Engineering Solutions for Citrus Greening: Targeted Peptides and Metabolic Modeling

By Lab to Farm Team

Citrus greening disease, also known as Huanglongbing (HLB), is one of the most devastating plant diseases worldwide. Spread by the Asian citrus psyllid and caused by the bacterium Candidatus Liberibacter asiaticus (CLas), HLB leads to stunted growth, bitter fruit, and ultimately tree death. Current treatments are ineffective, and no cure exists—yet. A new project aims to change that by combining advanced metabolic modeling and precision antimicrobial engineering to develop effective, sustainable solutions for citrus growers.

Designing Disease-Specific Therapies Using Targeted Peptides

One of the project’s most innovative strategies focuses on chimeric antimicrobial peptides (AMPs) that specifically target and kill the CLas bacterium without harming the citrus plant or its beneficial microbes.

Instead of using broad-spectrum antimicrobials, which can disrupt the plant’s microbiome and lead to resistance, researchers are engineering chimeric AMPs: molecules that combine a powerful antimicrobial with a peptide “aptamer” that recognizes and binds to CLas. These peptides are designed in collaboration with peptide engineering experts at the Forsyth Institute and are modeled after successful medical approaches used in the treatment of human infectious diseases.

In preliminary studies, these CLas-targeting AMPs were shown to kill over 500 times more bacteria than their non-targeted counterparts. The team is using a clever delivery system based on the citrus tristeza virus (CTV) to transport these peptides directly into the phloem, where CLas resides. Early tests have shown promising reductions in HLB symptoms when these peptides are expressed in citrus trees.

Boosting Discovery Through Metabolic Modeling

To complement the development of antimicrobial treatments, the team is also creating a systems-level metabolic model of the HLB pathosystem—one that integrates the pathogen (CLas), the host (citrus), and the vector (Asian citrus psyllid).

Using genome-scale modeling and omics data, scientists at UC San Diego are reconstructing metabolic networks that simulate how these organisms interact during infection. By identifying the essential metabolic pathways that CLas depends on, they can predict vulnerabilities—and design peptides that block those critical pathways.

This model-based approach also guides efforts to improve in vitro cultivation of CLas, which has historically been difficult due to the bacterium's fastidious nature. By supplementing growth media with model-predicted nutrients, researchers are working to grow CLas more efficiently in lab settings, which will accelerate drug discovery.

Testing Treatments in Hairy Root Cultures and Citrus Trees

Before moving into the field, peptides are screened using a novel hairy root culture assay, developed at Texas A&M. This in vitro system allows researchers to rapidly test the efficacy of anti-CLas peptides in living citrus tissue. By using the actual pathogen instead of less-representative bacterial relatives, the assay provides more accurate data—up to 4 times faster than mature plant tests.

The most promising candidates are then tested in live citrus plants using the CTV delivery system. If these peptides continue to show effectiveness, the next step is large-scale greenhouse and field trials in partnership with Southern Gardens Citrus.

Economic Feasibility and Grower Adoption

Developing powerful treatments is only part of the challenge—making sure they're economically viable is just as crucial. That’s why the team is conducting an economic analysis to compare the cost and benefits of single-application CTV-based therapies versus more frequent conventional treatments. Using dynamic modeling adapted from vineyard disease management, the analysis will help determine the long-term return on investment for citrus growers in different regions.

Broadening Impact Through Outreach

The project’s outreach team is dedicated to sharing its results with scientists, growers, and the broader public. Educational materials are being developed for backyard citrus growers, who played a key role in the early spread of HLB in urban California, and undergraduates from underrepresented backgrounds will be involved in research and extension efforts. The team hopes that by staying closely connected to stakeholders, they can ensure fast and meaningful adoption of new solutions.

A Path Forward for the Citrus Industry

This ambitious project blends molecular biology, computational modeling, plant pathology, and economics to tackle HLB from every angle. By designing highly specific antimicrobial treatments and uncovering the complex biological networks of HLB through modeling, the team is laying the groundwork for transformative solutions to one of agriculture’s most persistent problems.

Join the Conversation

While the science is groundbreaking, it's essential to have open discussions about the technologies used in agriculture. Innovations like metabolic modeling and potential future applications of genetic modification (such as CRISPR) raise important questions about ethics, safety, and environmental impact.

We value your opinions and invite you to share your thoughts.

Learn more about this project and others like it at Lab to Farm.

For more information on all the Metabolic Modeling related research projects of Dr. Borneman and his collaborators, visit https://www.labtofarm.org/bioengineering.