Model legumes in the limelight

Leguminous plants provide protein and oil to humans and animals and fix atmospheric nitrogen, and are thus of considerable economic, ecological and biological interest. Most crop legumes are polyploid, and are not amenable to molecular genetic studies. Two legume species, however - the fodder crop Medicago truncatula (barrel medic) and the wild species Lotus japonicus (trefoil) - have recently emerged as model legumes because of their short generation times, diploidy and small genomes. The availability of these legumes enable investigation into aspects of plant biology that Arabidopsis lacks, such as symbiosis between plants and microbes. Recent research on these model systems was reported at the meeting.Because of their association with nitrogen-fixing bacteria, legumes are crucial in understanding the interplay between symbiont and host plant in the establishment of symbiosis. Using arbuscular mycorrhizal (AM) fungi as the symbiont, Maria Harrison (Samuel Roberts Noble Foundation, Oklahoma, USA) has identified M. truncatula mutants that fail to support the formation of fungal hyphal ingrowths (arbuscules) in the plant's cells (Figure 1). She also found that some of these mutants fail to form a symbiotic relationship with the nitrogen-fixing bacterium Sinorhizobium meliloti, indicating an overlap between the two symbiosis pathways. Similarly, Martin Parniske (John Innes Centre, UK) reported the identification of sixteen L. japonicus mutants, falling into six complementation groups, all of which fail to form symbiotic relationships with both rhizobia and AM fungi. Igor Tikhonovich (All-Russia Research Institute for Agricultural Microbiology, Russia) described various pea (Pisum sativum) mutants that cease nodule development at different stages. By exploiting the natural genetic variability in M. truncatula, Jean Denarie and Thierry Huguet (CNRS-INRA, Tolouse, France) have identified ecotypes that fail to recognize different rhizobial nod (nodulation) factors, which