Abstract
Plant tissues host a variety of fungi. One important group is the dark septate endophytes (DSEs) that colonize plant roots and form characteristic intracellular structures – melanized hyphae and microsclerotia. The DSE associations are common and frequently observed in various biomes and plant taxa. Reviews suggest that the proportion of plant species colonized by DSE equal that colonized by AM and microscopic studies show that the proportion of the root system colonized by fungi DSE can equal, or even exceed, the colonization by AM fungi. Despite the high frequency and suspected ecological importance, the effects of DSE colonization on plant growth and performance have remained unclear. Here, we draw from over a decade of experimentation with the obscure DSE symbiosis and synthesize across large bodies of published and unpublished data from Arabidopsis thaliana and Allium porrum model systems as well as from experiments that use native plants to better resolve the host responses to DSE colonization. The data indicate similar distribution of host responses in model and native plant studies, validating the use of model plants for tractable dissection of DSE symbioses. The available data also permit empirical testing of the environmental modulation of host responses to DSE colonization and refining the “mutualism-parasitism-continuum” paradigm for DSE symbioses. These data highlight the context dependency of the DSE symbioses: not only plant species but also ecotypes vary in their responses to populations of conspecific DSE fungi – environmental conditions further shift the host responses similar to those predicted based on the mutualism-parasitism-continuum paradigm. The model systems provide several established avenues of inquiry that permit more detailed molecular and functional dissection of fungal endophyte symbioses, identifying thus likely mechanisms that may underlie the observed host responses to endophyte colonization.
Highlights
Dark septate endophyte (DSE) fungi colonize plant roots and form characteristic structures – melanized hyphae and microsclerotia – and often have variable effects on plant growth
The mutualism–parasitism paradigm has been used as a general framework to understand the mycorrhizal symbioses that have – to DSE symbioses – been considered variable when observed in different hosts or compared under different abiotic conditions
We primarily focus on Periconia macrospinosa and its close relatives from the prairie ecosystems (Mandyam et al, 2010) and acknowledge that our experiments do not include other common DSE fungi such as Phialocephala fortinii or Cadophora finlandica that tend to be common in boreal/temperate forest ecosystems (Jumpponen and Trappe, 1998b; Jumpponen, 2001; Grünig et al, 2008)
Summary
Dark septate endophyte (DSE) fungi colonize plant roots and form characteristic structures – melanized hyphae and microsclerotia – and often have variable effects on plant growth. This inter- and intraspecific variability in host responses has been hypothesized to be central to plant community structuring by mycorrhizal fungi (Wilson and Hartnett, 1998; Hartnett and Wilson, 1999; van der Heijden, 2002). Results from studies that use model and native plant systems provide unique empirical insights into the variability in host responses to DSE fungi drawn from populations of conspecific fungi We argue that these data permit empirical evaluation of the “mutualism-parasitism-continuum” paradigm (Johnson et al, 1997; Saikkonen et al, 1998). These isolates colonized roots intracellularly, suggesting an association distinct from www.frontiersin.org
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