The molecular arms race: mechanisms of fungal antiviral defense and viral counterstrategies.

Advances in high-throughput sequencing have accelerated the discovery of novel mycoviruses, shedding light on the dynamic coevolution between fungi and their viral partners. Recent studies indicate that mycoviruses act as key ecological drivers, modulating fungal growth, pathogenicity, and environmental adaptability. While most infections remain asymptomatic, some display a broad spectrum of interactions ranging from hypovirulence and hypervirulence to mutualism. Fungal hosts possess multilayered antiviral systems - such as vegetative incompatibility and RNA interference - that restrict viral replication and transmission. Beyond these canonical pathways, additional defense mechanisms have recently been identified, including RNA-directed DNA methylation, which targets and degrades viral genomes. Conversely, mycoviruses have evolved diverse counter-defense strategies, such as RNA silencing suppressors and the modulation of host immune responses and autophagy pathways, to facilitate viral persistence. These reciprocal interactions influence the replication and transmission of both homologous and heterologous viruses within fungal populations and their plant hosts. This review provides a comprehensive overview of the molecular mechanisms underlying fungal antiviral defenses and the corresponding counterstrategies employed by mycoviruses. Understanding these intricate molecular dialogues is essential for overcoming genetic barriers in mycovirus-mediated biocontrol and developing novel therapeutics against fungal pathogens.