Progress Towards the Development of Transgenic Disease Resistance in Citrus

Florida is the world’s third largest producer of citrus, behind Brazil and China. This 9 billion dollar industry is now under siege by two important diseases citrus greening, also known as huanglongbing (HLB), and citrus canker. Both these diseases are caused by gram negative bacteria. Canker is primarily affecting the grapefruit industry and poses problems with the establishment of new sweet orange groves. It can be managed by following a canker suppression program, whereas HLB affects all cultivated citrus varieties and cannot currently be controlled. Resistance to either HLB or canker is also not present in commercial orange and grapefruit cultivars. A major strategy is to develop resistant cultivars via genetic engineering by incorporating resistance genes not found in citrus. We have successfully cloned several natural and synthetic antibacterial genes and genes that have the potential to turn on SAR (Systemic Acquired Resistance) and made significant progress in introducing them into commercial sweet orange and grapefruit cultivars. We have regenerated hundreds of transgenic citrus plants, using both the standard Agrobacterium-mediated transformation system and the protoplast/GFP transformation system developed previously in our program. Genetic constructs containing promoters that target expression of the above gene(s) exclusively to the phloem tissue, where HLB resides, are also being utilized in efforts to minimize foreign gene expression in fruit or juice subsequently going to market. Progress towards the development and testing of such transgenic disease resistance in commercial citrus cultivars will be discussed. INTRODUCTION Florida has been dealing with several common citrus diseases for many years, including CTV-induced quick-decline, citrus blight (unknown pathogen) and Phytophthora root rot. The impact of these diseases has been minimized by rootstock selection and more rigorous nursery regulations. More recently, two much more serious diseases have found their way into Florida. The first to arrive was citrus canker, a disease that is severely damaging the Florida fresh grapefruit industry. It does not kill trees, but can reduce yield and marketability of fruit. An effort by the state government to eradicate this disease (Schubert et al., 2001) failed, and the disease has since been widespread by hurricanes. The second disease to enter Florida, along with its insect vector the citrus psyllid, is citrus greening, also known as huanglongbing (HLB), caused by the phloemlimited bacterium Candidatus Liberibacter asiaticus. This disease is truly industry threatening because it spreads quickly and renders trees worthless. It has already spread to all citrus producing areas of the state and devastated entire counties. Both diseases are caused by gram negative bacteria, and the development of resistant sweet orange and grapefruit cultivars by conventional methods is not an option. Thus, several biotechnology-based approaches are being utilized in efforts to develop mediumand long-term solutions to both of these diseases. We are using somatic cybridization in Proc. II IS on Citrus Biotechnology Eds.: A. Gentile and S. La Malfa Acta Hort. 892, ISHS 2011