Abstract

Many fungi contain dsRNA infections, sometimes associated with virus-like particles, but in most cases there is no obvious phenotypic expression of their presence. In Puccinia striiformis all isolates have identical dsRNA patterns within their respective formae speciales tritici and hordei. The dsRNA could function to generate phenotypic variation for selection through gene silencing whilst maintaining the genotypic integrity of a successful pathotype. Alternatively, or additionally, the dsRNA may function to suppress recognition by the host plant, or to prevent hybridisation between non-adapted genotypes.

Discussion

Fungal pathogens fall into a number of different categories describing their strategies for generating variation whilst maintaining fit pathotypes. In powdery mildew of cereals (Blumeria graminis) for example, huge numbers of asexual spores are dispersed efficiently and exposed to selection, then undergo sexual recombination before ‘overwintering' thereby generating maximum variation. In contrast, pathogens such as yellow rust (Puccinia striiformis), whilst generating less but still considerable numbers of asexual spores and dispersing over shorter distances, it does not undergo sexual recombination and has relatively few pathotypes. These examples are of obligate biotrophs, but amongst the necrotrophs there are similar contrasts. Stagonospora nodorum and Phytophthora infestans undergo sexual recombination as well as having asexual spore dispersal. They are also both capable of generating variation in their asexual state in culture where colony sectors are often found. Rhynchosporium secalis also generates much cultural variation and also pathogenic variation, but no sexual cycle has been found.

The mechanisms generating variation, particularly cultural, are not known. In S. nodorum a high proportion of auxotrophic mutants, both induced and spontaneous, are unstable, sometimes over 50% (Newton, 1988). Transposon activation and DNA methylation are possible explanations, but this high level of reversion is uncharacteristic of such mechanisms. Another explanation may be dsRNA-mediated gene silencing.

DsRNA in fungi is often associated with Virus-Like Particles (VLPs). VLPs have been found in all major fungal taxonomic groupings which include more than 100 species representing 60 genera (Buck, 1980; Zhang et al., 1994) including 40 which are plant pathogens (Lemke, 1977). At least 10% of randomly sampled fungal isolates contain VLPs (Bozarth, 1972), and many more contain dsRNA but no detectable VLP (Bevan & Mitchell, 1979; Koltin & Day, 1976a; 1976b; Newton, 1987). In some species every single isolate sampled worldwide contains dsRNA (Newton et al., 1985). In the case of wheat yellow rust (Puccinia striiformis f.sp. hordei) every isolate, worldwide, appears to contain identical dsRNA. This ubiquitous occurrence suggests an important role in the lifecycle of the fungus, or a very efficient transmission mechanism (Newton, 1986). For example, if they encode for a toxin such as those encoded by dsRNA in Ustilago maydis (Day & Dodds, 1979) or Saccharomyces cerevisiae (Bevan & Mitchell, 1978), then they could restrict hybridization to identical dsRNA types thus protecting an optimised genotype.

Hypovirulent, dsRNA-containing strains can be derived from virulent, dsRNA-free strains of Endothia parasitica through far-UV irradiation (Day & Dodds, 1979) indicating that the dsRNA may sometimes be present as a provirus integrated into the host genome and homology has been reported between dsRNA and nuclear DNA suggesting the presence of proviruses (Vodkin, 1977). Such an activation / inactivation mechanism may be stress-related, triggering variation when selection is advantageous.

The formae sceciales of P. striiformis are characterised by distinct ‘D group' dsRNA bands (Newton et al., 1985) which may ensure the integrity of their optimised germplasm pools. Alternatively it may function as a gene silencing mechanism which allows pathogen infection to proceed undetected in its host, a different set of dsRNA being needed for wheat infection compared with barley. The mechanism whereby dsRNA affects gene function is becoming clearer in plants, and reports of its occurrence are increasing. Genes which encode for post-transcriptional gene silencing have been characterised in plants and fungi (Dalmay et al., 2000; Cogoni & Macino, 1999), and one of these, an RNA-dependent RNA polymerase, is common to both. It is likely to function by synthesising a dsRNA initiator of post-transcriptional gene silencing (Dalmay et al., 2000).

Isolates of wheat yellow rust with appropriate matching virulence factors can infect barley and visa versa (Newton et al., 1986). However, infection levels are always much less than for the appropriate forma specialis for that host (Newton & Crute, 1989) which may be a reflection of the lack of the appropriate advantageous dsRNA complement to evade background level detection in that host. This dsRNA may be used as a tool to determine which genes are responsible for this type of pathogen recognition.

Whether dsRNA banding patterns encode for anti-fungal toxins, or they exhibit other fungal gene expression modifying characteristics, if they have a role then there is the potential to utilise them in plants to control infection by fungi. The VLPs which probably contain the dsRNA in rusts may be exploitable as components of a transient expression or transformation system in rust fungi. Once such is available in rusts then rapid progress may be made in determining the basis of host species specificity, and whether dsRNA has a role to play in this or other attributes of rust fungi as effective plant pathogens.

Acknowledgements

I am grateful to the Scottish Executive Environment and Rural Affairs Department for financial support, and to Dr Rients Niks who acted as Editor for this paper as the author is the current Editor of CRPMB.

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