Control of Root Nodule Initiation in Medicago

Root nodules, formed in Rhizobium-legumesymbiosis, provide an excellent model system to study plant developmental processes. These organs are initiated in a localized region of the root, in the emerging root hair zone, and their development is triggered by the Nod factors, under limiting supply of combined nitrogen in the soil. Nod factors induce reactivation of the cell cycle in differentiated cortical cells, leading to the formation of the nodule meristem. In Medicago, the Go arrested cells become activated by the R. meliloti Nod signals (Savoure et al., 1994; Yang et al., 1994). Later on, certain cells loose their ability to divide. These cells can be invaded by the bacteria and differentiation of both the plant cells and the bacteria begins, resulting in the formation of a highly organized nodule structure consisting of distinct zones. Nodule cells during this process enlarge and most of them undergo one or multiple rounds of endoreduplication cycles (Truchet, 1978). Thus, activation and at least partial inactivation of the cell division cycle are programmed during nodule organogenesis.