Localized expression of antimicrobial proteins mitigates huanglongbing symptoms in Mexican lime

Citrus huanglongbing (HLB) is a devastating disease associated with Candidatus Liberibacter asiaticus spp. (CLas), a bacterium restricted to the sieve tube system of the phloem that is transmitted by the psyllid vector, Diaphorina citri. In this study, the human antimicrobial peptides, lysozyme and β-defensin 2, were targeted to the vascular tissue of Mexican lime (Citrus x aurantifolia [Christm.] Swingle) by fusion to a phloem-restricted protein. Localized expression was achieved, via Agrobacterium tumefaciens-mediated transformation of the stem, which led to protein expression and mobilization within the vascular tissue of heterotrophic tissues. HLB-infected plants were monitored for 360 days. Lower bacteria titers were observed in plants expressing either β-defensin 2, lysozyme, or the combination thereof, and these plants had increased photosynthesis, compared to untreated control trees. Thus, targeting of antimicrobial proteins to the vascular tissue was effective in decreasing CLas titer, and alleviating citrus greening symptoms. Based on these findings, this strategy could be used to effectively treat plants that are already infected with bacterial pathogens that reside in the phloem translocation stream.