Differential Contribution of RNA Interference Components in Response to Distinct Fusarium graminearum Virus Infections
Authors: Yu Jisuk , Lee Kyung-Mi , Cho Won Kyong , Park Ju Yeon , Kim Kook-Hyung 
Published: February 2018

The mechanisms of RNA interference (RNAi) as a defense response against viruses remain unclear in many plant-pathogenic fungi. In this study, we used reverse genetics and virus-derived small RNA profiling to investigate the contributions of RNAi components to the antiviral response against Fusarium graminearum viruses 1 to 3 (FgV1, -2, and -3). Real-time reverse transcription-quantitative PCR (qRT-PCR) indicated that infection of
Fusarium graminearum
by FgV1, -2, or -3 differentially induces the gene expression of RNAi components in
F. graminearum
. Transcripts of the
DICER-2
and
AGO-1
genes of
F. graminearum
(
FgDICER-2
and
FgAGO-1
) accumulated at lower levels following FgV1 infection than following FgV2 or FgV3 infection. We constructed gene disruption and overexpression mutants for each of the Argonaute and dicer genes and for two RNA-dependent RNA polymerase (RdRP) genes and generated virus-infected strains of each mutant. Interestingly, mycelial growth was significantly faster for the FgV1-infected
FgAGO-1
overexpression mutant than for the FgV1-infected wild type, while neither FgV2 nor FgV3 infection altered the colony morphology of the gene deletion and overexpression mutants. FgV1 RNA accumulation was significantly decreased in the
FgAGO-1
overexpression mutant. Furthermore, the levels of induction of
FgAGO-1
,
FgDICER-2
, and some of the
FgRdRP
genes caused by FgV2 and FgV3 infection were similar to those caused by hairpin RNA-induced gene silencing. Using small RNA sequencing analysis, we documented different patterns of virus-derived small interfering RNA (vsiRNA) production in strains infected with FgV1, -2, and -3. Our results suggest that the Argonaute protein encoded by
FgAGO-1
is required for RNAi in
F. graminearum
, that
FgAGO-1
induction differs in response to FgV1, -2, and -3, and that
FgAGO-1
might contribute to the accumulation of vsiRNAs in FgV1-infected
F. graminearum
.

IMPORTANCE
To increase our understanding of how RNAi components in
Fusarium graminearum
react to mycovirus infections, we characterized the role(s) of RNAi components involved in the antiviral defense response against Fusarium graminearum viruses (FgVs). We observed differences in the levels of induction of RNA silencing-related genes, including
FgDICER-2
and
FgAGO-1
, in response to infection by three different FgVs.
FgAGO-1
can efficiently induce a robust RNAi response against FgV1 infection, but
FgDICER
genes might be relatively redundant to
FgAGO-1
with respect to antiviral defense. However, the contribution of this gene in the response to the other FgV infections might be small. Compared to previous studies of
Cryphonectria parasitica
, which showed dicer-like protein 2 and Argonaute-like protein 2 to be important in antiviral RNA silencing, our results showed that
F. graminearum
developed a more complex and robust RNA silencing system against mycoviruses and that FgDICER-1 and FgDICER-2 and FgAGO-1 and FgAGO-2 had redundant roles in antiviral RNA silencing.