Mature Citrus Transformation to Combat Citrus Diseases in Florida

Citrus canker, caused by Xanthomonas citri, and HLB or greening, thought to be caused by Candidatus liberibacter asiaticus, cause serious economic losses in the Florida citrus industry.   Traditional breeding programs are lengthy endeavors because the time for flowering and fruit production of new cultivars can range from 5 to 20 years. Mature tissue transformation is a relatively rapid means of genetic improvement that preserves early flowering and fruiting of the scion or rootstock. Mature rootstock and scion transgenics flower ~13 and 19 months after the transformation event, respectively. The transcriptional co-regulator, non-expresser of pathogenesis-related genes, NPR1, plays a central role in the activation of salicylic acid dependent defense genes which increase systemic acquired resistance (SAR).  NPR1 is a candidate gene for conferring disease tolerance to canker and greening. The Arabidopsis thaliana NPR1 gene was transformed into mature scion cultivars, Hamlin, Valencia and Pineapple, and the rootstock cultivars, Swingle and Carrizo, via Agrobacterium-mediated transformation. PCR amplification of NPR1 using gene specific primers indicated that all the transgenic lines contain the NPR1 transgenes. Southern blot analyses confirmed that the transgenes were primarily low copy number and integrated into the plant genomes. These transgenic lines will be challenged by the pathogens in the future and assessed for tolerance.