Using peptides as a preventive approach to target the psyllid and the pathogen

Peptides are short chains that come from naturally occurring proteins. Most organisms produce a special group of peptides that act as a first line of defense against bacteria, interfering with their ability to transmit disease.  USDA-ARS researcher, Dr. Robert Shatters, along with others, is looking at the possible use of antimicrobial peptides to stop the spread of the CLas bacterium that causes huanglongbing (HLB) disease of citrus, in concert with peptides that affect the Asian citrus psyllid’s ability to acquire and transmit the pathogenic bacterium. The quickest approach to use these peptides would be in topical applications to the leaves of citrus trees, while a longer term approach would be to engineer citrus with the genes to produce the peptides.

What is the technique?

Dr. Shatters and colleagues are pursuing three different application strategies to deliver a combination of peptides with the combined effect of killing the psyllid, or blocking the psyllid from acquiring and transmitting the bacterium, and/or directly killing the bacterium. The first peptide deliver strategy would be a spray application of peptides directly on the plant. This approach would be the quickest solution for growers; however, it would require repeated applications.  A second strategy involves engineering citrus to produce the peptides directly in the phloem, the part of the plant where the bacterium lives. This approach avoids the necessity of having to repeatedly applying the peptides. However, it would work only on new plantings of genetically engineered citrus trees, not on existing trees and the regulatory pathway to permit its commercial use would be longer than direct applications. A third approach , conducted collaboratively with Dr. Bill Dawson, at the University of Florida, involves inserting genes for the peptides into Citrus tristeza virus (CTV).  The virus would be transferred to the citrus tree by grafting CTV-infected tissue onto the tree. The engineered CTV would live in the phloem and produce peptides that would circulate through the phloem.  This strategy will require only a single application. It could be used on existing plantings and would require a longer regulatory process than the sprayable peptides.

Who is working on the project?

Dr. Robert Shatters, a Research Molecular Biologist at the USDA Horticultural Research Laboratory in Ft. Pierce FL, and his laboratory have demonstrated that by combining antimicrobial peptides with acquisition/transmission blocking peptides, up to 90% of the psyllids were killed in lab experiments and pathogenic bacteria were completely blocked from being acquired by the psyllids that survived.  This group is now moving this work to greenhouse and research field to determine how well these peptides function under whole-plant and field conditions.  This work is being conducted collaboratively with other scientists.  Dr. Michelle Cilia, USDA-ARS in Ithaca NY, is identifying the specific protein interactions involved with the gut binding peptides; Dr. Ed Stover, USDA-ARS, Fort Pierce FL, is working to produce transgenic citrus expressing these peptides; and Dr. Bill Dawson, University of Florida, is investigating use of the CTV virus to produce these peptides within infected citrus.

What are the challenges and opportunities?

Using peptides to reduce disease is not a new technique, as they are currently being used in human health to attempt to combat damage from cardiovascular disease.

Because citrus is constantly being re-infected with the CLas bacterium, this approach will most likely serve as a preventative. However, in California, where psyllid and tree infection rates are very low, it is possible this strategy could be a cure. Experimentally, bacterial populations were reduced by peptides in citrus leaves by >99% after five days (based on detectable bacterial DNA). Thus, over time, treatment with peptides could result in a cure, if peptides were shown to reach throughout the plant, including the roots. 

In order for these approaches to be effective in reducing HLB in citrus, a number of parts to this research puzzle need to fall into place.  First, the peptides have to be effective against the bacterium. Second, the delivery of the peptides into the psyllid needs to be effective and economically viable. Third, to be useful for growers, the regulatory pathway must be established that will permit these tools to be deployed in the field. It is encouraging that commercial peptide-based topical sprays were developed and used by growers in the 1990’s to induce plant defenses. And, since then, more economical methods of producing peptides have been achieved.  If these three hurdles can be overcome, peptides may become a useful tool to combat HLB.