Plant Virus Spillovers

Dear Colleagues,

Most previous plant virus research focused on viruses that damage economically important plants. Wild plant viruses are largely unknown to science, yet those that appear in crops have wild plant origins.

Some plant viruses that infect domesticated plants cause great economic losses, while others, probably the majority, cause negligible economic costs. Some of the most damaging plant viruses, ones that cost growers millions of dollars, include tobacco mosaic virus (TMV), cucumber mosaic virus (CMV), African cassava mosaic virus, and citrus tristeza virus. Every year, several papers are published that announce a ‘new’ virus discovered in a valuable crop species. Some of these viruses go on to wreak havoc across a region or the world, while others are never mentioned again. A recent example of the former is tomato brown rugose fruit virus that appeared for the first time in Israel in tomatoes in 2014 and has since travelled across the globe depleting tomato and capsicum harvests. Such viruses are not ‘new’ in the strictest sense of the word, they are old viruses that have emerged from wild plants growing at the interface between wild and cultivated plant communities. Less well known, but perhaps occurring just as frequently, are spillovers of viruses in the opposite direction—from crops to wild plant communities. And, of course, viruses also spillover from a wild species to another wild species and from one domestic species to another domestic species. 

Spillover and emergence are terms used to describe the process by which a virus successfully undertakes the colonization of a new host species. A challenging number of barriers must be negotiated before this is achieved: breaching the physical distance between the original and new hosts, gaining access to the appropriate cell type, successful interactions with the new cellular machinery, successful replication while not inducing apoptosis, cell to cell and systemic movement within the plant, and transmission to other individuals of the same species. Viruses such as TMV and CMV have successfully negotiated this process hundreds of times and are thereby referred to as ‘broad host-range’ viruses. Many other viruses remain marooned within a single host species and appear to pose a negligible risk of spillover. Why this is so is not clearly understood, but it may relate to an inability to negotiate one or more of the potential barriers listed above.

Wide spillovers of viruses between kingdoms have occurred over evolutionary time, such as with the endornaviruses that infect fungi, oomycetes, and plants. It is probable that ongoing human encroachment into the remaining wild communities, and the ecological changes wrought by a heating climate and the extinction crisis, may trigger further spillovers.

This Special Issue welcomes papers that describe new spillover events by plant, fungal, and oomycete viruses in wild and/or domesticated systems, those that increase our understanding of spillover mechanisms, and reviews addressing the relevant aspects of this topic.

Dr. Steve Wylie
Dr. Nuredin Habili
Guest Editors

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• spillover
• emergence
• replication
• wild-plant virus
• transmission
• virus-cell interaction
• vector