Endophytic colonization of plants by the biocontrol agent Rhizobium etli G12 in relation to Meloidogyne incognita infection.

The external and internal colonization of potato and Arabidopsis roots by t he biocontrol strain Rhizobium etli G12 containing a plasmidborne trp promo ter 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 pG T-trp was stable for more than 80 generations without selection and conferr ed sufficient fluorescence to detect single bacterial cells in planta. Alth ough bacteria were found over the entire rhizoplane, they preferentially co lonized root tips, the emerging lateral roots, and galled tissue caused by Meloidogyne infestation. Internal colonization of potato roots was mainly o bserved in epidermal cells, especially root hairs. G12(pGT-trp) colonizatio n was also observed in inner Arabidopsis root tissues in areas of vasculari zation. In the presence of M. incognita, G12(pGT-trp) colonized the interio r of nematode galls in high numbers. In some cases, bacterial colonization even extended from the galled tissue into adjacent root tissue. The interna lly colonized sites in roots were often discontinuous. Fluorescence microsc opy of gfp-tagged rhizobacteria was a sensitive and a rapid technique to st udy external and internal colonization of plant roots by bacteria interacti ng with nematodes.