Fungi: Recyclers, Pathogens,
and Plant Partners
31 Fungi: Recyclers, Pathogens, and Plant Partners
• General Biology of the Fungi
• Diversity in the Kingdom Fungi
• Fungal Associations
31 General Biology of the Fungi
• The fungi live by absorptive nutrition, secreting
digestive enzymes that break down large food
molecules and absorbing the breakdown
products.
• Some are saprobes (feeding on dead matter);
others are parasites.
• A few have mutually beneficial (symbiotic)
relationships with other organisms.
Figure 31.1 Parasitic Fungi Attack Other Living Organisms
31 General Biology of the Fungi
• The production of chitin is a shared derived trait
for fungi, choanoflagellates, and animals.
• The presence of chitin in fungi is evidence that all
fungi are more closely related to animals than to
plants.
• The kingdom Fungi consists of four phyla:
Chytridiomycota, Zygomycota, Ascomycota, and
Basidiomycota.
• The four phyla are primarily distinguished by their
methods and structures of reproduction.
Table 31.1 Classification of Fungi
31 General Biology of the Fungi
• Unicellular forms are found in all of the fungal
phyla.
• Those of the Zygomycota, Ascomycota, and
Basidiomycota are called yeasts.
• Yeasts may reproduce by budding, fission, or
sexual means.
Figure 31.2 Yeasts Are Unicellular Fungi
31 General Biology of the Fungi
• The vegetative body of a multicellular fungus is
called the mycelium (plural mycelia).
• The mycelium is composed of threadlike hyphae
(singular hypha).
• Within the hyphae of two clades, cell-like
compartments are formed by incomplete cross-
walls called septa (singular septum).
• Pores in septa allow free movement of organelles,
sometimes even nuclei, and other materials.
• Some hyphae are coenocytic—they have no
internal separations into distinct cells.
Figure 31.3 Most Hyphae Are Incompletely Divided into Separate Cells
31 General Biology of the Fungi
• The hyphae may be widely dispersed to forage for
nutrients or they may be clumped together in a
cottony mass to exploit a rich nutrient source.
• Sometimes the mycelium becomes reorganized
into a fruiting (reproductive) structure, such as a
mushroom.
• Rhizoids are modified hyphae, which anchor
Chytridiomycota to a substrate.
• These rhizoids are not homologous to the rhizoids
of plants because they are not specialized to
absorb water and nutrients.
31 General Biology of the Fungi
• Some parasitic fungi may have modified hyphae
that take up nutrients.
• Fungal parasites can invade wounds on plants
and grow mycelium throughout the plant.
• Some hyphae produce haustoria, branching
projections that push into the living plant cells and
absorb nutrients within them.
Figure 31.4 A Fungus Attacks a Leaf
31 General Biology of the Fungi
• The mycelium has a very high surface area-to-
volume ratio.
• Throughout the mycelium (except in fruiting
structures), all the hyphae are very close to their
environmental food source.
• Fungi are tolerant to highly hypertonic
environments.
• Many can tolerate temperatures as low as 5–6C
below freezing. Some can tolerate temperatures
as high as 50 C or more.
31 General Biology of the Fungi
• The majority of fungi are saprobes, living on dead
organisms.
• Saprobic fungi (along with bacteria) are Earth’s
primary decomposers.
• Fungi are the principal decomposers of cellulose
and lignin.
• Many fungi can use ammonium (NH4+) ions or
nitrate (NO3–) as a sole source of nitrogen.
• Most are unable to synthesize their own thiamin
or biotin, but they can synthesize some vitamins
that animals cannot.
31 General Biology of the Fungi
• Facultative parasites can attack living organisms
but they can also be grown on defined media.
• Obligate parasites grow only on their specific
host.
• Most predatory fungi secrete sticky substances
from the hyphae. Trapped prey are penetrated by
hyphae and eventually killed.
• Some species form a ring with modified hyphae
that constricts around nematodes.
• The crawling nematode triggers these rings to
swell and trap the worm. Hyphae quickly invade
and digest the worm.
Figure 31.5 Some Fungi Are Predators
31 General Biology of the Fungi
• Lichens are symbiotic associations of a fungus
with a cyanobacterium, a unicellular photosynthetic
eukaryote, or both.
• Mycorrhizae are mutualistic associations of fungi
and plant roots.
• The fungus obtains organic compounds, while the
plant is provided with water and soil minerals.
31 General Biology of the Fungi
• Asexual reproduction among the fungi includes:
The production of haploid spores within
sporangia.
