In situ localization and penetration route of an endophytic bacteria into roots of wheat and the common bean

Abstract

We examined the structural interaction of the green fluorescent protein−labeled bacterium Herbaspirillum lusitanum, strain P6-12, with Triticum aestivum L. and Phaseolus vulgaris L. The interaction was examined by fluorescence and confocal microscopy for 21 days after inoculation. As early as 0.5 h after inoculation, H. lusitanum P6-12 was observed between the loose peripheral cells of the root cap. This suggests that the bacterium entered the plants through the loose parenchyma of the root cap. One h after inoculation, H. lusitanum P6-12 was detected in the root division zone, and after one and a half h, it reached the elongation and maturation zones.By day 2 after inoculation, variously sized intracellular aggregates of bacteria were detected all over the roots, and by day 4 after inoculation, the bacteria were seen colonizing the shoots. In both plants, the number of viable bacteria in the roots and aerial parts peaked on day 7 after inoculation. A plant growth−promoting effect was clearly visible in all inoculated plants and did not depend on the specific host. The association of H. lusitanum P6-12 with Triticum aestivum L. and Phaseolus vulgaris L. significantly increased biomass yield in an environment without nitrogen supply. The biomass of inoculated P. vulgaris also increased during plant growth with nitrogen. H. lusitanum P6-12 inhibited the growth of root rot pathogens and had no effect on Vero cells; therefore, it can be a subject for further study of this bacterium as biocontrol agents. We conclude that H. lusitanum P6-12 has potential as a beneficial plant endophyte. Further research is required on the mechanisms by which H. lusitanum P6-12 promotes plant growth.