Molecular Interactions during Nodule Development

Abstract

Nodule development commences as a consequence of molecular communication between Rhizobium and its host legume. Flavonoids secreted by the legume, in concert with NodD, induce the nod genes; nod gene products synthesize Nod factor, a substituted oligoglucosamime with a fatty acid tail on one end of the molecule (Lerouge et al., 1990; Spaink et al., 1991; Sanjuan et al., 1992). Nod factor molecules depolarize root hair cell membranes (Ehrhardt et al., 1992), induce root hair deformation and cortical cell divisions (Truchet et al., 1991; Spaink et al., 1991), and elicit the formation of pre-infection threads in vetch (van Brussel et al., 1992). Some early nodulin genes are also activated by nod gene products, namely, ENOD2 (Govers et al., 1986) and ENOD12 (Scheres et al., 1990). However, the exact mechanism of Nod factor action on the legume host is unknown. Nod factor may directly activate genes involved in the biochemical and structural events described above, or alternatively, it may trigger a signal transduction pathway consisting of many steps, which then lead to early nodulin gene expression, root hair membrane depolarization, root hair deformation, and so on.