Fig. 1.

Scheme showing the infection process, the major differences between indeterminate and determinate nodules, and some key metabolic pathways in the nodule cells. (A) Briefly, the infection process is as follows: legume roots release flavonoids to the rhizosphere which induce the production of nodulation (Nod) factors in compatible rhizobia (depicted as red dots next to root hairs); Nod factors are recognized by root receptors that activate the symbiotic signaling pathway; rhizobia enter the root through hairs that curve and trap the bacteria inside a curl; invaginations of the cell membrane form infection threads that permit the invasion of the root cortex by rhizobia; a new nodule meristem forms underneath the site of infection; as the nodule grows, the bacteria are released into membrane-bound compartments, the symbiosomes, inside the nodule cells, where the bacteria differentiate into bacteroids and start N₂ fixation. As a result, two major types of nodules are formed. Indeterminate nodules of Medicago truncatula and crops such as pea and alfalfa contain a persistent meristem and are generally elongated with a longitudinal gradient of age. Four zones can be distinguished from the apex (distal) to the base (proximal) regions: zone I (meristem), zone II (infection), zone III (N₂-fixing), and zone IV (senescent). Determinate nodules of Lotus japonicus and of crops such as soybean and common bean lack permanent meristems and are usually spherical. In this case, N₂ fixation takes place in the central infected zone, which also contains uninfected or interstitial cells. Abbreviations of cell layers: c, cortex; e, epidermis; en, endodermis; p, pericycle; vb, vascular bundle. (B) Some key processes in the symbiotic nodule cells. Sucrose from the shoot is metabolized to malate that is transported into bacteroids through dicarboxylate transporters (Dct). In the bacteroids, malate is oxidized, providing energy through the tricarboxylic acid (TCA) cycle and the electron transport chain (ETC) for nitrogenase (N₂ase) activity. Fixed nitrogen in the form of ammonium is transported back to the plant where, with some exceptions, it is assimilated into the amides glutamine and asparagine in indeterminate nodules or ureides in determinate nodules. In the cytoplasm of infected cells, leghemoglobin (Lb) transports and delivers O₂ to the symbiosomes at a low steady concentration to avoid the inhibition of nitrogenase, but to simultaneously allow high rates of bacteroid respiration. After Oldroyd et al., (2011), Popp and Ott (2011), and Hichri et al., (2015).