Part A – The UF/CREC Core Citrus Improvement Program (Complementary to Part B – The UF/CREC Citrus Improvement Program’s Field Trial Evaluations)

Part A - The UF/CREC Core Citrus Improvement Program (Complementary to Part B - The UF/CREC Citrus Improvement Program's Field Trial Evaluations)

Report Date: 04/06/2020
Project: 18-011   Year: 2020
Category: Plant Improvement
Author: Fred Gmitter
Sponsor: Citrus Research and Development Foundation

1. Develop new rootstocks that impart HLB-tolerance to scion cultivars. Seeds of twelve new rootstock crosses made in spring 2019 were harvested, planted into the calcareous soil/ Phytophthora screen, the first stage of the “gauntlet” screening protocol, and good performers have been identified and moved forward in the Gauntlet process. Seeds harvested from the UFRs have been distributed to interested nurseries. Seeds from various unreleased rootstock candidates, showing good performance (HLB tolerance, good yields and fruit quality, some tree size controlling with more efficient canopies) in several field trials throughout the industry were harvested to be available for future trials with interested industry partners in Florida, as well as for new trials in other citrus production areas. 2. Develop new, HLB-tolerant scion cultivars from sweet orange germplasm, as well as other important fruit types such as grapefruit, mandarins, and acid fruit. Spring 2019 crosses for this objective were numerous. We harvested fruit from more than 2 dozen interploid crosses, including several designed to produce sweet orange-like fruit, using HLB tolerant parents selected by our program previously. In addition, selected HLB and canker tolerant pummelo breeding parents were crossed with diploid and tetraploid grapefruit, to produce new grapefruit hybrids with enhanced tolerance of HLB. Embryo rescue has been completed and plant production has proceeded. Over 100 new somaclones have been regenerated in vitro from EV1 and EV2, for further selection; these are being grown off for propagation and eventual field planting. Six new grapefruit hybrid selections were made in this autumn 2019 and January 2020 that produce fruit very similar to grapefruit in appearance, color and flavor, but with improved fruit quality attributes and substantially more HLB-tolerant trees; many of these have been entered into the DPI Parent Tree Program for cleanup. We worked together with USDA and citrus industry colleagues to write a manuscript titled: “Rationale for reconsidering current regulations restricting the use of hybrids in orange juice”; this was a joint effort led by Drs. Stover and Gmitter, and submitted to a major international horticulture journal, Horticulture Research (under the Nature banner). The objective of this manuscript is to work together with the Florida Citrus Processers Association, to lay the groundwork for a revision of the current Standards of Identity for Orange Juice, to enable use of HLB-tolerant sweet orange-like hybrids in OJ. We facilitated the commercial harvest of grove run Sugar Belle® fruit for commercial evaluation by a major juice processor, to assess its ability to be transported in trailers to the processing plant, and the quality of its juice through the processing process. This was done to support the interests of some industry players to consider this most HLB-tolerant variety for blending to improve OJ color, flavor, and overall quality 3. Screen our ever-growing germplasm collection for more tolerant types and evaluate fruit quality of candidate selections. We have collected extensive data using new approaches to quantify tree responses to HLB, in addition to the previously used subjective approaches. Specifically, we have measured photosynthetic parameters and leaf canopy indexes, to produce repeatable and reliable quantitative data in support of further genetic analyses of tolerant types. Analysis of these data is underway, and we are comparing the results from different methods. We are looking for consistency in the rankings and weighing this against the simplicity and efficiency of collecting data. Continuing through the fruit harvest season, we have evaluated fruit quality of the more tolerant types of sweet orange-like hybrids, as well as mandarins and grapefruit hybrids, and identified some worthy of further evaluation as potential new cultivars.  4. Conduct studies to unravel host responses to CLas and select targets for genetic manipulations leading to consumer-friendly new scion and rootstock cultivars. Using the quantitative data described in 3. above, we began additional GWAS to validate previously identified or to identify new genomic regions associated with HLB tolerance and/or sensitivity; results will be analyzed in the coming months. Several new genetic constructs have been developed using newly identified citrus specific promoters (phloem and root tissue), and new putative disease resistance genes, or downstream genes. Transgenic plants have been produced with some of these constructs, and additional transformation experiments were carried out.     

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