Multiple Approaches for Genetic Engineering of Citrus for Disease and Pest Resistance

Citrus in Florida is threatened by non-indigenous but now endemic bacterial pathogens causing the diseases Huanglongbing (HLB, caused by Candidatus Liberibacter asiaticus) and citrus canker (caused by Xanthomonas citri ssp. citri). These diseases have the potential to wipe out the entire industry due to unavailability of resistance in commercially cultivated Florida cultivars.  HLB is vectored by the Asian citrus psyllid (Diaphorina citri Kuwayama), while canker is dispersed by windblown rain, contaminated equipment or human activity.  In efforts to control the diseases, several transgenic strategies have been adopted. Citrus scion and rootstock cultivars were transformed with gene(s) encoding antimicrobial peptides, systemic acquired resistance (SAR) proteins or insecticidal proteins. Each gene was under control of an enhanced CaMV 35S promoter. Several genes were also under control of a phloem specific Arabidopsis SUC2 (AtSUC2) promoter as C. Liberibacter asiaticus is a phloem limited bacterium and targeting gene expression in the phloem potentially resolves issues of presence of transprotein in the fruit. Molecular analysis confirmed the presence of the gene(s) in the citrus genome. We employed a rapid micrografting technique on tender rootstocks to quickly propagate our transgenic lines. Results from preliminary disease resistance tests suggested that some of the antimicrobial peptides could provide varying levels of resistance against both HLB and canker. Several studies are currently underway to evaluate resistance to either of the pathogens and also to the insect vector.