Testing grapefruit trees expressing an anti-NodT antibody for resistance to HLB

Testing grapefruit trees expressing an anti-NodT antibody for resistance to HLB

Report Date: 10/01/2020
Project: 18-016   Year: 2020
Category: Plant Improvement
Author: Tim McNellis
Sponsor: Citrus Research and Development Foundation

 1. Please state project objectives and what work was done this quarter to address them: .This report covers the period of June 1 – August 31, 2020.  The objective of this project is to test transgenic ‘Ducan’ grapefruit trees expressing an anti-HLB antibody fused to the FT (Flowering Locus T) protein.  Work on the project was slowed a bit by COVID-19 restrictions.  However, tissue sampling and phenotypic analysis was continued on three HLB inoculation tests underway for the FT-scFv plants:  a field trial natural inoculation; an Asian citrus psyllid (ACP) infection in the greenhouse; and a graft challenge with FT-scFv scions grafted to HLB-infected rough lemon rootstocks.  Samples taken from the field trial trees were all negative for ‘Candidatus Liberibacter asiaticus’ (CLas) infection by qPCR at the 6-month point after planting the trees at the test site in November of 2019.  The graft transmission test has yielded promising initial data.  While 93% of the FT-scFv scions grew out and were healthy and vigorous with only slight to no HLB symptoms, only 67% of the control buds grew out, and those that did grow were relatively short and slow growing.  The non-growing buds remained green, indicating a successful grafting, but failed to grow.  This raises the possibility that the FT-scFv protein will counteract growth of CLas when the plant is mounting a defense reaction to Clas, as may be the case because rough lemon is tolerant to CLas.  Also during this reporting period, a manuscript was written, revised, and then accepted for publication in Plant Biotechnology Journal describing our finding that the FT-scFv transgenic plants have a strong precocious blooming phenotype (https://doi.org/10.1111/pbi.13463).  This is a very useful output of the project that will enable rapid-cycle breeding in citrus.  Citrus breeding programs are already using the technique developed through this project to accelerate breeding for HLB resistance.  Although this outcome was not an original objective of the project, it is a fortunate finding that solves a long-standing challenge of producing precocious edible citrus varieties.    2. Please state what work is anticipated for next quarter: Next quarter we anticipate that the graduate student will be able to visit ACP vector infected trees at Fort Pierce in November, 2020.  These trees will be moved from the United States Horticultural Research Laboratory to the Indian River Research and Extension Center next door the week of October 12-16, 2020.  This will allow us to have direct access to these trees.  We will also continue sampling of tissues an qPCR monitoring of CLas titers in trees from all tests, and measure phenotypes of FT-scFv scions grafted onto the HLB-positive rough lemon rootstocks, including shoot length and HLB symptom severity.  We also plan to set up a repeat of the rough lemon graft transmission experiment, given the promising results of the first run of the experiment.   3. Please state budget status (underspend or overspend, and why):  The budget is 40% spent.  This is partially due to an approximately 8 month delay to the start of the project while a graduate student was recruited to be the full-time researcher on the project.  Travel has been limited since March, 2020 due to COVID-19, resulting in reduced travel costs.  In addition, the project director elected to redirect all the budget for his salary to research support.  This project will need a one-year no-cost extension to complete thorough testing of the HLB resistance tess already initiated and to complete a repeat of the graft-inoculation test.  The existing budget will be sufficient to cover conclusion of all experiments and support the graduate student to the end of his M.S. degree.  


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