Characterization of LYM genes in Medicago truncatula.M. truncatula contains a large family of genes encoding receptor-like proteins containing the LysM domain (Arrighi et al., 2006). Additionally, multiple proteins containing one or more LysM domains can be predicted from available sequencing resources. Among these, two transcripts encoding proteins with structural homology to the chitin elicitor binding protein CEBiP from rice (Oryza sativa (Kaku et al., 2006)) were identified, MtLYM1 and MtLYM2 (Arrighi et al., 2006). The proteins deduced from these transcripts included signal peptides, indicating extracellular localization (Fig. 1). Additionally, both proteins were predicted to be modified by glycosylphosphatidylinositol (GPI) for cell surface anchoring after removal of the C-terminal transmembrane domain. The genes encoding MtLYM1 and 2 contained five exons/four introns and four exons/three introns, respectively (Fig. 1), from which only one intron position was conserved between the two genes. Interestingly, a small intron was present in MtLYM2 within the region encoding the three LysM domains, which has never been observed for other genes encoding LysM-containing proteins in M. truncatula (Arrighi et al., 2006). We analyzed the phylogeny of LYMs by performing a consensus search of fully sequenced plant genomes for encoded proteins with similarity to M. truncatula LYMs. Proteins that were not expected to contain a signal peptide or had fewer than 300 residues were removed to obtain a list of 53 LYMs (suppl table xx). Unlike LYPs, which are less strictly defined as extracellular LysM-containing proteins and have been detected in the moss Physcomitrella patens (Zhang et al., 2009), LYMs are present......middle of the item... ...n). The strongest promoter activity was observed at root tips and in lateral root emergences (Fig. 7A-D). The expression level and distribution were rather variable, with occasional staining of root hairs (Fig. 7A,D,F). During Sinorhizobium meliloti (pXLGD4) infection, GUS activity was detectable in developing nodules (Fig. 7E,F). Major nodules showed little GUS activity in the outer layers (Fig. 7G,H). Sectioning old nodules (4 weeks pi??) before GUS staining revealed a particular expression pattern of MtLYM2. Here, promoter activity was found to be restricted to vascular bundles only, with in some cases very little staining of the outer cortex (Fig. 7x). In summary, histochemical localization of GUS activity in roots and nodules expressing proLYM2::GUS-int confirmed our previous observation of reduced transcript and protein levels in nodules compared to roots...