolates with strong in vitro antagonistic activity toward Pythium debaryanum were more abundant in alfalfa plants established in a previously natural ecosystem while biosurfactant producers were less abundant in that location. A subset of isolates was selected for genotypic characterization by rep-PCR using BOX primers. Twenty-four genotypes were defined, sixteen from a single geographical origin and eight composed of isolates from multiple origins. Genotypic profiles correlated well with phenotypic traits. A subset of isolates was assayed to determine their ability to protect alfalfa against P. debaryanum damping-off and to promote plant growth. Five native Pseudomonas isolates showed significant effects on alfalfa by increasing plant biomass and/or protecting from pathogen infection. Plant growth promoting isolates from each location were genotypically similar. Our work contributes to the knowledge of the phenotypic and genotypic diversity of rhizospheric fluorescent pseudomonads of forage legumes and the frequency of plant growth promoting traits associated with this group of bacteria in different agricultural systems.