Characterization of LRP, a leucine-rich repeat (LRR) protein from tomato plants that is processed during pathogenesis

This paper describes the isolation and characterization of LRP, a new gene from tomato plants. The deduced amino acid sequence showed that the encoded protein is enriched in leucine, and contains interesting structural motifs. LRP contains four tandem repeats of a canonical 24 amino acid leucine-rich repeat (LRR) sequence present in different proteins that mediates molecular recognition and/or interaction processes. Genomic organization and intron-exon arrangement of LRP favor the hypothesis that the LRR domains present in LRP evolved by exon duplication and shuffling. LRP expression analysis and immunohistochemical localization studies of the encoded protein indicate that the gene is under developmental regulation exhibiting tissue-specificity, particularly in certain cell types of the stele, like phloem fibers, parenchyma cells of the protoxylem, and in the cell files that constitute the rays of the secondary xylem. It is shown that this gene is upregulated in diseased tomato plants infected with citrus exocortis viroid. However, in this pathogenic context, LRP is processed proteolytically to a lower molecular weight form by a host-induced extracellular protease. The structural characteristics of LRP, its spatio-temporal pattern of expression, and its post-translational processing during pathogenesis, suggest this protein as a candidate molecule that may mediate recognition and interaction events taking place in the plant extracellular matrix under normal and/or pathogenesis-related conditions.