Identification of a novel pathogen-induced gene encoding a leucine-rich repeat protein expressed in phloem cells of Capsicum annuum

The CALRR1 gene, expressed in pepper leaves following infection by Xanthomonas campestris pv. vesicatoria, encodes a secreted leucine-rich repeat (LRR) with five tandem repeats of a 24-amino-acid LRR motif. Northern blot analyses revealed that CALRR1 is not constitutively expressed in pepper plants, but is strongly induced upon the infection by X. campestris pv. vesicatoria, Phytophthora capsici, Colletotrichum coccodes and Colletotrichum gloeosporioides on leaves. CALRR1 was not systemically induced in upper leaves by bacterial infection. The inoculation of bacterial live cells, and treatment with dead cells and culture filtrates of pathogenic or nonpathogenic bacteria triggered the accumulation of CALRR1 transcripts. Treatment with signaling molecules, including salicylic acid (SA), ethylene (ET), methyl jasmonate (MeJA), DL-β-amino-n-butyric acid (BABA) and benzothiadiazole (BTH), did not activate the transcription of the CALRR1 gene, indicating that CALRR1 expression is not regulated by defense signaling pathways activated by these molecules. CALRR1 was induced by treatment with high salinity, abscisic acid (ABA) and wounding, but not by drought and cold stress. An in situ hybridization study showed that CALRR1 mRNA was localized in phloem tissues of leaves, stems, and green fruits of pepper plants during the pathogen infection and ABA exposure. The location characteristics and the spatio-temporal expression pattern of CALRR1 suggest that it may play a role in protecting phloem cells against biotic and abiotic stresses affecting phloem function.