Fungal endophytes and their toxins
In recent years the leaves and roots of many types of
plant have been shown to harbor fungi that grow as
symptomless endophytes. These fungi do not pene-
trate the plant cells; instead they grow as relatively sparse
hyphae in the spaces between the cells or within the
walls of the plant cells, utilizing low levels of nutrients
in the intercellular fluids and causing no obvious
damage. In many ways, this behavior is similar to the
way in which the vascular wilt fusaria colonize the low
levels of nutrients in the xylem fluids, discussed earlier.
The significance of fungal endophytes lies in the fact
that they produce a wide range of secondary metabo-
lites of potential commercial importance. In addition,
some of the endophytes confer abiotic stress tolerance
on their plant hosts, or act as insect antifeedants.
In the discussion that follows we will focus on one
particular group of fungal endophytes – those that grow
in some pasture grasses and have repeatedly been
shown to reduce insect damage (Bony et al. 2001).
The grass-associated endophytes grow very slowly in
laboratory culture and do not produce sporing stages.
In fact, some of them have probably lost this ability
altogether because they always grow within a plant and
are transferred from generation to generation by grow-
ing into the seed coat. This makes it easy to compare
the behavior of endophyte-infected and endophyte-free
populations of grasses. The few isolates that have been
induced to sporulate in culture produce a simple coni-
dial stage classified as Neotyphodium(previously called
Acremonium) and resembling the conidial stage of the
ergot fungus Claviceps purpurea. These “clavicipitace-
ous endophytes” (so-named because they are related to
the ergot fungus) are found in several important pas-
ture grasses such as Lolium(ryegrass), Festuca(fescue),
and Dactylis glomerata(cocksfoot) in the USA, Europe,
and New Zealand. They grow within the leaf tissues but
their growth seems to be tightly regulated because it
is synchronized with growth of the leaves, and often
ceases when the leaves mature (Clay 1989).
Endophyte-infested perennial ryegrass has been
shown to contain several mycotoxins, including three
that are particularly important:
1 The saturated aminopyrrolizidine alkaloids such as
lolitrem B, a compound that has been shown to be
responsible for the damaging “ryegrass staggers”
disease of grazing sheep and cattle in New Zealand.
These alkaloids can reach levels of up to 2% in the
grass biomass.
2 Ergovaline, which causes diseases such as “fescue
foot” and “fescue toxicosis” in grazing animals.
3 Peramine, a tripeptide that is repellant and toxic to
insects, but not to mammals.
The production of these compounds is probably
favored by the slow, substrate-limited growth of
endophytes within plant tissues, because secondary
metabolites characteristically accumulate in conditions
that restrict normal growth (Chapter 7). The multiple
effects of these mycotoxins raise dilemmas. On the one
hand, endophyte-infected grass varieties can pose seri-
ous threats to grazing livestock. On the other hand,
there are many reports that endophytes increase the
stress-tolerance of grasses, and confer a competitive
advantage over endophyte-free grasses by deterring
insect damage.
The endophyte Neotyphodium coenophialumis found
in tall fescue grassland and produces ergovalinein the
plant tissues. Ergovaline causes a range of symptoms
in grazing animals, including hyperthermia, weight
loss, reduced rates of pregnancy, decreased milk pro-
duction, and (in horses) birth defects and abortion.
The endophyte N. loliiin perennial ryegrass produces
lolitrem B as its major toxin. This neurotoxin is
responsible for a condition called ryegrass staggers,
where animals experience tremors and loss of co-
ordination. The potential extent of these problems
in the USA was revealed by a survey of horse pastures
(Fig. 14.18; USDA 2000). Nationally, 61.6% of sampled
pastures tested positive for endophytes, and a total
28.5% tested positive for toxin. Without interven-
tion, the infection rates would increase with time
because infected grasses can outcompete noninfected
grasses.
Because the clavicipitaceous endophytes never
leave the host plant, pasture and turf managers have
options. For grazing animals, a pasture can be re-
established with certified endophyte-free seed, whereas
managers of ornamental or recreational turf can elect
to sow endophyte-infected seed for improved grass
vigor and insect resistance. This has spawned a niche
market in the USA, where “endophyte-enhanced
grasses” are available through garden centers and
seed stores for use in amenity grasslands, golf-courses,
sports turf, etc.PhytophthoradiseasesThe genus Phytophthora contains more than 50
described species, most or all of which are plant
pathogens (Erwin & Ribeiro 1996). In fact the name
Phytophthora, derived from Greek, means literally plant
destroyer. Among the many species of Phytophthora,
the potato blight pathogen, P. infestans, has become
notorious, even though it has a narrow host range –
essentially potato plants and, to a lesser extent, tomato
(another member of the potato family). But several other
species have very broad host ranges and cause various
types of disease, including: