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: 08/13/2021
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. Stepped up 160 candidate rootstock hybrids from the first step of the “Gauntlet” screen (high pH, calcareous soil inoculated with Phytophthora), from different crosses into citripots; crosses were made using LB8-9 (Sugar Belle®) as a seed parent with either trifoliate hybrids or salt tolerant sour orange (pummelo-mandarin hybrids) types. Three new “Gauntlet” rootstock candidates, all showing very good HLB tolerance and possibly resistance, were entered into the PTP. These hybrids arose from similar crosses as described above, combining genetics from pummelo, Shekwasha mandarin, LB8-9, and trifoliate orange. All 3 hybrids are showing excellent health and ability to suppress CLas replication in their root systems and in grafted Valencia scion.  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 are in the process of removing previously planted and tested scions from our program that do not warrant further scrutiny, grown in the Trailer Park block. Newly available spaces will be planted in late summer with 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 have been grown on UFR-5 by a commercial nursery. Somaclone seedling derived populations of January-maturing OLL sweet orange clones are being propagated on UFR-4 rootstock, to attempt to produce an even earlier maturing clone; to date >100 individual seedlings have been propagated.3. Screen our ever-growing germplasm collection for more tolerant types and evaluate fruit quality of candidate selections. We have explored some new approaches to quantifying tree responses to HLB, in addition to the previously used subjective approaches. Specifically, we have begun measuring photosynthetic parameters and leaf canopy indexes, to produce repeatable and reliable quantitative data in support of further genetic analyses of tolerant types. Analysis of collected data is underway, in consultations with plant physiologists familiar with the techniques and data interpretation. This work will improve the precision with which we can define HLB tolerance genes. 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 are conducting additional GWAS to validate previously identified, or to identify new, genomic regions associated with HLB tolerance and/or sensitivity. 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 have been begun. Finally, the very early response of citrus to the CLas, vectored by infective ACP, was evaluated for the first time, thus allowing the changes in gene expression relating to the primary mechanisms of susceptibility and host -pathogen interactions to be studied, and without the secondary effects caused by the development of complex whole plant symptoms; please see doi: 10.3389/fpls.2021.635153.    

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