Chapter Six - Differentiation of symbiotic nodule cells and their rhizobium endosymbionts

International audienceThe symbiotic nodule cells are at the heart of the rhizobium-legume symbiosis. They harbor thousands of nitrogen-fixing rhizobia, called bacteroids, within symbiosome vesicles. The symbiotic cells differentiate from meristem cells and this process is driven by endoreduplication and a transcriptome switch, establishing an optimal environment for the maintenance and the metabolic integration of the nitrogen-fixing bacteroids. The differentiating symbiotic cells uptake rhizobia by an endocytosis-like process and the resulting symbiosomes are matured by a specific exocytotic pathway. Finally, the rhizobia within the symbiosomes differentiate into bacteroids involving a metabolic transition controlled by the low oxygen conditions in the symbiotic cells. Depending on the host plant, the bacteroid differentiation involves in addition an irreversible morphological and cytological transition, which is driven by host-produced peptides, called Nodule-specific Cysteine-Rich peptides or NCRs. The parallel differentiation of the host cells and their bacterial endosymbionts results in a unique cellular configuration that is extremely efficient for symbiotic nitrogen fixation