Elaine Larsen

Zoom Recording

Fungi as Vectors of Plant-Pathogenic Viruses

Viruses are the most abundant biological entity on Earth, infecting all cellular organisms across terrestrial and marine ecosystems [1]. Plants are known to be infected by diverse viruses and viroids (i.e., virus-like, unencapsidated, single-stranded RNA particles) belonging to at least 35 families [2]. Several viral families contain structurally related viruses that infect either plants or fungi (i.e., mycoviruses), and several viral genera infect both plants and fungi [2]. The majority of research on plant viruses has focused on pathogenic viruses that cause disease in agricultural crops and account for the annual global loss of ca. $30 billion [3]. Plant pathogenic viruses are typically vectored by sap-sucking arthropods [4], but the potential for fungi to act as vectors for plant-pathogenic viruses was discovered as early as the 1970s, when tobacco mosaic virus (TMV) was identified in the asexual spores (conidia) of a powdery mildew fungus [2]. Since then, several plant viruses have been successfully inoculated into fungi and oomycetes, with some resulting in stable infections [5-8]. Similarly, studies have shown that viroids can infect yeasts ( Saccharomyces cerevisiae ), plant-pathogenic filamentous fungi, and the oomycete pathogen Phytophthora infestans [9-11]. Viroids also have been transmitted successfully between fungal hyphae via hyphal fusion and persist in spores after asexual reproduction [11]. Although plant viruses could exist within fungal cells, they were not known to undergo replication [2]. However, it is now recognized that some plant viruses and viroids can replicate within fungal cells, suggesting that fungi may both vector and amplify viral loads [5-11]. Beyond simply being vectored by fungi, viruses and viroids can modulate the pathogenicity of infected fungi [2]. In some cases, the severity of plant disease caused by plant-pathogenic fungi has been shown to either increase or decrease when fungi are infected by plant viruses [7]. While most viroid infections appear to be asymptomatic in fungi, hop stunt viroid (HSVd) reduced growth and pathogenicity in the plant-pathogenic fungus Valsa mali [11]. Mycoviruses may also reduce the pathogenicity of pathogenic fungi and have been researched as potential biological control agents [2,12]. Virus-induced hypovirulence has been observed in Cryphonectria parasitica , the causative agent of chestnut blight, and Ophiostoma ulmi , which causes Dutch elm disease [2,12]. Demonstrating the potential for both pathogenic and non-pathogenic fungi to serve as vectors of pathogenic viruses, a recent study found that fungal endophytes isolated from the leaves of plants exhibiting symptoms of viral infection carried a number of plant-pathogenic viruses [8]. Overall, the long evolutionary history of close endosymbiotic relationships between plants and fungi—ranging from parasitic to mutualistic interactions—appears to have shaped the evolution, host specificity, and transmission of plant viruses [2]. However, more research is needed to understand the role of pathogenic and non-pathogenic fungi in vectoring plant-pathogenic viruses and the implications of fungus-virus interactions on plant disease management.