20237002941309

Huanglongbing (HLB) is the most devastating disease of citrus in the U.S. Economic losses over the past five years in FL alone amount to over $1 billion per year, with nearly 5,000 jobs lost annually.

The U.S. citrus industry is in dire need of new citrus cultivars with stable and broad-spectrum resistance/tolerance to HLB as well as desirable horticultural characteristics, crop yield, and fruit quality. The other major citrus states are at high risk for a similar disastrous situation if suitable remedies are not forthcoming soon.

Neither a simple cure nor successful management strategies have been identified even after 15+ years of the disease's presence in Florida.

Previous efforts to create new HLB resistant/tolerant cultivars were focused on conventional hybrid breeding or the development of transgenic plants. Unfortunately, the approaches used in those projects are inadequate to meet the current crisis because of the lack of sources of resistance in citrus, time constraints associated with citrus breeding and genetic engineering, and regulatory obstacles for transgenic citrus.

Meanwhile, more than 15 years of HLB epidemics and tens of millions of infected trees in groves across Florida provide an unprecedented opportunity and high probability for selecting new mutant scions that are resistant or at least highly tolerant.

Over the past years, our group has selected multiple citrus varieties with field resistance or tolerance to HLB from Florida groves. A recent study showed that HLB is a pathogen-triggered immune disease. These natural variants may have HLB resistance/tolerance through evasion of the immune response or loss of susceptibility genes.

Although the exploitation of natural resistance/tolerance provides a key solution to developing new HLB-resistant/tolerant citrus scions and rootstocks, it still requires a significant amount of time to confirm stable resistance of the selected varieties. It is critical to develop rapid methods to shorten the selection process, to provide solutions within a shorter time frame to ensure the survival of the citrus industry, especially in Florida.

Our previous research shows that transposon activities are one of the important drivers of diversification in sweet oranges. By monitoring the transposon activities in selected varieties, we may be able to determine their genetic stability, and then facilitate selection.

Furthermore, there are many available sequences of citrus genomes, including several HLB-resistant/tolerant citrus plants and their counterparts that our group and others have sequenced. It is critical that we can integrate all genome-based information to understand the HLB-resistant/tolerant or susceptibility-related genes. With more sequences of HLB-resistant/tolerant citrus plants and their counterparts in this project, we can build a pangenome that includes all HLB-resistant/tolerant related variants. This should help identify HLB-resistant/tolerant or susceptibility-related genes.

This project addresses the development of commercial citrus varieties (rootstocks and scions) for both fresh and processed markets with genetic tolerance and resistance to HLB using traditional breeding techniques and/or gene editing, which is one of the ECDRE focus areas.

In this proposal, we first present a systematic approach with emphasis on the rapid selection, assessment, and delivery of new HLB-resistant/tolerant citrus variants as new scion and rootstock cultivars. Second, we will develop a novel pangenome to identify HLB resistance/tolerance or susceptibility-related genes. Moreover, with the HLB susceptibility-related genes identified in our previous study, we will develop new varieties via gene editing using CRISPR-Cas and test their resistance to HLB. We will conduct multistate field trials of our previous selected varieties and evaluate their horticultural performance and economic impacts. Meanwhile, we will conduct active outreach and extension to disseminate our project results to growers, stakeholders, and the public.

Our practical methods for rapidly selecting, assessing, and delivering stable and broad-spectrum HLB-resistant/tolerant citrus variants will allow HLB-resistant/tolerant scion and rootstock cultivars to be developed and deployed much quicker without regulatory constraints. The integrated citrus pangenome can help identify the genetic elements associated with HLB resistance/tolerance or susceptibility. The identified HLB resistance/tolerance or susceptibility-related genes will provide a foundation for understanding the molecular mechanisms of LAS infection and HLB symptom progression. Precise gene editing based on the CRISPR system will enable targeted disruption of citrus genes that are essential to LAS infection, multiplication, and pathogenesis, production of loss-of-function mutants, and development of new HLB resistant/tolerant citrus cultivars that are free of GMO fingerprints and potentially free of regulatory obstacles to the market.

Horticulture and socio-economics-based support tools will be developed for the industry to adopt new cultivars with confidence. The selected citrus varieties with HLB resistance/tolerance are expected to be the most durable, eco-friendly, and cost-effective solution to this devastating disease.

The successful achievement of our goals and objectives will broadly impact the entire US citrus industry, by maintaining the viability of industries not yet fully impacted as in Florida, and by providing the plant materials that will encourage Florida growers to replant with confidence. The combined economic impacts of our US citrus industries will be measured in billions of dollars. The many communities throughout the country having economies built around viable citrus production will avoid the major disturbances that will come from the economic disasters that HLB incurs. The success of this project will set up a model for the citrus community across the globe to follow and solve the HLB problems, in the not-too-distant future.

Clemson University

10.309 - Specialty Crop Research Initiative

Institute of Food Production and Sustainability (IFPS) [USDA - NIFA]

Clemson, South Carolina 29634-0001 United States

Single Zip Code

USDA-NIFA-SCRI-009843