Biological and molecular characterization of linalool-mediated field resistance against Xanthomonas citri subsp. citri in citrus trees

The biological and molecular traits of the Ponkan mandarin (Citrus reticulata Blanco) were characterized in an investigation of the mechanisms of field resistance against citrus canker disease caused by the bacterial pathogen, Xanthomonas citri subsp. citri (Xcc). Various conventional citrus varieties that show diverse responses to Xcc were investigated, and the temporal changes in Xcc titer in response to linalool concentrations among the varieties revealed differences in Xcc proliferation trends in the inoculated leaves of the immune, field-resistant and susceptible varieties. In addition, increased linalool accumulation was inversely related to Xcc titers in the field-resistant varieties, which is likely caused by host--pathogen interactions. Quantitative trait locus (QTL) analysis using the F1 population of the resistant Ponkan mandarin and susceptible ‘Harehime’ (‘E-647’ × ‘Miyagawa-wase’) cultivar revealed that linalool accumulation and Xcc susceptibility QTLs overlapped. These results provide novel insights into the molecular mechanisms of linalool-mediated field resistance to Xcc, and suggest that high linalool concentrations in leaves has an antibacterial effect and becomes a candidate-biomarker target for citrus breeding to produce seedlings with linalool-mediated field resistance against Xcc.