New insights into the resistance of Nagami kumquat to canker disease

Asiatic citrus canker, caused by the bacterial species Xanthomonas axonopodis pv. citri (Xac A), has had a detrimental impact on citrus production in various regions of the world. It has been suggested, based on reported field observations and other anecdotal evidence, that some citrus relatives, e.g. kumquats, have higher levels of tolerance to Xac A compared to commercial varieties. However, this has not been confirmed with rigorous scientific evidence. In this study, the growth rate of Xac A in challenged kumquat leaves was significantly hindered after 4 days of infection compared to that in susceptible grapefruit leaves. Cell death was observed in kumquat leaves however, typical symptoms of a hypersensitive response (HR) were not apparent until 3–4 days post-inoculation. Abscission of the leaves occured 5–7 days after infection, depending on the age of the leaf. The HR was accompanied by differential gene expression: two suppression subtractive cDNA libraries were constructed using Nagami kumquat mRNA to identify genes that were differentially expressed due to infection. A number of cDNA clones were randomly selected for sequencing. Homologues to components such as transcription factors, receptors and defense genes known to be involved in plant–pathogen interactions in general and plant cell death were identified.