Genetic Variability and Evolutionary Implications of RNA Silencing Suppressor Genes in RNA1 of Sweet Potato Chlorotic Stunt Virus Isolates Infecting Sweetpotato and Related Wild Species
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Date
2013
Journal Title
Journal ISSN
Volume Title
Publisher
PLoS ONE
Abstract
The bipartite single-stranded RNA genome of Sweet potato chlorotic stunt virus (SPCSV, genus
Crinivirus; Closteroviridae) encodes a Class 1 RNase III (RNase3), a putative hydrophobic protein (p7) and a 22-kDa
protein (p22) from genes located in RNA1. RNase3 and p22 suppress RNA silencing, the basal antiviral defence
mechanism in plants. RNase3 is sufficient to render sweetpotato (Ipomoea batatas) virus-susceptible and
predisposes it to development of severe diseases following infection with unrelated virus. The incidence, strains and
gene content of SPCSV infecting wild plant species have not been studied.
Methodology/Principal Findings: Thirty SPCSV isolates were characterized from 10 wild Ipomoea species,
Hewittia sublobata or Lepistemon owariensis (family Convolvulaceae) in Uganda and compared with 34 local SPCSV
isolates infecting sweetpotatoes. All isolates belonged to the East African (EA) strain of SPCSV and contained
RNase3 and p7, but p22 was not detected in six isolates. The three genes showed only limited genetic variability and
the proteins were under purifying selection. SPCSV isolates lacking p22 synergized with Sweet potato feathery
mottle virus (SPFMV, genus potyvirus; Potyviridae) and caused severe symptoms in co-infected sweetpotato plants.
One SPCSV isolate enhanced accumulation of SPFMV, but no severe symptoms developed. A new whiteflytransmitted
virus (KML33b) encoding an RNase3 homolog (<56% identity to SPCSV RNase3) able to suppresses
sense-mediated RNA silencing was detected in I. sinensis.
Conclusions/Significance: SPCSV isolates infecting wild species and sweetpotato in Uganda were genetically
undifferentiated, suggesting inter-species transmission of SPCSV. Most isolates in Uganda contained p22, unlike
SPCSV isolates characterized from other countries and continents. Enhanced accumulation of SPFMV and
increased disease severity were found to be uncoupled phenotypic outcomes of RNase3-mediated viral synergism in
sweetpotato. A second virus encoding an RNase3-like RNA silencing suppressor was detected. Overall, results
provided many novel and important insights into evolutionary biology of SPCSV.
Description
Keywords
Genetic Variability, RNA Silencing Suppressor Genes, Sweet Potato Chlorotic Stunt Virus
Citation
Tugume AK, Amayo R, Weinheimer I, Mukasa SB, Rubaihayo PR, et al. (2013) Genetic Variability and Evolutionary Implications of RNA Silencing Suppressor Genes in RNA1 of Sweet Potato Chlorotic Stunt Virus Isolates Infecting Sweetpotato and Related Wild Species. PLoS ONE 8(11): e81479. doi:10.1371/journal.pone.0081479