Molecular characterization of a novel pathogen-responsive receptor kinase-like in Citrus limon

Infection by pathogenic microorganisms is a source of biological stress in plants. Understanding the interaction between plants and microbial infection at molecular level might contribute to our understanding for the effective control of the disease. Here, we isolated a novel putative receptor kinase-like protein (Citrus limon P5) that is the first Lec-receptor kinase-like protein isolated in lemon during a pathogen infection. C. limon P5 cDNA fragment was detected by differential display assay in C. limon during Capnodium citri (sooty mold) infection. The deduced amino acid sequence of P5 full-length cDNA revealed a 83% sequence homology with a receptor kinase-like protein from Arabidopsis thaliana characterized by a N-terminal lectin domain and C-terminal serine/threonine conserved domain. The inhibition of the pathogen-responsive P5 by the protein kinase inhibitor staurosporine and by a mitogen-activated protein kinase inhibitor (PD 98059) indicated a defense mechanism in C. limon against pathogens mediated via signal transduction pathway. To our knowledge, this is the first evidence that C. limon uses this putative defense mechanism against pathogens. The role of this protein is discussed as a starting point to understanding the molecular mechanisms in the C. limon in response to C. citri infection.