2018 ASHS Annual Conference

Candidatus Liberibacter asiaticus (CLas) is the associated agent of Huanglongbing (HLB), the most serious and impactful disease of citrus worldwide. In previous work, we identified a population of antigen-binding fragments that interact with two CLas proteins that were predicted to play a vital role in the infection process. These proteins are homologs of the virulence gene InvA facilitating bacterial invasion of tissue layers and TolC which mediates removal of toxins from the bacterial cell. These antigen-binding fragments were transformed into Carrizo citrange under control of a modified 2x35s promoter. Confirmed transgenic plants were subjected to infestation and subsequent bacterial infection by CLas carrying Asian Citrus Psyllids (ACP), the insect vector of HLB. Bacterial titer was measured over a nine month period. Transgenic plants showed significantly reduced bacterial titers as measured by quantitative PCR analysis. Expression of the antigen-binding fragments resulted in average reductions in bacterial titer by 8.5 Ct (99.8 %) for InvA and 6 Ct (99%) for TolC targets. Transgenic populations also showed a much higher proportion of plants with non-detected CLas when compared to wild type controls, and these were not included in the statistical analysis. Transgenic plants expressing five additional antigen-binding fragments are currently undergoing ACP inoculation. Transgenic rootstocks are also being grafted with wild type scions for greenhouse and field testing of HLB-resistance.