Sclerotinia
sclerotiorum and Nectria
radicicola: Understanding Mycoviruses
Cedric
NDINGA MUNIANIA
Western
Illinois University
My name
is Cedric NDINGA MUNIANIA, a graduate student in the Biological Sciences Department at Western Illinois University.
This blog was developed to help inform people who are interested in
fungal pathogenesis and myxovirus. I will use Nectria radicicola and Sclerotinia sclerotiorum as model organisms to illustrate the topic, but these properties extend to other fungi as well. I hope this would be
beneficial to you and will increase your interest in the subject.
Mycoviruses
As the name explains, mycoviruses are viruses
that affect fungi. They are widely spread throughout the different taxonomic
groups of fungi including, pathogenic or symbiotic fungi (Ghabrial et al. 2009).
They differ from other viruses because they lack an extracellular stage in
their life cycle and remain in the infected fungus cytoplasm for their entire
life (Cañizares et al. 2014). The effect of mycoviruses on fungi ranges from
ineffective (asymptomatic) to very effective which includes phenotypic and
physiological changes (Webster & Weber, 2007) (Fig 1).
History and Distribution : scientists came across mycoviruses for the first time in 1962 following an aggressive fungal infection known as La France disease, which was affecting the commercial mushroom Agaricus bisporus and causing a malformation of the fruiting bodies leading to low harvesting (Ghabrial et al. 2009). Analysis of the mushroom showed dominance of three types of viruses specific to fungi which would later be referred as mycoviruses (Ghabrial et al. 2009). After this case, a large number of cases involving mycoviruses have been reported and mycoviruses have been found in every corner of the globe were fungi occur and they affect a large number of fungal taxonomic groups across all phyla.
Viral structure and mode of transmission:
Most
mycoviruses have a double stranded RNA (dsRNA) embedded in a membrane, but
lack a capsid, a viral protein coat (Webster & Weber, 2007). However some
specific mycoviruses have genome sequences for particular single-stranded RNA
(ssRNA) that make dsRNA replicative intermediates (Cañizares et al. 2014). Due
to their obligate intracellular life and the lack of a capsid, mycoviruses cannot efficiently be transmitted like other viruses but instead they are
restricted to be transferred using intracellular mechanisms (Cañizares et al.
2014), which in most cases, include hyphae anastamosis or fusion, and asexual sporulation (Cortesi et al. 2001). Due to these reasons, along with the fact that
some of the infected fungi could be asymptomatic, it is difficult to
study the prevalence and rate of infection of mycoviruses in fungal communities (Ghabrial et al.
2009).
Fungal immune system response and gene regulation
The
fungal immune system acts in different ways to fight against the virus and
prevents it from spreading to the entire mycelium and other fungi occurring in
the same area. In order to control the spread of virus, two molecular mechanisms are utilized, somatic incompatibility (SI)
and RNA silencing (Nuss 2011). SI is the primary defense system used by the
fungi to prevent spreading of the virus throughout the population. As
previously described, mycoviruses mainly spread through the process of hyphal fusion or
anastamosis. However, vegetative cell fusion is controlled by a set of loci referred as the Het Genes (HG). Therefore the interaction between two incompatible mycelia usually result in competition for substrate and cell death, preventing fusion of the two fungi (Glass & Dementhon 2006). This limits the transmission of the virus to only compatible
mycelia, which in some cases have to come from the same fungus. The
virus, however, has evolved as a response to this mechanism and behaves very differently when
different alleles of the HG are expressed (Nuss 2011). For example, in C. parasitaca,
the virus transmission rate is almost non-affected by the given allelic
combination of the HG even though cell death is caused by SI (Nuss 2011). Also, it is observed that the virus transmission lowers when there is occurrence of rapid cell death (Nuss 2011). This tells us that not only the virus can be dependent to allelic variability
of the HG but also to the rate of cell death for specific fungi (Biella et al. 2002).
The
second mechanism of fungal defense is through the route of RNA silencing. This well known process observed in animal and plant cells, is also done by fungi (Göhre et al. 2013).
In this process the host IS cleaves the dsRNA in to small fragments of siRNA and miRNA using enzymes known as dicer which has a RNAse-III domain. These
fragments are then bound by the RNA Induced Silencing Complex (RISC) which
blocks protein synthesis of viral RNA as well as its replication and spreads in
the organism (Baulcombe 2004) (Fig 2). Fascinatingly, the virus has evolved some strategies to overcome RNA silencing. One efficient mode by which the virus performs these strategies is by the use of viral suppressors of RNA silencing (VSR), which requires a set of
different mechanisms and proteins to suppress the RNA silencing (Nuss 2011). One
major protein used to achieve this goal is protein P19, a viral protein that binds to
ds siDNA with high affinity, preventing further binding with the RISC factor and
thus preventing silencing of the viral RNA (Burgyan & Havelda 2011). Interestingly, the P19 protein, in addition to stopping the RNA silencing, acts as a coat protein and helper components for
viral transmission or transcriptional regulators (Burgyan & Havelda 2011).
