Within the plant kingdom, legumes are unusual in their ability to form nitrogen-fixing nodules in symbiosis with certain bacteria in the family Rhizobiaceae (rhizhobia). Genes that are required for signaling between plant and symbiont, and for the development and maintenance of the nodule, were either created de novo or adopted from other plant pathways. Only in recent years have genome-scale sequence data from legumes made it possible to identify large, novel families of genes probably evolved to function in nodulation. Members of these novel families are expressed in seeds or nodules, and are homologous to defense-related proteins. Perhaps the most striking example is a large family (of more than 340 members) of cysteine cluster proteins that have homology to plant defensins.
Bibliographical noteFunding Information:
This work was supported by a National Science Foundation (NSF) Plant Genome Research Program award on Medicago truncatula genomics (DBI no. 0110206; Principal Investigator Douglas R Cook), an NSF Genes and Genome Systems award on Defensin-like Genes in Two Model Plant Species (IOB no. 0516811) and by funds from the University of Minnesota College of Biological Sciences.