We have searched the Arabidopsis and rice (Oryza sativa) genomes for homologs of LRX1, an Arabidopsis gene encoding a novel type of cell wall protein containing a leucine-rich repeat (LRR) and an extensin domain. Eleven and eight LRX (LRR/EXTENSIN) genes have been identified in these two plant species, respectively. The LRX gene family encodes proteins characterized by a short N-terminal domain, a domain with 10 LRRs, a cysteine-rich motif, and a variable C-terminal extensin-like domain. Phylogenetic analysis performed on the conserved domains indicates the existence of two major clades of LRX proteins that arose before the eudicot/monocot divergence and then diversified independently in each lineage. In Arabidopsis, gene expression studies by northern hybridization and promoter::uidA fusions showed that the two phylogenetic clades represent a specialization into "reproductive" and "vegetative" LRXs. The four Arabidopsis genes of the "reproductive" clade are specifically expressed in pollen, whereas the seven "vegetative" genes are predominantly expressed in various sporophytic tissues. This separation into two expression classes is also supported by previous studies on maize (Zea mays) and tomato (Lycopersicon esculentum) LRX homologs and by information on available rice ESTs. The strong conservation of the amino acids responsible for the putative recognition specificity of the LRR domain throughout the family suggests that the LRX proteins interact with similar ligands.With the completion of the Arabidopsis genome sequence, it became clear that many Arabidopsis genes are members of multigene families. Although this had already been suggested by the analysis of expressed sequence tag (EST) databases and by classical gene searches, the availability of the full gene set of a plant provides the unique opportunity to get a complete inventory of all the members of a gene family. Among the 25,500 genes predicted in the Arabidopsis genome, 65% are members of a multigene family and 37% belong to families of more than five members (Arabidopsis Genome Initiative, 2000). Although the predicted total gene number of Arabidopsis is significantly larger than that of other sequenced multicellular eukaryotes such as Caenorhabditis elegans (19,000; C. elegans Sequencing Consortium, 1998) or Drosophila melanogaster (13,600; Adams et al., 2000), the absolute number of gene families and singletons (11,601 in Arabidopsis) is comparable in all these organisms (Arabidopsis Genome Initiative, 2000). This indicates that frequent gene duplications and consequently large gene families are a distinctive feature of the Arabidopsis genome, and possibly of all plant genomes. With the recent publication of a high-quality draft from two different subspecies, rice (Oryza sativa) is the second plant whose genome can be comprehensively investigated. Depending on the stringency applied in gene prediction, the rice genome contains between 32,277 and 61,668 genes (Goff et al., 2002;Yu et al., 2002). With 77% of the genes distributed in about 15,000 m...