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: 12/02/2021
Project: 18-011   Year: 2021
Category: Plant Improvement
Author: Fred Gmitter
Sponsor: Citrus Research and Development Foundation

1. Develop new rootstocks that impart HLB-tolerance to scion cultivars. Seed from a first group of rootstock crosses was harvested and planted in the calcareous/Phytophthora soil as the first step in the `gauntlet’ screen; parents included several previously selected but unreleased HLB-tolerant rootstocks, as well as some of the UFRs, HLB-tolerant pummelos, and US-897 and US-942.  Seed were harvested from a second group of crosses, using LB8-9 Sugar Belle® as a seed parent with pollen from various hybrids of Poncirus trifoliata with citrus accessions, Citrus ichangensis (Ci), different Cleopatra mandarin x Ci hybrids, and a Palestine sweet lime x Ci hybrid; these will be planted next into the `gauntlet’ screen. In collaboration with researchers at IFAPA in Spain, new information has been generated regarding performance of selected UFRs and other unreleased rootstock hybrids from our program in response to drought and flooding, Phytophthora, and salinity. Three new rootstock candidates were entered into the Parent Tree Program, including the first LB8-9 Sugar Belle® x trifoliate orange hybrid selected through the `gauntlet’ pipeline.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. We removed previously planted and tested scions from our program, grown in the Trailer Park block, that do not warrant further scrutiny. We planted 60 new scion selections from the program that have gone through the DPI PTP cleanup and certification; these include true oranges and orange-like hybrids, grapefruit and hybrids, mandarins, lemons, pummelos, and acid fruit. All trees are on UFR-5 rootstock. Following extensive phenotyping of a replicated planting of hybrids between Monreal Clementine and an accession of Citrus latipes (perhaps the most HLB-tolerant citrus), we have found at least two hybrids that remain PCR-negative after 6 years under high pressure in the field. They fruited last season and produced large fruit, somewhat resembling sweet orange but with high acidity. Pollen was collected from these and used to make crosses this past spring with two low-acid selections in our breeding program. Several thousand seeds were produced and will be planted soon. We plan to screen the seedlings with markers previously developed for fruit quality, acid content, and potential HLB tolerance. We completed micrografting somaclone seedling-derived populations of early maturing (January), high soluble solids OLL sweet orange clones to UFR-4 rootstock liners, in efforts to generate an even earlier maturing OLL clone.  More than 100 individual seedlings successfully grafted to UFR-4 were stepped up to 4×4 citripots for spring planting at St. Helena. Parent Tree Program scion entries included 2 HLB-tolerant Valencia mutants produced by irradiation and selected from a small, replicated trial, and 1 September-maturing mandarin hybrid. Finally, 125 triploid hybrids, including sweet orange-like, mandarins and grapefruit, were prepared for field planting at the CREC.  3. Screen our ever-growing germplasm collection for more tolerant types and evaluate fruit quality of candidate selections. We have more than 70 5-year-old-trees of `Marathon’ mandarin on sour orange, that set huge crops of fruit this season. Although all trees have HLB, there are few to no obvious disease symptoms in fruit, leaves, or canopy, demonstrating a high degree of tolerance thus far. We have followed closely their performance, and individual trees yielded more than 300 pounds of fruit. A first harvest was made in late September, and another planned for October. We will determine fruit size distribution, and we will follow post-harvest behavior and fruit quality. 4. Conduct studies to unravel host responses to CLas and select targets for genetic manipulations leading to consumer-friendly new scion and rootstock cultivars. 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 are underway with several sweet oranges, grapefruit, and rootstocks. Finally, we have completed metabolomic studies, in collaboration with Dr. Y. Wang, to gain insight into the underlying mechanisms of HLB-tolerance and sensitivity; please see doi: 10.3389/fpls.2021.710598 and doi: 10.1021/acs.jafc.1c02875.   

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