Recent “ecogenomic” surveys have revealed that a diversity of novel virus taxa are present in communities of native or naturalized plants. Many of these non-crop associated (“wild”) viruses occur in plants without symptoms, which has prompted some plant-virologists to hypothesize that wild viruses form commensal or mutualistic symbioses with plants. The recent discovery of Switchgrass mosaic virus (SwMV), which occurs both symptomatically and asymptomatically in Panicum virgatum (switchgrass), represents an opportunity to study wild plant-virus interactions in a wide-ranging, co-dominant prairie grass.
We evaluated the effects of SwMV in P. virgatum by comparing growth and reproduction in symptomatic and non-symptomatic tillers (~400 total) growing in a large research plot in East Lansing, Michigan. Roughly half the plants in the plot had been naturally infected with SwMV, which is a leafhopper vectored virus. Over the course of the 2017 growing season, we tracked symptom expression by categorically assessing green, expanded leaves of tillers, and we measured tiller height, panicle development, and, after the first frost of the growing season, dry biomass. Additionally, in late-summer, we measured photosynthesis in top-most expanded leaves to examine the relationship between symptom severity and gas exchange rates. We used RT-PCR to confirm SwMV infection in symptomatic tillers and to estimate rates of asymptomatic infection.
Results/Conclusions
Symptomatic tillers were significantly shorter and produced fewer panicles than non-symptomatic tillers. Symptom severity was associated with lower rates of photosynthesis and higher SLA values in leaves. In a subset of non-symptomatic tillers, 27.5% tested positive for SwMV; all of the symptomatic tillers that we tested were positive (20/20).
Our results suggest that SwMV is a parasitic symbiont associated with growth and fitness declines in P. virgatum tillers. In future studies, we aim to investigate the context-dependent effects of this symbiosis, on both plant and prairie plant community.