Localization of RrF4 in the root hair zone of barley primary roots. (a, b) Root cross-sections showing the colonization of the root surface and the cortical tissue 7 dpi with (a) GUS- and (b) GFP-tagged RrF4. (c–e) Light microscopic (c) and TEM (d, e) images of ultra-thin cross-sections of the primary root shown in (a) illustrating the intercellular localization of RrF4 in the cortex; (c) overview; (d) RrF4 cells in the intercellular space; (e) zoom to single intercellular rod-shaped RrF4 cells. (f, g) Cross-sections showing colonization of the central cylinder at 21 dpi by GUS- (f) or GFP-tagged RrF4 cells (g). (a, f) Light microscopic and (b, g) confocal laser scanning images. Epidermis (ep), root hair cells (rh), cortex (cx), cortex-cells (cxc), endodermis (en), central cylinder (cc), peripheral xylem vessels (pX) central metaxylem vessels (cmX). Right arrows highlight bacterial cells.