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. Legumes comprise several evolutionary lineages derived from a common ancestor 60 million years ago (Myr ago). Papilionoids are the largest clade, dating nearly to the origin of legumes and containing most cultivated species. Medicago truncatula is a long-established model for the study of legume biology. Here we describe the draft sequence of the M. truncatula euchromatin based on a recently completed BAC assembly supplemented with Illumina shotgun sequence, together capturing ∼ 94% of all M. truncatula genes. A whole-genome duplication (WGD) approximately 58 Myr ago had a major role in shaping the M. truncatula genome and thereby contributed to the evolution of endosymbiotic nitrogen fixation. Subsequent to the WGD, the M. truncatula genome experienced higher levels of rearrangement than two other sequenced legumes, Glycine max and Lotus japonicus. M. truncatula is a close relative of alfalfa (Medicago sativa), a widely cultivated crop with limited genomics tools and complex autotetraploid genetics. As such, the M. truncatula genome sequence provides significant opportunities to expand alfalfa's genomic toolbox.
Bibliographical noteFunding Information:
FOOD-CT-2004-506223; to G.E.D.O. and J.R. from BBSRC BBS/B/11524; to F.D. and F.Q. from ANR project SEQMEDIC 2006-01122; to R.G. from the Dutch Science Organization VIDI 864.06.007, ERA-PG FP-06.038A; to Y.V.d.P. from the Belgian Federal Science Policy Office IUAP P6/25, Fund for Scientific Research Flanders, Institute for the Promotion of Innovation by Science and Technology in Flanders and Ghent University (MRP N2N); to D.R.C. from NSF IOS-0531408, IOS-0605251; to D.J.S.,B.C.M.and P.J.G.fromUSDACSREES2006-03567;toJ.Gouzyfrom‘Laboratoire d’Excellence’ (LABEX) TULIP (ANR-10-LABX-41). We also acknowledge technical support from the University of Minnesota Supercomputer Institute and thank Y. W. Nam for a BamHI BAC library used by Genoscope, S. Park and M. Accerbi for RNA isolation, T. Paape for statistical consulting, and M. Harrison for supplying myc infected and control root tissues used to make small RNA libraries.