Two-Pronged Approach to Suppress the Asian Citrus Psyllid Vector of HLB

What is the technique?

The Asian citrus psyllid (ACP) inoculates citrus trees with the CLas bacterium that causes huanglongbing (HLB). Effective tools to reduce psyllid populations are important for management of this complex disease. Certain pesticidal proteins, produced by the soil-borne bacterium Bacillus thuringiensis (Bt), are toxic to ACP.  A next step in developing effective strategies against ACP was to test the effects of these pesticidal proteins in combination with gene silencing RNAs, small molecules that prevent expression of targeted ACP genes, essential for psyllid survival. Importantly, both the pesticidal proteins (Jurat-Fuentes and Crickmore 2017) and the gene silencing RNAs are not harmful to nontarget organisms.

How would this work?

Although the reason for this effect is currently unclear, Bt pesticidal proteins and gene silencing RNAs have been shown to be significantly more effective when used in combination than individually (Fig. 1).

Figure 1. Bt proteins and gene silencing RNAs are individually effective tools for killing Asian citrus psyllid. An approach combining the two techniques for increased effectiveness is shown.

First, when ingested by ACP, the Bt pesticidal proteins bind to the wall of the insect gut and create a hole. The insect stops feeding as a result of the damage and ultimately dies. Second, the gene silencing RNAs shut down the targeted “survival” genes in the ACP gut so that production of proteins from those genes is blocked. One possible explanation for the enhanced combined effect is that the damage caused by the pesticidal Bt protein makes it easier for the gene silencing RNAs to enter into cells lining the psyllid gut.

Disease management applications

When the best combination of Bt pesticidal proteins and gene silencing RNAs has been identified, an appropriate delivery system to ACP will be needed. To be effective, both control agents need to be in the plant sap so that they are ingested as the psyllid feeds. One option is to use a naturally occurring plant virus (Citrus tristeza virus), that is known to reside in plant sap, for delivery of these agents. A second option is to engineer the citrus plant to make these compounds or to put them in a trap plant such as Indian curry that is more attractive to psyllids than fruit-producing citrus (Fig. 2). In that situation, psyllids attracted to trap plants are killed by the combined Bt pesticidal protein and gene silencing RNAs before they are able to reach fruit-producing citrus trees. Also, the engineered genes would not be present in those citrus trees.

Figure 2. One potential method to deploy this technology to protect commercial citrus groves is trap crops expressing Bt and gene silencing RNA, which intercept and kill the mobile ACP before they invade groves.

Who is working on the project?

Professor Bryony Bonning, leading the team based at University of Florida (UF), is conducting research on the Bt pesticidal proteins active against Asian citrus psyllid. This is in collaboration with Nabil Killiny, Associate Professor, Choaa El-Mohtar, Research Assistant Scientist, and Lukasz Stelinski, Professor, all at UF.  The latter are examining the impacts of combinations of pesticidal proteins and gene silencing RNAs on ACP. Manjul Dutt, Research Assistant Scientist at UF, is assessing the best strategies for production of Bt proteins and silencing RNAs in citrus sap.

What are the challenges and opportunities?

A key challenge for this strategy is appropriate and effective delivery to ACP of both the pesticidal protein and gene silencing RNAs. Delivery of both components has been achieved for another major insect pest, the corn rootworm, and should therefore be feasible for targeting Asian citrus psyllid, but optimization for adequate production of both components in plant sap will be required. An additional approach would be to increase the toxicity of the pesticidal proteins thereby reducing the amount of protein that needs to be produced by the plant. Bonning’s lab is using an approach that creates an artificial anchor to increase the amount of protein that binds to the gut wall to increase its negative impact on ACP.

Funding source: This material is based upon work that is supported by the National Institute of Food and Agriculture, U.S. Department of Agriculture, under award number 2020-70029-33177. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.

Jurat-Fuentes, JL & N.Crickmore (2017) Specificity determinants for Cry insecticidal proteins: Insights from their mode of action J. Invertebr. Pathol. 142:5-10. 10.1016/j.jip.2016.07.018