Figure 2.

Cytoarchitecture of a Legume (Medicago truncatula) and an Actinorhizal Nodule (Alnus glutinosa).

(A) A cross section of an M. truncatula nodule visualizing a large central zone of infected cells and five peripheral vascular bundles.

(B) A cross section of an A. glutinosa nodule visualizing a central vascular bundle surrounded by infected cortical cells.

(C) Infected M. truncatula nodule cells containing hundreds of elongated rhizobia as transient nitrogen-fixing organelle-like structures.

(D) Infected A. glutinosa nodule cells containing fixation threads and tens of densely stained nitrogen-fixing Frankia vesicles.