Characterization of Pantoea sp. endophytes from rice seeds

RUIZ, D; DE WERRA, P; WALL, LG; VALVERDE, C

La Plata

Congreso; 2da Reunión de la Sociedad Argentina de Microbiología General; 2005

SAMIGE

Those microorganisms (bacteria and fungi) that colonize the interior of plants without causing damage to hosts and that live most of their life inside plant tissues, are referred to as endophytes. Rhizospheric bacteria that can establish an endophytic relationship with plants can also have benefitial effects on plant growth or health; these bacteria are known as plant growth promoting rhizobacteria (PGPR). In our lab, when trying to obtain rice seedlings (var. CT6919) free of bacteria to be used as traps for specific colonizers from soil, we have observed that every surface-sterilized coated seed was colonized by yellow pigmented bacteria when germinated in water agar and then spotted in rich medium. Colonizers were isolated and two of them were identified by 16S rDNA sequencing as Pantoea sp. (formerly Enterobacter sp.). One of the isolates was transformed with a plasmid expressing GFP in order to study its root colonization behaviour. Already at two days after inoculation of axenic seedlings growing in soft agar with 105 CFU/ml of GFP-tagged Pantoea sp. 1, cells were observed to be concentrated at the site of emergence of lateral roots. At 5 days after inoculation and after vigorous shaking of inoculated roots in water, fluorescent microcolonies and single cells remained firmly attached to the base of lateral roots, suggesting that these are preferential colonizing sites for the Pantoea sp. 1 endophyte. Apart from studying the endophytic properties of these isolates, we are interested to investigate if isolated Pantoea sp. strains display plant growth promoting features. A fungal isolate was also obtained from germinating dehusked CT6919 seeds showing symptoms of disease. This isolate was identified morphologically as Curvularia sp. and was pathogenic to CT6919 seedlings when inoculated as a spore suspension onto roots growing on soft agar plates. Both Pantoea sp. isolates resulted to be antagonistic to Curvularia sp. in co-culture. Experiments are also under way to determine if Pantoea sp. can fix atmospheric nitrogen, solubilize recalcitrant phosphates and if promotes rice seedling development in controlled conditions. The hypothesis behind is that CT6919 rice seeds obtain benefits from carrying bacterial endophytes that will be the first organisms in making contact with emerging roots during germination allowing a rapid colonization of the root system.