Light micrographs (a,b,d,e) and transmission electron micrographs (TEMs) (c,f) of colonization of poaceous crops by endophytic diazotrophic bacteria. (a) Longitudinal section of a rice (Oryza sativa) root (r) being invaded by H. seropedicae (arrows) at a lateral root (asterisk) junction. Bacteria have colonized intercellular spaces deep within the cortex of the main root (arrowhead). (b) Transverse section of a rice root (r) showing that the intercellular spaces surrounding the point of emergence of an emerging lateral root (asterisk) are heavily colonized by H. seropedicae (arrows). (c) Transmission electron micrograph of rice root cells that are densely colonized by H. seropedicae, labeled by gold particles attached to an antibody against H. seropedicae strain Z67. The host cytoplasm is no longer present, but the bacteria are instead surrounded by exopolysaccharide material that is strongly labeled (see  for details). w, plant cell. (d) Longitudinal section of a sugarcane (Saccharum sp.) root tip being invaded by G. diazotrophicus. The bacteria (arrows) are present in the root cap cells (rc), some of which are broken, but the newly divided cells of the meristem (m) remain uninvaded. (e) Transverse section of a sorghum (Sorghum bicolor) leaf. The protoxylem and associated lacunae are densely colonized by H. rubrisubalbicans (arrows), but the metaxylem (mx) remains uninvaded. p, phloem. (f) TEM of sugarcane leaf xylem colonized by G. diazotrophicus; the bacteria are healthy in appearance but are embedded in a gum (g) that was produced by the plant as a defense response to their presence. The bacteria accordingly have released exopolysaccharide to protect themselves from this gum, thus creating 'halo-like' electron-transparent regions around them (arrows). The scale bars represent (a) 50 μm, (b) 20 μm, (c) 1 μm, (d) 10 μm, (e) 20 μm and (f) 1 μm.