University of Maryland virologist Anne Simon and the company she founded, Silvec Biologics, have successfully vaccinated laboratory hosts against citrus tristeza virus (CTV). They are now focusing on HLB, also called citrus greening. The vaccination induces trees to produce their own therapeutic agents.
Silvec’s vaccination concept is based on the discovery of a novel infectious RNA that the company calls an independently mobile RNA (iRNA). The first iRNA was discovered in four California limequat trees in the 1950s by University of California Riverside (UCR) and more recently characterized by UCR professor Georgios Vidalakis.
The Silvec team was the first to realize the unique properties of iRNAs and their therapeutic potential. iRNAs are relatively easy to modify by inserting targeted anti-pathogenic agents called small interfering RNAs (siRNAs). siRNAs can be used to target viruses, bacteria and host messenger RNAs (mRNAs) by bonding with them in a manner that triggers a host response where enzymes are sent to chop them up.
Simon says siRNAs can be designed that are highly effective against a single target organism with minimal interference with almost any other type of organism. “This makes them far safer and more effective to use than other anti-pathogenic agents like antibiotics that may indiscriminately target pathogens along with much of the plant’s microbiome, as well as consuming organisms and their microbiome,” Simon says.
Silvec is working to insert multiple siRNAs into a single iRNA to target the major bacterial, fungal or viral diseases of a given species of tree. Once grafted into a mature tree or seedling, the iRNA systemically travels throughout the tree and induces cells to replicate it. As it is replicated, so are the inserted siRNAs, which then break off and circulate throughout the tree. “This is expected to occur for the remainder of the tree’s lifespan” and could offer a lifetime of protection from a single inoculation, Simon says. She adds that Silvec’s technology could potentially be used to vaccinate most trees against most diseases.
Simon’s company is working in partnership with UCR and California citrus growers, which have provided funding for the project along with the U.S. Department of Agriculture’s National Institute of Food and Agriculture. It has also attracted several major agricultural venture capital investors.
Simon says Silvec believes that its technology represents the best hope in saving citrus trees from HLB. She adds that the company’s efforts to vaccinate against CTV will hopefully allow growers to eventually return to sour orange rootstock. Sour orange was a very popular rootstock in the past, but trees on sour orange are highly susceptible to CTV. In addition, the Silvec approach should work with all varieties of citrus so as not to limit growers’ options.
“My lab and Silvec Biologics scientists are working seven days a week trying to provide a novel solution to this terrible problem (HLB) because there is such an urgent need,” Simon says.