Overexpression of the Arabidopsis NPR1 protein in citrus confers tolerance to Huanglongbing

Huanglongbing (HLB), also known as citrus greening, is one of the most destructive diseases of citrus worldwide. It is caused by unculturable phloem-limited bacteria that belong to the Candidatus Liberibacter genus including Ca. L. asiaticus ( C Las), Ca. L. africanus, and Ca. L. americanus. Currently, there is no effective control strategy for HLB and no known cure for the disease. We have previously generated transgenic ‘Duncan’ grapefruit and ‘Hamlin’ sweet orange expressing the Arabidopsis NPR1 ( AtNPR1 ) gene, which encodes a master regulator of systemic acquired resistance. Characterization of the transgenic lines indicated that overexpression of AtNPR1 confers resistance to citrus canker, another serious bacterial disease in citrus. In this study, we intensively screened these transgenic lines for resistance or tolerance to HLB under greenhouse conditions. Three independent transgenic lines (one line in the ‘Duncan’ grapefruit background and two lines in the ‘Hamlin’ sweet orange background) only occasionally displayed mild HLB symptoms in the presence of C Las and have kept growing normally over a period of nine years. Significantly, all vegetatively propagated progeny plants of these lines have retained the same levels of HLB tolerance. Furthermore, immunoblot analysis revealed that the three transgenic lines accumulate high levels of AtNPR1 protein. These results, together with the previous finding that the same three transgenic lines are resistant to citrus canker, demonstrate that overexpression of the AtNPR1 protein in citrus is able to provide robust tolerance to HLB.