The production of naked spores at the tips of
hyphae (not within sporangia) called conidia.
Cell division by unicellular fungi—either equal
division (fission) or production of a daughter
cell (budding).
Simple breakage of the mycelium.
31 General Biology of the Fungi
• Sexual reproduction involves fusion between
different mating types.
• Some fungi have more than two mating types.
• Mating types cannot be distinguished
morphologically.
• Mating can occur only between different mating
types, which prevents self-fertilization.
• Fungi reproduce sexually when hyphae (or motile
cells in chytrids) of different mating types meet
and fuse.
31 General Biology of the Fungi
• In many fungi, the zygote nuclei are the only
diploid nuclei of the life cycle.
• These nuclei undergo meiosis, producing haploid
nuclei.
• Haploid spores divide mitotically to form haploid
hyphae.
• This type of life cycle is called a haplontic life
cycle and is a characteristic of many protists.
31 General Biology of the Fungi
• Fungal pathogens are a major cause of death
among people with compromised immune
systems.
• Most patients with AIDS die of fungal diseases
such as Pneumocystis carinii.
• Candida albicans and other yeasts also cause
severe diseases in those with AIDS.
• Other less severe and common diseases include
ringworm and athlete’s foot.
• Plant diseases include black stem rust and others.
Figure 31.6 Phylogeny of the Fungi
31 Diversity in the Kingdom Fungi
• The chytrids (phylum Chytridiomycota) are the
earliest diverging fungal lineage.
• They are aquatic microorganisms, formerly classed
with protists but now classed with fungi because of
the chitin in their cell walls.
• They are the only fungi that have flagella at any
stage of the life cycle.
• Chytrids are parasitic or saprobic, but some are
found in the rumen of ruminants. Most live in fresh
water or moist soil; some are marine.
• Some are unicellular; others have coenocytic
hyphae.
• They reproduce both sexually and asexually.
31 Diversity in the Kingdom Fungi
• Allomyces displays alternation of generations.
• A haploid zoospore germinates to form a haploid
organism, which later forms female and male
gametangia. Both have flagella.
• The female gametes produce a pheromone that
attracts male gametes. The male and female
gametes fuse to produce a diploid organism, which
produces numerous diploid flagellated zoospores.
These disperse and produce more diploid organisms.
• These eventually produce resistant resting sporangia
that can survive dry and freezing weather.
• The nuclei in sporangia eventually undergo meiosis
to produce haploid zoospores.
Figure 31.7 Reproductive Structures of a Chytrid
31 Diversity in the Kingdom Fungi
• Zygomycetes (phylum Zygomycota) have
coenocytic hyphae; they have only one diploid
cell, the zygote.
• Most form occasional stalked reproductive
structures called sporangiophores.
• Sporangiophores may have one or many
sporangia.
• One group are the fungal species in the most
common mycorrhizal associations.
• Black bread mold is Rhizopus stolonifer.
Figure 31.8 A Zygomycete
Figure 31.9 Sexual Reproduction in a Zygomycete (Part 2)
31 Diversity in the Kingdom Fungi
• The ascomycetes (phylum Ascomycota) are a
large and diverse group with septate hyphae, and
distinguished by the production of asci (singular
ascus).
• The ascus contains the products of meiosis.
• There are two groups of ascomycetes:
Those with an ascocarp are called
euascomycetes (―true ascomycetes‖).
Those without are called hemiascomycetes
(―half ascomycetes‖).
Figure 31.10 Asci and Ascospores
31 Diversity in the Kingdom Fungi
• Most hemiascomycetes are microscopic and
some are unicellular.
• Baker’s or brewer’s yeast (Saccharomyces
cerevisiae) is an ascomycete.
• Hemiascomycete yeasts reproduce asexually by
budding or fission.
• Sexual reproduction occurs when two haploid
cells of opposite mating types fuse.
• In some, the zygote immediately undergoes
meiosis. The entire cell becomes an ascus.
• Four or eight ascospores are produced
depending on whether the cells divide once after
meiosis.
31 Diversity in the Kingdom Fungi
• The euascomycetes include some of the fungi
known as mold. Neurospora is pink bread mold.
• Many euascomycetes are plant parasites such as
chestnut blight and Dutch elm disease.
• Powdery mildews infect cereals, lilacs, roses, and
other plants.
• Cup fungi such as morels and truffles are
euascomycetes. These produce huge numbers of
spores and can be several centimeters in
diameter.
Figure 31.11 Two Cup Fungi
31 Diversity in the Kingdom Fungi
• Penicillium is a genus of green molds. Some
species produce the antibiotic penicillin.
