Development, evaluation and delivery of citrus HLB management approaches by targeting its nature as a pathogen-triggered immune disease

Research by: University of Florida, University of California Riverside, University of California Davis, Texas A&M University-Kingsville, Soilcea, Southern Gardens Citrus, LLC

Article written by: Nian Wang
Article reviewed by: Peggy G. Lemaux, Ed Stover, Lukasz Stelinski


What is the research?

Recent work demonstrated that CLas stimulates a systemic and chronic immune response in the plant transport system (phloem) of HLB-sensitive citrus, causing cell death in the phloem and subsequent HLB disease symptoms (Figure 1). The hypothesis is that HLB symptoms are due to this systemic and chronic immune response and that the severity of the response can be controlled by reducing the reactive oxygen species (ROS), produced as a defense mechanism in the response to the HLB-pathogen

Genome editing (https://ucanr.edu/sites/scienceforcitrushealth/Research_Snapshots/General_Topics/LemauxCRISPR/) will be used to generate non-GMO citrus with reduced immune responses, resulting in HLB-resistant/tolerant citrus varieties.  We developed tools for generating gene-edited citrus plants using special lab-cultured plant cells. This approach has been successfully used to generate non-GMO canker-resistant citrus and will be used here to generate non-GMO HLB-resistant/tolerant citrus varieties for use in new plantings. For established HLB-infected groves, genes will be expressed in trees for making antioxidants, that are introduced using the Citrus tristeza virus (CTV). This virus is already present in many commercial citrus groves but the strain we use produces minimal symptoms and in our work was modified to express genes to protect against HLB. It has been shown that such modified CTVs can express a foreign gene for more than a decade in citrus and such modified CTVs have already been given regulatory approval to deliver antimicrobial peptides in commercial citrus groves in Florida.

What are the objectives of this research?

We will control HLB using three approaches. (1) Develop integrated horticultural approaches to mitigate ­CLas-triggered ROS production, which will involve optimized applications of micronutrients, gibberellic acid (GA) and antioxidants to lessen HLB symptoms. (2) Protect citrus plants from CLas-triggered ROS by expressing antioxidants and silencing genes involved in CLas-triggered ROS production. (3) Generate non-GMO HLB-resistant/tolerant citrus using editing of genes required for HLB disease.

Who is working on the project?

Nian Wang of the University of Florida (UF); Danelle Seymour of the University of California, Riverside (UCR); Abhaya Dandekar of UC Davis; Madhurababu Kunta of Texas A&M University-Kingsville; Dusica Coltrane of Soilcea, LLC; Jude Grosser of UF; Ashraf El-Kereamy of UCR; Choaa El-Mohtar of UF; Zhengfei Guan of UF; Davie Kadyampakeni of UF; Martha Orozco-Cardenas of UCR; Chris Oswalt of UF; Tripti Vashisth of UF; Yu Wang of UF; Mongi Zekri of UF; and Mike Irey of Southern Gardens Citrus, LLC.

What are the challenges?

Genome editing technology, using CRISPR, has been successfully used to generate non-GMO  canker resistant citrus varieties but remains a laborious process with low throughput, and needs to be conducted separately on each cultivar of interest. The CTV technology described will provide a potential management approach to control HLB-diseased trees in the field. Each modified CTV will likely be useful across most citrus cultivars; however, CTV cross-protection, which hinders new infections in CTV-infected trees, may prevent use on many field trees that are already infected with CTV.  State regulatory agencies may also require use of CTV strains that are already widespread in each state.

Funding source: USDA NIFA ECDRE program, project #2022-70029-38471