Engineering Resistance Against Citrus Disease Using Candidate Genes

Citrus canker is a devastating disease caused by Xanthomonas axonopodis pv. citri (Xac).  The NPR1 gene plays a pivotal role in systemic acquired resistance (SAR) in Arabidopsis.  We report the isolation and characterization of an NPR1 homolog from citrus, namely Citrus NPR1 homolog 1 (CtNH1).  When over-expressed in citrus, CtNH1 confers resistance to Xac and leads to constitutive expression of the pathogenesis-related (PR) gene chitinase 1 (Chi1), suggesting that CtNH1 is orthologous to NPR1.  In addition, we recently identified two closely-related citrus genes, named Xbct31 and Xbct32.  Database searching and sequence analyses reveals that other plant species, including rice, Arabidopsis, tomato, and Nicotiana benthamiana, contain homologs of these two citrus genes and they share high levels of sequence identity at the amino acid level.  When overexpressed in Nicotiana benthamiana via Agrobacterium infiltration-mediated transient expression, the citrus genes as well as their closely-related homologs from rice and Arabidopsis all trigger (HR)-like cell death hypersensitive response.  Since HR is often associated with resistance (R) gene-mediated immunity, our data suggests Xbct3s represent a family of evolutionarily conserved defense regulators and could be used to heighten defense against citrus diseases including greening and canker