Viticulture is an important agronomic sector that has the potential to greatly benefit by improvements in our understanding of grapevine cultivation. Although conventional viticulture relies to a great extent on pesticide and fertilizer application, more sustainable approaches involve management practices that favor plant–fungus interactions that have positive effects on the nutritional quality of the grapes and reduce production costs (i.e., of pesticides and fertilizers) and thus reduce the negative effects on the environment. Fungal endophytes in grapevine belong to different taxa, with the majority of reports focusing on fungi that form arbuscular mycorrhizal associations. These fungal endophytes have been demonstrated to confer beneficial growth and nutrition effects to their plant hosts via improved exploitation of the substrate and improved tolerance of the grapevine to abiotic and biotic stresses.
Arbuscular mycorrhizal fungi
The arbuscular mycorrhizal fungi (AMF) are among the most important beneficial fungal root endophytes, and they are known to colonise the roots of the majority of land plants, including grapevines (Karagiannidis and Nikolaou 1999; Schreiner and Mihara 2009; Likar et al. 2013). The AMF comprise of some 270 species of fungi from phylum Glomeromycota (Schüßler et al. 2001; Schüßler 2017), which associate with 80% of vascular plant species (mostly herbaceous plants, but also various woody plant families). AMF are obligate biotrophs, relying on living root tissue for carbohydrate supply. AMF colonize short roots and replace the function of root hairs. They supply water and nutrients, particularly phosphate but also nitrogen and other minerals including zinc, to the host plant. Inside the roots they form arbuscules and coils that improve the exchange between the symbionts through increased area of contact (Smith and Read 2008).
Jacott et al. (2017) DOI:10.3390/agronomy7040075 provide a good review on the importance of AMF in agro-ecosystems.
Dark septate endophytes
In addition to AMF, fungi that form septate hyphae with melanized cell walls are common colonizers of plant roots in natural environments. These fungi are referred to as dark septate endophytes (DSEs), and they represent a heterogeneous group of ascomycetes that occur across a wide range of terrestrial ecosystems, although they are most numerous in extreme habitats (for a review see Mandyam and Jumpponen 2005, DOI:10.3114/sim.53.1.173). Among the DSEs, members of the order Helotiales are commonly observed, although frequently these isolates are not fully identified or they are grouped with the anamorphic genera Phialocephala, Rhynchosporium, and Phialophora.
The knowledge on involvement of DSEs in plant nutrition is still very limited. The first reports on DSEs showed that they can promote the uptake of nitrogen and phosphorus into plants, although their effects on the overall plant biomass appeared to be dependent on host–symbiont combinations and soil conditions. With further studies, a wider range of effects on host physiology have been reported, from negative or negligible to positive (Newsham 2011).
Fungal endophytes in grapevine
Vineyard soils also support fungal endophytes like AMF and DSEs, including endophytes in grapevine (Likar et al. 2013; Oehl et al. 2005; Radić et al. 2014). AMF can have positive effects on grapevine performance (Biricolti et al. 1997; Linderman and Davis 2001). Furthermore, low root density and coarse root texture of grapevines suggest that they are highly dependent on symbiotic fungi, making these an important factor for grapevine’s growth and development. This is especially true in organic vineyards with low-input practices, as these support higher abundance and diversity of AMF (Oehl et al. 2004).
The presence of DSEs on grapevine is still not fully confirmed. The bulk of our knowledge on DSE fungi is restricted to the colder and temperate zones. Although DSEs are found throughout the world, studies performed in Mediterranean have failed to observe DSE taxa that dominate the colder regions (Girlanda et al. 2006). Therefore DSE communities in vineyards, which are by nature based in warmer climates, might be formed by completely different fungi. Consequently their absence in grapevine could be due to the fact that DSE species from warmer climates are not yet known.
For a review on fungal endophytes in grapevine see Likar and Regvar (2017) DOI: 10.1007/978-3-319-53064-2_13 that reviews current knowledge on the importance and potential of these diverse fungal groups for grapevine production and expose the gaps in our understanding of possible functions of fungal groups that are currently little studied.
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