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division. Some fungi (including many human pathogens) can adopt
filamentous or yeast-like growth, with morphology depending on
environmental conditions. All cells and organisms are polarized, that
is, they generate and maintain specialized regions that are essential
for movement and differentiation. Processes related to cell polarity
are fundamentally important in many areas of biology. Hyphal
cytoplasm has an extreme level of spatially and temporally predictable
polarity. As the hyphal tip extends, the apical cytoplasm secretes
wall-building materials and components for nutrient acquisition that
were synthesized in basal regions. A saprotrophic fungus like
Aspergillus nidulans, which forms green mold colonies on bread,
has hyphae that are 3 μm in diameter and grow up to 1 μm/min;
there are both larger and faster-growing species. The green center of
these colonies is due to colored asexual spores generated for survival
and dispersal (see Fig. 3 in Ref. 11). Consistent with fungal walls
being essential for defining the cell form and as an interface with
their environment, about 20 percent of the Aspergillus genome is
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suggested to have wall-related functions. Within the wall, organelle-
rich cytoplasm migrates toward the tip, keeping pace with growth,
and subapical regions become filled with vacuoles that contribute
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other metabolic functions. Taken together, fungal hyphae have
pronounced structural and functional polarization that unlike many
biological systems is relatively predictable and produces cells with
simple geometrical forms.
Like animals, fungi acquire their nutrition from other organisms.
Saprotrophic fungi consume dead organisms, particularly plants, and
have essential roles in recycling. Biotrophic fungi have more or less
long-term relationships with living organisms, again particularly
with plants. Some saprotrophic and some biotrophic fungi cause
disease; however, many others are essential symbionts. Mycorrhizal
fungi are associated with the roots of at least 90 percent of plant
families, trading minerals for carbohydrates created by photosynthesis.
Fossils from 450 million years ago showing these associations are some
of the evidence that suggested mycorrhizae might have been necessary
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for the colonization of land. More recently, endophytic fungi that
live within healthy plants have been proposed to be an equally
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ancient relationship. Some fungal endophytes confer tolerance
to environmental stress, and have been shown to be, at the least,
very widely distributed. 15
Fungi are important for ecosystem stability, as threats to the human
food supply, as emerging threats to human health, and for their roles in
ancient and modern biotechnology. Many fungal species have short
life cycles, relatively simple genomes, and are experimentally tractable.
Their underlying physiological similarities with animal systems, pre-
dictable growth patterns, and myriad ecological and technological
impacts, make fungi exemplary systems for scientific inquiry and for
assessing the biological relevance of certain analytical methods.