Expression of nodulin genes during nodule development from effective and ineffective root nodules

Root nodule formation in the Rhizobium-legume symbiosis is the result of a series of plant-Rhizobium interactions, in which each induces gene expression in the other. One of the steps in this process is the release of rhizobia into the cytoplasm of host cells, whereafter the infected cells differentiate into non-dividing ones, which form the nodule tissue. Furthermore, the rhizobia differentiate into the characteristic bacteroid forms. During the formation of a root nodule several host genes are specifically induced. Immunological techniques have been used to detect the occurrence of nodule specific polypeptides, the so-called nodulins in soybean and pea root nodules (Legocki, Verma 1980; Bisseling et al., 1983). Leghemoglobins (Lbs) are of course an example of nodule specific proteins, but many more (about 25) nodulins have now been identified. These findings were confirmed by hybridization analysis of nodule mRNA, which indicated the presence of several nodule specific mRNA sequences (Auger et al., 1981).