Colonization of Arabidopsis roots by GFP-tagged RrF4. Images were taken at 7 (a, b) and 21 (c-e) days after root dip-inoculation. (a) RrF4 colonization of the root surface in the root hair zone with single attached bacteria, thin biofilms and dense locally restricted aggregates (arrows). (b) Colonization of the root hairs mainly at the cell bottom (arrows). (c) Localization of bacteria forming biofilms and aggregates at the root surface of the primary root and at the sites of lateral root protrusions (arrows). (d, e) Bacterial cells inside the plant's vascular system (arrow: xylem). (c–e) Different layers of a confocal laser scanning microscopy screen through a root, (c) surface, (d, e) central layer showing the central vascular system.