2

Kingdom Fungi Objectives

After studying this chapter, the reader should be able to:

• Describe the major features of the kingdom Fungi and its three subkingdoms.

• List distinguishing characteristics for each major group of fungi.

• Trace the life cycles of representative fungi.

• Describe the ecologic and economic importance of fungi.

• Discuss the mutualistic relationships formed by lichens and mycorrhizae.

I. Fungal Structure, Nutrition, and Ecology

A. General information

1. Fungi historically have been classified with plants, but major differences between

these organisms necessitated the formation of a separate kingdom

2. Fungi are eukaryotic, usually multicellular, multinucleate organisms

3. They obtain nutrients by secreting enzymes into their substrate and absorbing

the digested materials

4. Most fungal cell walls contain the polysaccharide chitin

5. The fungal structure is characterized by long slender filaments called hyphae

(see Types of Fungal Hyphae for an illustration)

a. Coenocytic hyphae consist of long cellular strands with many nuclei, which

are not contained within individual cells

b. Hyphae with complete septa (barriers) are separated into individual cells;

septate hyphae may contain a single nucleus in each cell or two nuclei in each cell (called dikaryotic hyphae)

c. Hyphae with incomplete septa allow cytoplasm to flow freely from cell to cell

d. Some parasitic fungi have haustoria, which are specialized hyphae that ex-

tend into individual host cells

6. The mycelium is the mass of hyphae that constitutes the body of a fungus; it is

the most recognizable structure of a mushroom (toadstool)

B. Modes of nutrition and metabolism

1. Fungi (along with bacteria) are the principal decomposers in every ecosystem

2. They can break down lignin, a major component of wood

3. Some fungi also attack living organic matter, causing agricultural damage and de-

stroying food stores; food contaminated by fungi may be unpalatable or poison­ous to humans

4. Warm, moist, dark conditions are most favorable for fungal growth; however, fungi

can grow in various other habitats

5. If conditions become too stressful, fungi survive by producing spores, which are

resistant to temperature and moisture extremes

6. Parasitic fungi use haustoria to penetrate individual host cells and absorb nutri-

ents directly from the cytoplasm

C. Ecology

1. Fungi typically form symbiotic relationships with plants or algae

2. Two types of fungal symbionts are lichens and mycorrhizae

3. Lichens are a combination of fungi (Ascomycetes or Basidiomycetes) and green

algae (phylum Chlorophyta) or cyanobacteria (kingdom Monera) a. The algae portion of the lichen provides food through photosynthesis; the fun­gal portion acts as a living sponge to improve water retention

(1) Some evidence suggests that the fungal partner parasitizes the algae

in a controlled fashion and sometimes may even destroy algal cells

(2) The fungal component of lichens rarely grows independently, although

the algal component may do so

b. Lichens inhabit cold, dry, and generally harsh environments, where they help

break rock surfaces and prepare the habitat for other organisms

c. They survive harsh or adverse conditions by becoming dehydrated, which

subsequently slows their metabolism

(1) When the lichen's water content drops dramatically, the upper portion

of the thallus becomes opaque enough to exclude light from the photo-synthetic algae

(2) Dehydrated lichen are unaffected by most environmental extremes be-

cause they are temporarily dormant and do not engage in photosyn­thesis

d. Lichens absorb nutrients from rain and air; for this reason, they are very sen-

sitive to air quality and are among the first organisms to perish in a pol­luted environment

e. Lichens are grouped into three major growth forms

(1) Crustose lichens attach to or embed in their substrate and often form

brightly colored, crusty patches on bare rocks and tree bark

(2) Foliose lichens have leaflike thalli, are weakly attached to their sub-

strate, and have edges that are crinkly or divided into lobes

(3) Fruticose lichens resemble miniature upright shrubs or may hang from

tree branches; their thalli are usually branched and cylindrical 4. Mycorrhlzae are a combination of fungi and plant roots

a. Mycorrhizae enhance the absorption of essential nutrients by plant roots

(1) They also may provide protection against the effects of acidic soil and

may make a plant more resistant to drought, cold, and harsh condi­tions

2. They can prevent the accumulation of toxic metals in plants

3. They can help plants to grow better in poor soils

4. They can speed the seed germination of orchids

b. About 90% of all plants have a symbiotic relationship with mycorrhizae

c. Plants with mycorrhizae develop fewer root hairs than those without mycorrhi-

zae; the mycorrhizae perform the same functions as root hairs, making them less necessary

d. Mycorrhizae are highly susceptible to acid rain, which may negatively affect

the growth of some plants

e. The two forms of mycorrhizae are endomycorrhizae and ectomycorrhizae

(1) Endomycorrhizae, the more common variety, are characteristic of

many crop species and develop when the fungal hyphae penetrate the outer root cells

(2) Ectomycorrhizae, the less common variety, are characteristic of shrubs

and trees and develop when the hyphae surround, rather than pene­trate, the root cells