Developing novel virus-like agents to combat viral, bacterial, and other pathogens of trees to prevent a future without citrus, chocolate, grapes and olives

Dr. Anne Simon
University of Maryland
Tuesday, November 30, 2021 - 4:00pm Password: spls2021

Bacterial, fungal and viral pathogens are causing the deaths of billions of trees and vines every year.  Although siRNAs could be the key to controlling pathogens and the insects that deliver them, lack of a cost-effective delivery system for siRNAs and anti-bacterial peptides into the phloem has been a major impediment for mitigating the harm caused by biotic and abiotic stresses including climate change. Viruses are well known to be excellent vectors for producing siRNAs as RNAi is a major defense pathway in plants.  However, only a few viruses have been successfully developed into vectors (and for laboratory use only) due to pervasive issues with stability of inserted sequences, limited host ranges and virus-induced symptom expression.  Umbravirus-like subviral RNAs (ulaRNAs) have the potential to be game changers as vectors due to their extensive (unlimited?) host range, symptom-free infections (in nearly all hosts), lack of plant-to-plant transmission, and vascular location (where most of the worst pathogens reside).  Many ulaRNAs only encode an RNA polymerase and associated replication-required protein and thus lack any encoded movement proteins, coat proteins and silencing suppressors that were thought to be prerequisite for all viruses that are independently infectious. We have been developing the ulaRNA Citrus yellow vein-associated virus (CYVaV) as an siRNA vector for trees and vines.  CYVaV, whose host range includes (so far) all citrus species, papaya, cucumber, tomato and Nicotiana benthamiana, is only 2/3 the size of the smallest plant viruses and is independently mobile due to the use of a host RNA-binding movement protein called Phloem Protein 2 (PP2).  PP2, a highly expressed, ancient ubiquitous protein of unknown function in vascular plants, binds specifically to CYVaV and its derived defective (D)RNAs, forming a high molecular weight complex that can protect the enveloped RNA from RNase treatment.  The secondary structure of the CYVaV genome was solved using SHAPE structure probing combined with extensive phylogenetic comparisons and knowledge of the structure was key to understanding how to stabilize inserts in six locations.  A CYVaV vector was used to keep N. benthamiana from being infected by citrus tristeza virus and is being developed to mitigate symptoms and prevent infection of citrus trees by Candidus Liberibacter asiaticus, the causal agent of citrus greening, which is devastating citrus orchards world-wide. Surprisingly, bacteria that reside in the phloem or xylem can be targeted by siRNAs, opening up CYVaV as a potential treatment for many of the most devastating tree and vine diseases.
Anne Simon received her BA from the University of California, San Diego and her PhD from Indiana University. She was a professor at the University of Massachusetts for 13 years and has been in the Department of Cell Biology and Molecular Genetics at the University of Maryland for 21 years.  Dr. Simon teaches the Introductory Majors Biology course, for which she has been awarded the highest university teaching honor at both of her institutions.   Dr. Simon recently stepped down as the head of the University of Maryland Virology Program, a joint program with the NIH, after 18 years.  Dr. Simon was a recipient of  the International Franski Prize for Research in Plant Virology, the Norma Allewell Prize for Entrepreneurship  and in 2014 was elected a fellow of the American Academy of Microbiology.  For the past 15 years, she has been a senior editor of Journal of Virology after a decade as editor of Virology.  In 2020, Dr. Simon and her brother, Dr. Rafael Simon, founded Silvec Biologics, a company located in Gaithersburg Maryland that focuses on siRNA treatments for tree and vine diseases.