Introduction to Fungi, Third Edition

CLAVICIPITALES
359
Fig12.31 Endophytic hyphae of
Neotyphodium lolii growing
intercellularly between epidermal
cells of Lolium perenne.
Photographed from material kindly
provided by P. J.Fisher.
economic impact of fescue toxicosis to livestock
producers in the USA has been estimated at
$50 200 million annually (Siegel et al., 1984).
Perennial ryegrass (Lolium perenne) containing
N. lolii is associated with ryegrass staggers in
sheep in New Zealand (Fletcher & Harvey, 1981).
Various alkaloids are present in Neotyphodiuminfected
grasses. These belong to several groups,
including the ergopeptide-type ergot alkaloids
which we have already encountered in Claviceps
purpurea (Figs. 12.28, 12.32a), the lolitrems
(Fig. 12.32b), the lolines (Fig. 12.32c) and peramine
(Fig. 12.32d). The primary causal agent
of ryegrass staggers is the neurotoxin lolitrem B
(Siegel & Bush, 1997; Kuldau & Bacon, 2001).
In addition to their toxicity to mammals,
endophyte-infected grasses have deleterious
effects on insects feeding on them. The loline
alkaloids are primarily insecticidal. The endophytes
are therefore regarded as mutualistic
symbionts with their grass hosts, protecting
them against mammalian and insect herbivory
whilst themselves gaining nutrients and a means
of dissemination (Clay, 1988; Schardl & Clay,
1997). Endophyte-infected grasses are also more
resistant than uninfected hosts against attack by
certain fungal pathogens (Christensen, 1996) and
nematodes.
There are other effects of infection on the
grass hosts. Infection causes enhanced production
of biomass, increased tillering, drought
resistance and competitiveness. These are valuable
attributes of turf grasses and attempts have
been made to enhance the agronomic value of
turf grasses by deliberately infecting them with
endophytic fungi (Bacon et al., 1997), including
genetically modified strains. Attempts have also
been made to free seeds and seedlings of pasture
grasses from endophytes by fungicidal or heat
treatment. Prolonged storage may also achieve
this goal because the endophytes do not retain
viability for as long as the seeds. However,
endophyte-free plants are often less competitive
than their infected counterparts.
The origin of grass endophytes
The most convincing evidence that Neotyphodium
species are the anamorphic state of Epichloe is
molecular (Glenn et al., 1996; Kuldau et al., 1997).
Some of the endophytic Neotyphodium species are
believed to be directly related to a species of
Epichloe, e.g. N. lolii which is apparently derived
from E. festucae, whilst others are of hybrid
origin (Tsai et al., 1994; Schardl & Moon, 2003).
Interspecific heterokaryon formation has been
observed in culture (Chung & Schardl, 1997b).
Hyphal anastomosis and heterokaryosis may
take place within grass shoots infected with
more than one species of Epichloe, and the parasexual
origin of some Neotyphodium species is
a possibility (Tredway et al., 1999).