Figure 1.

Overview of the stages in the development of indeterminate nodules on plants, such as Medicago (papilionoid legume) or Mimosa (mimosoid legume). (A) Light microscopic (LM) view of the colonization of the root surface by rhizobia. The arrowheads point to polarly attached rhizobia. (B) Transmission electron micrograph (TEM) showing polar attachment of a rhizobial cell to a root hair. (C) Scanning electron micrograph (SEM) of the start of shepherd's crook formation. (D) LM of green-fluorescent protein (gfp)-labeled rhizobia within an infection thread with a shepherd's crook. (E) TEM of an infection thread within a developing nodule cell. (F) LM showing young nodule primordium; the rhizobia are labeled with gusA and stain blue. (G) Confocal laser scanning micrograph (CLSM) showing fluorescently labeled rhizobia within a young nodule. (H) SEM of a nodule cell containing elongating bacteroids. (I) A mature nodule containing lacZ-labeled rhizobia. (J) Two views (LM and SEM) of nitrogen-fixing bacteroids within a nodule cell. Modified from Niner and Hirsch.⁷⁴