• P. roquefortii and P. camembertii provide the
flavors to the cheeses Roquefort and Camembert.
• Aspergillus tamarii is used to ferment soybeans to
make soy sauce. A. oryzae is used in brewing the
Japanese alcoholic beverage sake.
• Some Aspergillus species that contaminate
peanuts and pecans produce powerful mutagens
called aflatoxins.
Figure 31.13 The Life Cycle of a Euascomycete
31 Diversity in the Kingdom Fungi
• About 25,000 species of basidiomycetes
(phylum Basidiomycota) have been described.
• They produce a wide variety of fruiting structures
(basidiocarps): puffballs, mushrooms, and giant
bracket fungi.
• There are more than 3,250 species of
mushrooms.
• Agaricus bisporus is the common edible one;
some Amanita mushrooms are deadly poisonous.
• Bracket fungi are tree parasites.
• Smut fungi parasitize cereal grains.
• Basidiomycetes have septate hyphae.
Figure 31.14 Basidiomycete Fruiting Structures (Part 1)
Figure 31.14 Basidiomycete Fruiting Structures (Part 2)
Figure 31.14 Basidiomycete Fruiting Structures (Part 3)
Figure 31.15 The Basidiomycete Life Cycle
31 Diversity in the Kingdom Fungi
• Fungi not yet placed in any existing phyla are
grouped as imperfect fungi or deuteromycetes.
• Deuteromycetes currently include 25,000 species.
• The sexual cycle has yet to be observed in these
species.
• DNA sequences can now be used to determine
actual relationships between deuteromycetes and
other fungi.
31 Fungal Associations
• Almost all tracheophytes have mycorrhizae, which
help make water and minerals more available to the
plant.
• Ectomycorrhizae are fungi that wrap around the
root tips and acts as a sponge.
• Endomycorrhizae infect the interior of the root.
• The fungi get sugars, amino acids, and some
vitamins from the plant.
• The fungi might supply growth hormones as well,
and protect the plant against attack by
microorganisms.
• Fungal–plant root interactions have existed for
hundreds of millions of years.
Figure 31.16 Mycorrhizal Associations
31 Fungal Associations
• Lichens are a meshwork of two different
organisms. One is a fungus, and the other is a
photosynthetic organism.
• Lichens can survive harsh environments.
• In spite of this hardiness, lichens are sensitive to
air pollution because they cannot excrete toxic
substances. Hence they are good biological
indicators of air pollution.
• The fungi of most lichens are ascomycetes.
• The photosynthetic component may be either a
cyanobacterium or a unicellular green alga.
31 Fungal Associations
• There are about 13,500 ―species‖ of lichens.
• The fungal components cannot grow
independently of their photosynthetic partners.
• The reindeer ―moss‖ is a lichen that is very
important in the diet of large mammals in the
arctic, subarctic, and boreal regions.
• Lichen growth forms include crustose, foliose,
and fruticose.
Figure 31.17 Lichen Body Forms (Part 1)
Figure 31.17 Lichen Body Forms (Part 2)
31 Fungal Associations
• The most widely held interpretation is that the
lichen relationship is a mutualistic one.
• The algal cells in a lichen ―leak‖ photosynthetic
products at a greater rate than do similar cells
growing on their own.
• On the other hand, the photosynthetic cells from
lichens grow more rapidly on their own than when
combined with a fungus.
• Therefore, we could consider lichen fungi as
parasitic on their photosynthetic partners.
31 Fungal Associations
• Lichens can reproduce simply by fragmentation of
the vegetative body called the thallus.
• They can also reproduce by means of specialized
structures called soredia (singular soredium).
• These are composed of fungal hyphae and a few
photosynthetic cells.
• These become detached, are dispersed by air
currents, and then develop into a new lichen.
• If the fungal partner is an ascomycete or a
basidiomycete, the fungus may undergo a sexual
process, but the spores are released alone into
the environment and fail to reestablish the lichen
relationship.
Figure 31.18 Lichen Anatomy (Part 1)
Figure 31.18 Lichen Anatomy (Part 2)
31 Fungal Associations
• Lichens are often the first colonists on newly
exposed areas of rock.
• They satisfy most of their nutritional needs from
air, rainwater, and from the minerals absorbed
from dust.
• A lichen usually begins to grow shortly after a rain
event. Eventually, as the lichen grows, its water
content drops to less than 10 percent of its dry
weight, and it becomes highly tolerant of
temperature extremes.