When plants are Trojan horses for viruses

Vertical and horizontal transmission of plant viruses: two extremes of a continuum?

Parasites have a variety of mechanisms to be transmitted to new susceptible hosts, which can be largely grouped in two main modes: vertical (i.e., from parents to the offspring) and horizontal (i.e., between hosts regardless of descent). Because between-host dispersal is a key trait for parasite fitness, scientists studying host-parasite interactions have been long interested in understanding the evolution of their transmission mode(s). Most work in this regard has been theoretical, which resulted in the development of the so-called Continuum hypothesis. This theory states that because vertically transmitted parasites require the host to reproduce, the evolution of this mode of transmission will involve reduced virulence (i.e., the effect of infection on host fecundity) in order to allow maximal host viable progeny production. Conversely, the evolution of horizontal transmission does not have this limitation and parasites with this mode of transmission will evolve higher virulence. Therefore, a trade-off between both modes of transmission across a continuum of virulence values is predicted, with each transmission mode located at the extremes of the continuum. Using plant viruses as a focal parasite, here we review existing theory surrounding the Continuum hypothesis and the experimental work testing the predictions of the theory. Finally, we briefly discuss molecular mechanisms that may explain the existence of vertical-to-horizontal transmission trade-offs and potential implications for the management of virus epidemics.

A Distinct Arabidopsis Latent Virus 1 Isolate Was Found in Wild Brassica hirta Plants and Bees, Suggesting the Potential Involvement of Pollinators in Virus Spread

During our search for aphid-pathogenic viruses, a comovirus was isolated from wild asymptomatic Brassica hirta (white mustard) plants harboring a dense population of Brevicoryne brassicae aphids. The transmission-electron-microscopy visualization of purified virions revealed icosahedral particles. The virus was mechanically transmitted to plants belonging to Brassicaceae, Solanaceae, Amaranthaceae, and Fabaceae families, showing unique ringspot symptoms only on B. rapa var. perviridis plants. The complete viral genome, comprised of two RNA segments, was sequenced. RNA1 and RNA2 contained 5921 and 3457 nucleotides, respectively, excluding the 3' terminal poly-adenylated tails. RNA1 and RNA2 each had one open-reading frame encoding a polyprotein of 1850 and 1050 amino acids, respectively. The deduced amino acids at the Pro-Pol region, delineated between a conserved CG motif of 3C-like proteinase and a GDD motif of RNA-dependent RNA polymerase, shared a 96.5% and 90% identity with the newly identified Apis mellifera-associated comovirus and Arabidopsis latent virus 1 (ArLV1), respectively. Because ArLV1 was identified early in 2018, the B. hirta comovirus was designated as ArLV1-IL-Bh. A high-throughput-sequencing-analyses of the extracted RNA from managed honeybees and three abundant wild bee genera, mining bees, long-horned bees, and masked bees, sampled while co-foraging in a Mediterranean ecosystem, allowed the assembly of ArLV1-IL-Bh, suggesting pollinators' involvement in comovirus spread in weeds.