Effects of an Alphasatellite on the Life Cycle of the Nanovirus Faba Bean Necrotic Yellows Virus.

Nanoviruses are plant viruses with a multipartite single-stranded DNA (ssDNA) genome. Alphasatellites are commonly associated with nanovirus infections, but their putative impact on their helper viruses is unknown. In this study, we investigated the role of subterranean clover stunt alphasatellite 1 (here named SCSA 1) on various important traits of Faba bean necrotic yellows virus (FBNYV) in its host plant Vicia faba and aphid vector Acyrthosiphon pisum, including disease symptoms, viral accumulation, and viral transmission. The results indicate that SCSA 1 does not affect the severity of symptoms nor overall FBNYV accumulation in V. faba, but it does change the relative amounts of its different genomic segments. Moreover, the association of SCSA 1 with FBNYV increases the rate of plant-to-plant transmission by a process seemingly unrelated to the simple increase of viral accumulation in the vector. These results represent the first study on the impact of an alphasatellite on the biology of its helper nanovirus. They suggest that SCSA 1 may benefit FBNYV, but the genericity of this conclusion is discussed and questioned. IMPORTANCE Alphasatellites are circular single-stranded DNA molecules frequently found in association with natural isolates of nanoviruses and some geminiviruses, the two ssDNA plant-infecting virus families. While the implications of alphasatellite presence in geminivirus infections are relatively well documented, comparable studies on alphasatellites associated with nanoviruses are not available. Here, we confirm that subterranean clover stunt alphasatellite 1 affects different traits of its helper nanovirus, Faba bean necrotic yellows virus, both in the host plant and aphid vector. We show that the frequencies of the virus segments change in the presence of alphasatellite, in both the plant and the vector. We also confirm that although within-plant virus load and symptoms are not affected by alphasatellite, the presence of alphasatellite decreases within-aphid virus load but significantly increases virus transmission rate, and thus it may confer a possible evolutionary advantage for the helper virus.