Endophytic Colonization of Plants by the Biocontrol Agent Rhizobium etli G12 in Relation to Meloidogyne incognita Infection.

ABSTRACT The external and internal colonization of potato and Arabidopsis roots by the biocontrol strain Rhizobium etli G12 containing a plasmidborne trp promoter green fluorescent protein transcriptional fusion, pGT-trp, was studied in the presence and absence of the root-knot nematode Meloidogyne incognita. Plant colonization behavior and biocontrol potential of the marked strain G12(pGT-trp) was not altered compared with the parental strain. Plasmid pGT-trp was stable for more than 80 generations without selection and conferred sufficient fluorescence to detect single bacterial cells in planta. Although bacteria were found over the entire rhizoplane, they preferentially colonized root tips, the emerging lateral roots, and galled tissue caused by Meloidogyne infestation. Internal colonization of potato roots was mainly observed in epidermal cells, especially root hairs. G12(pGT-trp) colonization was also observed in inner Arabidopsis root tissues in areas of vascularization. In the presence of M. incognita, G12(pGT-trp) colonized the interior of nematode galls in high numbers. In some cases, bacterial colonization even extended from the galled tissue into adjacent root tissue. The internally colonized sites in roots were often discontinuous. Fluorescence microscopy of gfp-tagged rhizobacteria was a sensitive and a rapid technique to study external and internal colonization of plant roots by bacteria interacting with nematodes.