Nodulated Tree Legumes and Their Symbiotic Bradyrhizobium in African and South-American Tropical Rainforests

Abstract

Leguminosae, the third largest family of angiosperms, are of major agricultural, ecological and economic importance. Several recent studies have clarified the taxonomic and phylogenetic relationships among the 19,400 species that constitute this family and traced the "road map of legume diversity" (Doyle and Lückow, 2003). Following the work of Doyle et al. (1997) on the rbcL gene, Wojciechowski et al. (2004) analysed the plastid matK genes of 330 legume species and brought new insight on clade organization within this family. Using the phylogenetic tree obtained by these latter authors, we superimposed on each plant species the taxonomic identity of its symbiotic N 2-fixing partner (from the databank http://www.ncbi.nlm.nih.gov/). We also checked the nodulation status of each plant species (from http://www.ars-grin.gov/ ~sbmljw/cgi-bin/taxnodul.pl). Surprisingly, the bacterial genera are randomly distributed among the plant clades and the genus Bradyrhizobium is increasingly represented from the Milletioid to the Caesalpinioid clades. Although these analyses have to be confirmed on a larger number of plants and bacteria, with respect to each plant clade size, diversity, and natural geographic distr i-bution, they reveal the need for plant and bacterial phylogenists to work together hand-in-hand and to seek the help of botanists and foresters. Care should be taken with the protocols used to obtain the bacterial isolates, i.e., directly from field collected nodules or through baiting with homologous or heterologous (i.e., Macroptilium atropurpureum) species. Molecular characterization of bacterial DNA from nodule extracts can complement the surveys when bacteria failed to be isolated. through Sustainable Agriculture.