136 Elena Fernández-Miranda Cagigal
(Wilcox & Wang, 1987 Phialocephala fortinii in association with Pinus
resinosa and Picea rubens; Stoyke & Currah, 1993 P. fortinii in association
with Menziesia ferruginea; Tellenbach et al., 2011 P. fortinii in association
with Picea abies) to positive (Newsham, 1999 P. graminicola in association
with Vulpia ciliata spp. ambigua; Usuki & Narisawa, 2007 Heteroconium
chaetospira in association with Brassica rapa). Furthermore, it has also been
suggested that the effect of dark septate endophyte colonization can vary along
a continuum from parasitism to mutualism, much as the effect of mycorrhizal
symbioses (Jumpponen, 2001).
Figure 3. A) Ectomycorrhiza caused by Helotial sp. in Betula celtiberica, B) Cross-
Section stained with blue cotton.
Several hypotheses have been put forward to explain the observed positive
responses to root endophyte colonization, being the two most prominent: 1)
modulation of plant growth via nutrient mineralization (as in mycorrhizae)
(Jumpponen, 2001; Mandyam & Jumpponen 2005; Upson et al., 2009;
Newsham, 2011) and 2) production of plant growth promoting phytohormones
(Mucciarelli et al., 2002; Schulz & Boyle, 2005; Schulz, 2006).
In view of the above evidence, a meta-analysis performed by Mayerhofer
et al., (2013) showed that identity of the inoculated endophyte affects plant
response, as was the case for plants inoculated with Phialocephala, which tend
to have smaller biomass than controls (Tellenbach et al., 2011; Reininger et
al., 2012). Not to mention differences in experimental conditions, which
undoubtedly contribute to the high levels of variability in plant response seen
in the literature. In conclusion, it appears that the effects depend on the host-
symbiont combination, and therefore more studies are necessary to clarify
which species inside this genus could be considered mycorrhizal fungi and
which not.