Legumes (Fabaceae or Leguminosae) are unique among cultivated plants for their ability to carry out endosymbiotic nitrogen fixation with rhizobial bacteria, a process that takes place in a specialized structure known as the nodule. Legumes belong to one of the two main groups of eurosids, the Fabidae, which includes most species capable of endosymbiotic nitrogen fixation 1. Legumes comprise several evolutionary lineages derived from a common ancestor 60 million years ago (Mya). Papilionoids are the largest clade, dating nearly to the origin of legumes and containing most cultivated species 2. Medicago truncatula (Mt) is a long-established model for the study of legume biology. Here we describe the draft sequence of the Mt euchromatin based on a recently completed BAC-assembly supplemented with Illumina-shotgun sequence, together capturing ~94% of all Mt genes. A whole-genome duplication (WGD) approximately 58 Mya played a major role in shaping the Mt genome and thereby contributed to the evolution of endosymbiotic nitrogen fixation. Subsequent to the WGD, the Mt genome experienced higher levels of rearrangement than two other sequenced legumes, Glycine max (Gm) and Lotus japonicus (Lj). Mt is a close relative of alfalfa (M. sativa), a widely cultivated crop with limited genomics tools and complex autotetraploid genetics. As such, the Mt genome sequence provides significant opportunities to expand alfalfa’s genomic toolbox.

Arbuscular mycorrhiza (AM) is a root endosymbiosis between plants and glomeromycete fungi. It is the most widespread terrestrial plant symbiosis, improving plant uptake of water and mineral nutrients. Yet, despite its crucial role in land ecosystems, molecular mechanisms leading to its formation are just beginning to be unravelled. Recent evidence suggests that AM fungi produce diffusible symbiotic signals. Here we show that Glomus intraradices secretes symbiotic signals that are a mixture of sulphated and non-sulphated simple lipochitooligosaccharides (LCOs), which stimulate formation of AM in plant species of diverse families (Fabaceae, Asteraceae and Umbelliferae). In the legume Medicago truncatula these signals stimulate root growth and branching by the symbiotic DMI signalling pathway. These findings provide a better understanding of the evolution of signalling mechanisms involved in plant root endosymbioses and will greatly facilitate their molecular dissection. They also open the way to using these natural and very active molecules in agriculture.

Rhizobia are symbiotic bacteria that elicit the formation on leguminous plants of specialized organs, root nodules, in which they fix nitrogen. In various Rhizobium species, such as R. leguminosarum and R. meliloti, common and host-specific nodulation (nod) genes have been identified which determine infection and nodulation of specific hosts. Common nodABC genes as well as host-specific nodH and nodQ genes were shown recently, using bioassays, to be involved in the production of extracellular Nod signals. Using R. meliloti strains overproducing symbiotic Nod factors, we have purified the major alfalfa-specific signal, NodRm-1, by gel permeation, ion exchange and C18 reverse-phase high performance liquid chromatography. From mass spectrometry, nuclear magnetic resonance, (35)S-labelling and chemical modification studies, NodRm-1 was shown to be a sulphated beta-1,4-tetrasaccharide of D-glucosamine (Mr 1,102) in which three amino groups were acetylated and one was acylated with a C16 bis-unsaturated fatty acid. This purified Nod signal specifically elicited root hair deformation on the homologous host when added in nanomolar concentration.