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: 06/03/2020
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. Five rootstock 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. Similar crosses made in previous years have yielded several good candidates through the Gauntlet screen, and these new crosses were intended to explore new families or to expand on previously fruitful combinations. Two refereed manuscripts were published describing the positive impact of HLB-tolerant rootstocks from our program on juice quality and the metabolome, and the changes in the proteome of Valencia on tolerant vs. sensitive rootstocks. 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 2020 crosses for this objective were numerous. Forty-two interploid crosses were made for improving sweet orange-like, mandarin, grapefruit and acid fruit hybrids. Twelve crosses were made at the diploid level targeting sweet orange-like hybrid development. Embryo rescue from 2019 crosses resulted in shoots from >1750 germinating embryos from 36 interploid crosses (including 11 targeting sweet orange-like hybrids, and 8 targeting grapefruit improvement), all to be micrografted to rootstocks. Cybridization experiments were conducted to combine Meiwa kumquat cytoplasm with OLL and EV sweet oranges, to attempt improvements in citrus canker resistance; embryos have been recovered for next steps in plant regeneration. Somatic hybridization of Tango and W. Murcott suspension lines with leaf protoplasts of several CREC and other public cultivars, and advanced selections was attempted; several combinations already have produced embryos and a few shoots. Other materials, including grapefruit cybrids with Meiwa, and vigorous Vernia seedling selections, have been propagated for future field plating.  3. Screen our ever-growing germplasm collection for more tolerant types and evaluate fruit quality of candidate selections. We have explored new approaches to quantifying tree responses to HLB, in addition to the previously used subjective approaches. Specifically, we measured photosynthetic parameters and leaf canopy indexes, to produce repeatable and reliable quantitative data in support of further genetic analyses of tolerant types. Objective quantitative data of tree responses provides more reliable information that improves the precision with which we can associate genome regions with tolerance or sensitivity; see Obj 4. To finish the current fruit season, we have evaluated fruit quality of the more tolerant types of sweet orange-like hybrids, as well as mandarins and grapefruit hybrids, and selected candidates in all categories 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. We identified a set of ~ 500 individuals for GWAS studies, using the data referred to in Obj 3 above. Despite UF-CREC closure, we were able to collect leaf samples from these trees and DNA samples have been prepared to send to a commercial outfit to perform the SNP chip analysis. This work will validate previously identified, or to identify new, genomic regions associated with HLB tolerance 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 carried out. Finally, a renewal application has been submitted to the USDA for our multi-location transgenic field permit, to enable the program to continue to explore the impact of certain genetic modifications on HLB incidence, disease development, and potential tolerance or resistance, under real world field conditions.       

Your browser does not support pdfs, click here to download the file.