Localization of GUS- and GFP-expressing RrF4 in barley roots. GUS-expressing RrF4 on a primary root at 5 (a) and 14 dpi (b), and on lateral protrusions and hair zones of secondary roots at 21 dpi (c). GFP-expressing RrF4 on hair zones (30 dpi) visualized by epifluorescence microscopy (d). (e–g) Single (dead) rhizodermal cells colonized by GUS-tagged RrF4 at 21 dpi. Light microscopy (e) and TEM (f, g) of GUS-tagged RrF4. Dark blue stained plant cells (arrows) are densely packed with bacteria. Dashed box: area heightened in (f); arrows: bacterial cells.