Progress Using Biotechnological Approaches to Improve the Disease Resistance of Commercial Citrus

The 9 billion dollar/year Florida citrus 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 non-GMO biotechnology-based strategy for the development of canker resistant grapefruit is to transfer the cytoplasm of kumquat (believed to contain a gene(s) for canker resistance) to grapefruit by the process of somatic cybridization. Cybrid citrus plants can be produced as a by-product of somatic fusion. Putative cybrid grapefruit plants from several commercial cultivars containing kumquat cytoplasm have been produced and are being characterized. Some of these plants are showing significantly improved canker resistance in a detached leaf assay. Progress with this work will be presented. Another primary 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 (greenhouse and field) of such transgenic disease resistance in commercial citrus cultivars will be discussed.