Huanglongbing (HLB) has become the dominant disease threatening the Florida citrus industry. HLB affects all cultivated citrus varieties and causes a rapid decline of trees and produces unmarketable fruit. This disease is caused by a phloem limited nonindigenous bacterial pathogen and causes substantial economic losses to every commercial citrus cultivar. Incorporation of gene(s) via genetic engineering can potentially confer resistance in susceptible cultivars, while maintaining the varietal fidelity. Several antimicrobial peptide gene constructs (AMPs) and the SAR inducing genes NPR1 and SABP2 have been tested in commercial sweet orange cultivars with varying degrees of success. Field trials in an HLB endemic environment as well as greenhouse based insect vector transmission trials have identified a few genes that can potentially confer tolerance to HLB. Initially, the AMP genes looked promising, but the tolerance response faded over time. The most successful genes to date are the SAR induction genes NPR1 and SABP2, driven either by a constitutive d35S promoter or a phloem specific Arabidopsis SUC2 promoter. Independent transgenic lines with these genes continue to show tolerance to HLB after four years in the field at a challenging field site. We will also discuss our current citrus improvement strategies to develop more consumer-friendly transgenic plants by the utilization of plant based genetic constructs. We have developed a transformation system to generate reporter gene expression free citrus by coupling a visual citrus derived anthocyanin producing transcriptional factor gene with an embryo specific promoter to regenerate genetically modified plants that have the marker gene switched off. In addition, we are also evaluating a large number of citrus derived genes, promoters, genes and terminators to create an all citrus transformation vector. These constructs are also being incorporated using our protoplast transformation system, which allows the incorporation of linear DNA pieces into the citrus genome. HLB-resistant transgenic citrus containing all plant DNA should have quicker regulatory approval as well as improved public acceptance.

Progress on transgenic approaches to solving citrus greening disease

252nd National Meeting (2016)

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