1a. Objectives (from AD-416):
Develop new citrus rootstocks with resistance or tolerance to Huanglongbing disease.
1b. Approach (from AD-416):
Sexual hybridization, transformation, laboratory testing, greenhouse screening, field testing.
3. Progress Report:
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. Fruit quality, yield, tree health, and tree size data were collected from eight rootstock field trials. All trials appear significantly affected by huanglongbing (HLB). Some selections, including US-942, appear to perform better than other standard rootstocks in locations being affected by huanglongbing (HLB). Rootstocks that appear to have more tolerance to HLB were identified, and selected rootstocks propagated for increase and inclusion into more and larger scale field trials. Fruit quality data was collected at multiple harvest dates for a large grapefruit rootstock trial in Indian River County. Significant differences were observed among the rootstocks for many fruit quality traits. For example, sour orange induced an intermediate level of total soluble solids early season and relatively high solids late season. US-897 induced the highest soluble solids early season, and US-812 induced the highest soluble solids late season, while X-639 and US-852 rootstocks induced the lowest soluble solids throughout the season. A field trial with new supersour rootstocks was planted at a site in Indian River County and will be used to evaluate trees for tolerance of Diaprepes, Phytophthora, and huanglongbing (HLB). Rootstock liners were budded in preparation for planting in three new rootstock field trials later this year. Several thousand propagations of supersour rootstocks were prepared for budding and use in field trials. Cooperative work continued with a commercial nursery to multiply promising supersour rootstocks to prepare trees for medium-scale commercial plantings. Work continues to assess supersour tolerance of Citrus tristeza virus (CTV), salinity, and calcareous soils. The most promising new USDA rootstocks were identified for a cooperative effort to place new rootstocks with HLB tolerance into larger scale commercial plantings. 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 huanglongbing (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. Presentations were made at Florida Citrus Show, Ft. Pierce and Florida Citrus Growers Institute, Avon Park, with exciting new information regarding rootstock effect on tree performance with HLB. Progress was monitored by conference calls, emails, meetings, and reports to the original funding agency, Citrus Research and Development Foundation.