Introduction

Nitrogen-fixing symbioses between legumes and the soil bacteria Azorhizobium, Bradyrhizobium, Mesorhizobium and Rhizobium (collectively rhizobia) contribute substantially to plant productivity. Legumes and rhizobia have vastly different genomes. All rhizobia have a chromosome along with none to many plasmids which, combined, total six to nine mega-base pairs (Mbp) (Perret et al. 2000). In contrast, genomes of legumes are much larger with many chromosomes and total DNA contents that range from about 450 Mbp/lC to 4500 Mbp/lC. As examples, the model legumes Lotus japonicus, Medicago truncatula and Phaseolus vulgaris have haploid genomes made up of six, eight or eleven chromosomes with DNA contents of ± 450 to 550 Mbp/lC. Legume genomes are thus at least 50 times larger than those of their micro-symbionts. Despite these differences, available information suggests that the contributions of both symbiotic partners are roughly equal. Like most plants, legumes release a variety of compounds into the soil surrounding their roots (the rhizosphere). Amongst these substances, phenolic compounds and especially flavonoids are perceived by rhizobia as inducers of nodulation («o£/)-genes. In turn, rhizobia secrete a variety of lipochito-oligosaccharides called Nod factors. By themselves Nod factors are fully able to mimic the initial effects of rhizobia on legumes. Once rhizobia enter the infection thread, Nod factors seem to turn their guiding role over to other molecules however. This second set of rhizobial "factors" include extra-cellular polysaccharides as well as proteins and are perceived by different plants in various ways (see Broughton et al. 2000). Their possible roles are discussed below.

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