Legume models strut their stuff.

Legumes have been an integral part of sustainable agriculture for hundreds, if not thousands, of years, and they are likely to become even more important in the future. This is because legumes are able to form nitrogen-fixing symbioses with rhizobia that allow them to grow in the absence of added nitrogen fertilizers. In addition, legumes are able to form mycorrhizal symbioses with fungi that provide the plants with phosphorous and other soil nutrients, which significantly contributes to the agricultural output of legumes. The symbiotic phenotypes of legumes set them apart from other model plant species such as Arabidopsis. In June 2000, a group of 120 scientists from around the world met at the John Innes Centre (JIC) in Norwich, U.K., to discuss recent developments in the molecular genetics of model legumes. The topics included comparative and functional genomics, the genetics of nitrogen-fixing and mycorrhizal symbioses, pathogen interactions, metabolism, signal transduction, development, agriculture and breeding, and the molecular systematics of legumes. Model legumes fall into two broad classes: those that are of major importance to agriculture and those that are not. The former have been the subject of scientific investigation for many decades because of their ubiquity and economic value and include such crops as pea, soybean, and alfalfa. Unfortunately, agriculturally important legumes generally have large, complex genomes and are difficult to transform, which makes them less than ideal for molecular genetics and genomics. For this reason two species, Lotus japonicus and Medicago truncatula, have been adopted by a growing number of researchers in recent years. These models they have relatively small and simple genomes (three to four times the size of the Arabidopsis genome), are easy to transform, and have a number of other positive attributes for classical and molecular genetics and symbiosis research (Barker et al. 1990; Handberg and Stougaard 1992). Leguminosae is the third-largest and possibly the most diverse family of angiosperms. Species range from small herbaceous plants to giant rainforest trees, a diversity that was beautifully illustrated in Toby Pennington’s (Royal Botanic Garden, Edinburgh, U.K.) opening talk. Pennington also discussed how molecular analysis is reorganizing legume systematics, providing new insights into the evolution of important traits such as nodulation. Interestingly, it appears that the common ancestor of all legumes was not a nodulator and that this feature probably evolved independently several times in this family. The ability to nodulate also appears to have been lost from many species in multiple genera over time, possibly as a result of an increased availability of mineral or organic nitrogen in the niches occupied by legumes. The humble pea has been a model for genetic studies since the time of Mendel and an important crop species for a much longer period. It has a relatively large and complex genome that is peppered with highly polymorphic retrotransposons (RTs). Noel Ellis (JIC) described how these RTs are being used in the genetic mapping of the pea genome. He also demonstrated substantial colinearity between the genetic linkage maps of pea and alfalfa. This theme of macroand microsynteny between legume genomes was one that emerged throughout the meeting. For instance, there is a high degree of synteny between species in the Medicago genus (Thierry Huguet, CNRS-INRA, Toulouse, France) as well as between the Medicago species and pea (György Kiss, Institute of Genetics, Szeged, Hungary). Nevin Young (University of Minnesota, St. Paul, U.S.A.) also described microsynteny between M. truncatula and soybean. Synteny between the smaller genomes of model species such as M. truncatula and L. japonicus and the much larger genomes of agricultural mainstays such as soybean and pea should help to accelerate the isolation of agronomically important genes in the latter. To facilitate the isolation of genes and quantitative trait loci involved in symbiosis and other important agricultural traits, much effort has been invested in mapping several legume genomes. The genetic map of diploid alfalfa (Medicago sativa, 2n = 16) now contains 1,500 markers distributed over 754 cM (Kiss). M. truncatula (2n = 16) has at least 384 molecular markers distributed over 1,400 cM of the eight linkage groups (Huguet), whereas the L. japonicus (2n = 12) genetic map contains 500 molecular markers over 378 cM (Niels Sandal, Aarhus University, Denmark). Large bacterial artificial chromosome genomic libraries of L. japonicus (N. Sandal; Shinji Kawasaki, National Institute of Agrobiological Resources, Tsukuba, Japan; Peter M. Gresshoff, University of Queensland, Brisbane, Australia) and M. truncatula (Doug Cook, Texas A&M University, College Station, U.S.A.) will enable physical mapping of these genomes and accelerate map-based cloning of genes. Chemical mutagenesis has generated large collections of interesting mutants in the various model legume species. Many genes involved in the development of functional symA Report on the Molecular Genetics of Model Legumes Conference, held June 2000 at the John Innes Centre, Norwich, U.K.