POSITIONAL CLONING AND ANALYSIS OF THE Citrus tristeza Virus RESISTANCE GENE

¹ Department of Plant Pathology and Microbiology, Texas A&M University Agricultural Experiment Station, 2415 East Hwy 83, Weslaco, TX, 78596, USA

² Department of Botany and Plant Science, University of California, Riverside, CA 92521, USA

Citrus tristeza virus (CTV) can be a devastating disease of citrus, causing economic losses by killing trees or reducing fruit size. Aphids transmit the virus making control difficult. All commercially grown citrus varieties are susceptible to CTV, but the level of damage varies with cultivar and viral strain. Development and use of resistant varieties could minimize damage to new plantings. The overall objective of this project is to use positional cloning methods to isolate a dominant gene (Ctv) from the trifoliate orange that causes resistance to the virus. The gene will then be transformed into CTV-susceptible citrus cultivars to produce virus-resistant plants. Initial genetic and physical mapping work completed during this project delimited the region that must contain Ctv to a genomic region of 300 kb, and complete sequencing of this region was completed to 8X coverage. New genetic markers identified from sequencing this region have allowed to us further delimit the region that must contain Ctv to 173 kb and that contains only 12 candidate genes. Further genetic mapping and phenotypic analysis of new plants segregating for CTV resistance has allowed us to even further delimit the region that must contain Ctv to a genomic region of 121 kb that contains only 10 genes. Transformation with the initial 12 candidate genes has led to the regeneration of over 100 shoots that are presumably transgenic. Progress to evaluate the effects of these transgenes on resistance to CTV will be presented.