919 Evaluation of Transgenic Citrus for Disease Resistance to HLB and Canker Manjul Dutt 1,* , Gary A. Barthe 1 , Vladimir Orbovic 1 , Michael Irey 2 and Jude W. Grosser 1 1 University of Florida, IFAS, Citrus Research and Education Center, Lake Alfred, FL, USA 2 US Sugar Corporation, Clewiston, FL, USA *Corresponding author: manjul@ufl.edu Abstract Sweet orange and grapefruit cultivars were transformed with gene(s) encoding antimicrobial peptides or systemic acquired resistance (SAR) proteins. Each gene was under control of an enhanced CaMV 35S promoter. Several genes were also under control of a phloem specific Arabidopsis SUC2 (AtSUC2) promoter. A number of clones of each transgenic line were evaluated for resistance to Huanglongbing (HLB, caused by Candidatus Liberibacter asiaticus) and citrus canker (caused by Xanthomonas citri ssp. citri). Six hundred trees planted in spring 2009 in a heavily HLB and canker infected Martin County, FL grove were tested using qPCR for HLB after 30 months in the field. Three hundred ninety-six trees tested negative for the HLB bacterium. Approximately 200 PCR negative trees were observed to be healthy and flushing after 40 months in the field and were again evaluated using qPCR. In addition, we observed several trees containing one of our antimicrobial constructs to be tolerant to citrus canker. These results suggested that some of the antimicrobial peptides/SAR inducing genes could provide varying levels of resistance against both HLB and canker. Keywords: Agrobacterium mediated transformation, citrus bacterial diseases Huanglongbing INTRODUCTION In recent years, two serious citrus diseases have found their way into Florida. These diseases, Huanglongbing [HLB, caused by Candidatus Liberibacter asiaticus (CLas)] and citrus canker (caused by Xanthomonas citri ssp. citri), are incited by gram negative bacteria and result in substantial economic losses for the citrus industry. Eradication of citrus canker has failed, and the disease is now widespread (Schubert et al., 2001). The other disease, HLB is truly industry threatening because it spreads quickly. This disease results in the development of bitter-tasting and undersized fruit, thereby rendering it worthless. None of the commercial citrus cultivars are resistant to HLB. All grapefruit (Citrus paradisi Macf.) and most sweet orange (C. sinensis (L.) Osb.) cultivars are susceptible to canker, whereas mandarins (C. reticulata Blanco) are tolerant and kumquat (Fortunella spp.) is resistant (Schubert et al., 2001; Brunings and Gabriel, 2003). Therefore, the rapid development of canker- and/or HLB-resistant sweet orange and grapefruit cultivars by conventional breeding methods is not an option. The development of transgenic citrus, which is the result of the deliberate introduction of one or more traits through genetic transformation, offers an opportunity for the improvement of a desirable cultivar without otherwise changing its genotypic and phenotypic makeup. Genetic engineering also presents the ability to produce citrus plants with resistance to bacterial disease(s) by the incorporation of resistance gene(s) from sexually compatible or incompatible plant species or other organisms. Recently, several antimicrobial peptides have been demonstrated to neutralize bacteria and fungi. An important group of antimicrobial peptides is the cecropins, which are the base of several of the gene constructs we have developed (except the AtNPR1 gene). In general, gram-negative bacteria are more sensitive to cecropins than gram-positive ones. In addition, cecropin derivatives, like CEME and CEMA, permeabilize bacterial outer membranes of gram- Proc. XII th Intl. Citrus Congress Eds.: B. Sabater-Muñoz et al. Acta Hort. 1065, ISHS 2015