Commonality of root nodulation signals and nitrogen assimilation in tropical grain legumes belonging to the tribe Phaseoleae

The tribe Phaseoleae (family Leguminosae) is home to many of the annual food legumes cultivated in the tropics. Cowpea (Vigna unguiculata (L.) Walp.), Bambara groundnut (Vigna subterranea (L.) Verdc.), Kersting's bean (Macrotyloma geocarpum L.), mung bean (Vigna radiata (L.) Wilczek) and common bean (Phaseolus vulgaris L.), all belonging to subtribe Phaseolinae, and together with soybean (Glycine max (L.) Merr., subtribe Glycininae) and pigeon pea (Cajanus cajan L., subtribe Cajaninae), are important members of the tribe Phaseoleae. These legumes are unique in their use of identical root chemical molecules to induce the expression of nodulation genes in their respective homologous microsymbionts during nodule formation. Of those studied so far, common bean, soybean, Bambara groundnut, Kersting's bean and cowpea all use the isoflavones daidzein, genistein and coumestrol as root exudate signals to induce the expression of nod genes in their rhizobial partners. Additionally, members of the Phaseoleae tribe are easily recognised on the basis of their tropical biogeographic origin, broad host nodulation habit, route of Rhizobium entry into roots, chemotaxonomy and use of a common isoflavone biosynthetic pathway, determinate nodulation phenotype and internal nodule anatomy, xylem composition and transportable solutes of fixed N, site of NO3- reduction and metabolic response of N-2(-) fed plants to NO3- supply. These shared traits and their potential application for agriculture are discussed in this review.