Colonization of barley primary roots by RrF4 analyzed by scanning (a–f) and transmission (g) electron microscopy. (a, b) RrF4 at the root tip (b is a zoom out of a showing RrF4 cells at cracks of the root cap cells, arrows). (c) RrF4 cell aggregates at the root tip. (d) Single RrF4 cells attached to the root surface distal to the tip area. Bacterial cells are cross-linked by fiber-like structures. (e) Different stages in biofilm formation at the rhizoplane of the root hair zone: single RrF4 cells attached to the rhizoplane (s); micro-colonies formed through multiplication of single attached cells (m); larger cell aggregates (a), thick surface attached biofilm (b); root hairs (rh). (f) RrF4 cell aggregates around the root hair protrusion site; assumed area of penetration into the root tissue (arrow). (g) Surface-attached dense biofilm of RrF4 cells. Bacterial cells are embedded in an extracellular polysaccharide (EPS)-like structure forming a four to five cell layers thick biofilm at the root surface (b).