This means that they are enhancing virulence of the mycovirus. In addition to the VSR, a mutation or recombination
in the fungi leading to synthesis of different dicer or argonaute proteins
could result in failure of silencing by the host immune system (Nuss 2011).
Mycovirus Effects on Fungi and Benefits
Depending
on the fungus and stages in its life cycle, a virus can have alternating effects and
some time a single virus or two closely related viruses can have different effects on two different fungi (Ahn & Lee 2000). In most cases mycovirus
infections are asymptomatic and do not cause cell death (Asensio et al. 2011). However true, the virus could also display aggressive behaviors and cause major changes, such as change in hyphae shape or structure, low reproduction or infertility
(Fig 3), lower mycotoxin production and hypovirulence or hypervirulence
(Ghabrial et Suzuki 2009). Although there are a plethora of distinguishable effects displayed by mycoviruses in which focus could be given, they could also be used as a biological control for hypovirulence, a type of
infection where the virus lower virulence or pathogenicity of a pathogenic
fungus; or hypervirulence, where the virus actually enhances virulence of a
given fungus. The last is not well studied due to the fact that agricultural
studies are majorly focusing on how to control pathogenic fungus affecting crops.
Phylum: Ascomycota
Class: Sordariomycetes
Order: Hypocreales
Family: Nectriaceae
Genus: Nectria
Nectria radicicola (anamorphic Cylindrocarpon destructans) is
an ascomycetes homothallic fungus that causes seedling blight and roots rot in
a variety of plants (Webster &Weber 2007). This fungus is particularly
pathogenic to ginseng plants (Ahn & Lee 2000). Ginseng required about six
years of cultivation before harvesting; therefore infection of the crops by this
fungus causes major problems for agriculture in ginseng-growing regions(Ahn & Lee 2000).
Genetics analysis of N. radicicola isolated
from ginseng have shown that mycoviruses with a specific sequence L1 dsRNA, enhances the virulence factor of this fungus, causing higher degradation of
the crop. Thus, selection of mycovirus to be used as biocontrol should exclude L1 dsRNA carrying mycoviruses. Interestingly these same
mycoviruses have the opposite effect on Cryphonectria parasitica, a
fungus that belongs to Cryphonectriaceae where infection with virus
carrying this gene lead to hypovirulence (Ahn & Lee 2000).
Hypovirulence:
Sclerotinia sclerotiorum (Fungorum 2014)
Phylum: Ascomycota
Class: Leotiomycetes
Order: Helotiales
Family: Sclerotiniaceae
Genus: Sclerotinia
Sclerotinia sclerotiorum is a pathogenic ascomycetes placed with in the
Sclerotinia genus. This fungus mainly reproduces asexually throughout the year
and starts developing sexual fruiting bodies in the spring (Ghabrial et Suzuki
2009). This fungus represents a big problem for crop harvesting and infects mainly
sunflower, soybean and oilseed rape. One major way to improve yield is to use
mycoviruses as biocontrol which could make the fungus less virulent. In this specific
case, Sclerotinia sclerotiorum debilitation-associated RNA-virus is used
because this mycovirus infects the S. sclerotiorum with high
affinity. The strain Ep-1PN causes
hypovirulence of the S. sclerotiorum (Ghabrial et Suzuki 2009). This is
a very useful tool for biological control of agricultural crops (Fig 4).
Other interesting facts
Mycoviruses
can actually be involved in a three way symbiosis to allow plant growth in
drought conditions. A good example is the mycovirus Curvularia thermal
tolerance virus (CthTV) infection on the fungus Curvularia protuberata. CthTVcan activate specific genes on the fungus which now gives it the ability to transfer heat resistance to plants
(Marquez et al. 2006). Another interesting fact is that some mycovirus strains
were classified and observed to belong to the same viral family or clade with
animal and human pathogens. Mycoviruses such as SsDRV SSRV-L and SsRV-L are all
included in the polyphyletic group of Rubi-Like Viruses, a groups which includes the human pathogen Hepatitis E Virus (Liu et al. 2009).
Conclusion
In retrospect, mycoviruses
are very diverse and can affect a variety of fungi. These mycoviruses usually asymptotic can be aggressive and even lead to cell death and hypo/hypertolerance. For these reasons, mycoviruses are used as biological control in the agricultural and food
industry to lower the effect of pathogenic fungi which can cause very dramatic
consequences. In order to understand the full effect of mycoviruses in the environment, more in depth research needs to be performed, which will furthermore expand our
knowledge on these organisms and their life cycle and biochemical processes.
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