Switchgrass as a reservoir for Barley yellow dwarf viruses: Is plant genotype important?

Background/Question/Methods

Panicum virgatum (switchgrass) is being considered as a source of cellulosic material for biofuel production, and if switchgrass monocultures become widely planted, pathogen dynamics in the landscape could be altered.  Barley yellow dwarf viruses and Cereal yellow dwarf viruses (Luteoviridae: BYDVs and CYDVs; hereafter jointly referred to BYDVs) are aphid-vectored RNA viruses that infect wild grasses and cereal crops worldwide.  Switchgrass could serve as a reservoir from which BYDVs could spillover into cereal crops, lowering their productivity.  The severity of this spillover may be mediated by plant genotypes, with particularly strong effects if susceptible cultivars are widely planted.  To assess whether BYDVs commonly infect field-grown switchgrass, we sampled individuals in switchgrass fields and prairie restorations across the southern portion of Michigan.  To assess whether varieties of switchgrass vary in susceptibility to BYDVs, we conducted a greenhouse inoculation experiment in which we caged viruliferous aphids on seedlings from four different populations of switchgrass. Seedlings were exposed to the aphids for six days, and tissue was harvested from the plants ten days after the inoculation period.  In both experiments, total RNA was extracted from samples, and an RT-PCR assay was run to determine infection status. 

Results/Conclusions

We found that BYDVs are present in field-grown switchgrass at significant levels: initial results from switchgrass fields indicate infection rates ranging from 9% to 28%.  In the greenhouse experiment, we were able to infect a portion of plants from each of the four seed sources with BYDV-PAV, but we found that populations differed in their infection rate.   The least susceptible population (12% infection) was a Michigan genotype produced by a local grower without any selective breeding.  The most susceptible population (68% infection) was the cultivar Trailblazer, which is recognized for high digestibility as a forage grass, indicating the possibility that selection for digestibility has reduced defensive traits.  Further work will assess whether there is a correlation between susceptibility and measures of digestibility as well as a comparison of infection rates in switchgrass fields as compared to more diverse prairie.  Our findings indicate that switchgrass could act as a reservoir for BYDVs and suggests that selection of biofuel cultivars for digestibility could inadvertently increase cultivar susceptibility to virus infection, with potential consequences for regional virus dynamics.