218 CHAPTER 11
tissues, through to the final stages of decomposition
that result in the production of soil humus – a highly
complex mixture of organic compounds that persist for
tens or hundreds of years and that play a vital role in
soil fertility. For reviews of decomposition sequences
the following texts are strongly recommended: Carroll
& Wicklow (1992), Harper & Webster (1964), and
Hudson (1968).
The resident fungal population of living
tissues
The surfaces of all higher organisms support a popula-
tion of bacteria and yeasts that grow on the simple,
soluble nutrients that leak from the host tissues. This
is as true of humans as it is of the living leaves of plants
- it is estimated that, of all the cells in a human
body, fewer than 0.1% are human cells! Normally,
these resident microbes cause no harm and can even
be beneficial in suppressing invasion by potential
pathogens. But they can become invasive as the host
tissues begin to senesce, initiating the decomposition
process.
The yeasts and yeast-like fungi that colonize the
surfaces of living plant leaves have been studied in
detail. They grow in an environment termed the
phyllosphere, where the conditions are relatively
hostile because of exposure to UV irradiation and wide
fluctuations in temperature, moisture and nutrient
levels. Several common yeasts are found in this envir-
onment, including both Ascomycota and Basidio-
mycota, such as species of Candida, Cryptococcus,
Rhodotorula, Torulopsis, and Sporobolomyces. Many of
them have carotenoid pigments, which quench the toxic
effects of reactive oxygen species (Chapter 8). Several
also produce capsules and slime for adhesion to the
plant surfaces. One of the basidiomycetous yeasts,
Filobasidiella (Cryptococcus) neoformans, is notable
because it has become one of the most serious life-
threatening pathogens of immunocompromised people
in western societies, and a different form of this
fungus (variety gattii) causes an endemic disease of
immunocompetent people in Australia, Papua New
Guinea, and parts of Africa, India, south-east Asia
and Central and South America (Chapter 16). This
variety of the fungus is commonly associated with
eucalypt trees, and the extensive plantations of euca-
lypts that have been established outside of Australia
may provide the inoculum source for spread of this
human disease.
In stark contrast to this, the yeast Candida oleophila,
which grows on leaf and fruit surfaces, has been
marketed commercially as a biological control agent.
It is applied to fruit surfaces, where it competes for
nutrients, helping to reduce the incidence of fruit-rot
fungi that invade through minor wounds (Chapter
12).
The distribution of leaf surface yeasts can be studied
by pressing leaves against the surface of malt-extract
agar, then removing the leaves and incubating the agar
plates (Fig. 11.6). The surfaces of young leaves support
a relatively low microbial population, but as the
leaves start to senesce the population increases and is
often dominated by dimorphic fungi such as Sydowia
polyspora(Aureobasidium pullulans; see Fig. 6.7) and a
yellow Candidaspecies that produces a fringe of fun-
gal hyphae with clusters of yeast cells (Fig. 11.6).Endophytes, weak parasites, and
pathogensIn recent years, many fungi have been discovered to
grow as symptomless endophytes within plants that
show no obvious sign of infection or distress. Often
they occur sparsely as hyphae in the intercellular fluids
and wall spaces of their plant hosts. A specialized group
of these fungi – the “clavicipitaceous endophytes” –
are known to produce toxic alkaloids in grasses
(Chapter 14). Other fungal endophytes have attracted
interest as potential sources of novel metabolites
(bioprospecting), although recent studies of the endo-
phytic Colletotrichumisolates from tropical rainforests
in Guyana suggest that the diversity of endophytic fungi
(and thus of their products) may have been overesti-
mated: most of the endophytes were nonhost-specific
and were found repeatedly in different samples (Lu
et al. 2004).
Pathogens and weak parasites colonize living plant
tissues or tissues that are beginning to senesce. They
gain an advantage over purely saprotrophic fungi
because they tolerate or overcome the host defense
factors. They can remain active for a time after the
tissues have died, usually by exploiting the more
readily utilizable nutrients. Several mitosporic fungi
that produce melanized hyphae and spores are often
seen at this stage. They include Alternariaspp. and
Cladosporium spp., and a range of similar fungi (Fig. 11.7)
which utilize sugars and other soluble nutrients,
including the sugar-rich “honeydew” that aphids
excrete when they tap into plant phloem. Several
of these fungi grow well in laboratory culture when
supplied with soluble cellulose sources such as
carboxymethylcellulose, but they seldom grow well
on “crystalline” cellulose (filter paper, cotton fabric, etc.).
This suggests that they might utilize the swollen
(hydrated) cellulose and hemicelluloses in moist plant
tissues, as explained in Chapter 6.
Some plant pathogens that attack living plant tissues
can continue to grow after the plant has died, by
slowly using the dead tissues that they colonized