DEVELOPMENT OF PROMISING SUPERSOUR AND OTHER ROOTSTOCKS RESISTANT TO HLB (HUANGLONGBING)

The project includes four components with interrelated objectives: 1) Develop promising supersour rootstocks through sexual hybridization, screening and selection of progeny, and long term field trials; 2) Improve disease resistance of citrus through manipulation of Poncirus- related defense genes; 3) Produce and test transgenic rootsocks; 4) Evaluate high grafting as a management strategy for HLB (Huanglongbing) disease control. This research related to all of the objectives of the inhouse project: 1. Create new genetic combinations of citrus germplasm via conventional breeding, mutation, and transformation. 2. Screen germplasm for important traits and select superior individuals. 3. Evaluate selections for field performance and other traits. Detailed evaluation of specific defense-related citrus genes continued, including genes identified by expression studies as being associated with huanglongbing (HLB) response, such as CtCDR1 and CtPDF2. Constructs designed to alter expression of these citrus defense genes are being used to transform citrus for improvement of HLB tolerance and resistance, and derived transgenics will be tested using the pathogen. More than 200 new transgenic rootstock selections with potential resistance to HLB were produced this year, targeting increased expression of the citrus resistance genes CtNPR1, CtEDS1, CtMOD1, CtEDS5, CtPAD4, CtNDR1, and CtACD1. Eighteen new transgenic rootstocks with selected antimicrobial genes were propagated and entered into a replicated greenhouse test with Asian citrus psyllid (ACP) inoculation to assess tolerance to HLB. Evaluation and indexing of three previous groups of transgenic rootstocks under test with HLB continued, with selected transgenics showing some promise. A paper was published describing the overexpression of a citrus NDR1 ortholog and increased disease resistance.