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IV. FUNGI AS SYMBIONTS AND COMMENSALS TOP A. Symbionts of Plants 2. Lichens 3. Symbiosis and Evolution of Land Plants
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2. Lichens An association between a fungus and an alga that develops into a unique morphological form that is distinct from either partner is termed lichen (Fig. 14-70).
Fig. 14-70. Cladina stellaris is a common lichen found on sandy, barren habitats. There are three basic types of lichens based upon their superficial morphology. They are: fruticose (Fig. 14-70) in which the thallus is upright and usually looks like coarse hairs, crustose (Fig. 14-71) in which the thallus is crustaceous and grows flat on the substrate; and foliose (Fig. 14-72) in which the thallus is leaflike and attached to the substrate.
Fig. 14-71. Flat, crustose lichens are common on a large number of tree and shrub species.
Fig. 14-72. A foliose lichen in which the mycobiont is a discomycete, note the yellowish apothecia. Microscopically, one can observe an intermingling of the fungus and alga in the lichen association. The algal partner in the lichen association is called the phycobiont (Fig. 14-73) and the fungal partner, the mycobiont.
Fig. 14-73. A thin section through a lichen showing the green phycobiont and the filamentous mycobiont.
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Close to 25 genera of algae are members of lichen
associations, but the majority belongs to the green algal genus Trebouxia
and most of the remainder to the blue-green alga (cyanobacterium) Nostoc.
It has been repeatedly shown that the fungus derives nutrients from its
algal partner through haustorial penetration but it is not easy to determine the possible advantages the algal
partner receives. The alga may store foods that it manufactures in the
fungus that it can use whenever environmental conditions are not
conducive to photosynthesis. The fungus, however, may have some inherent
ability to absorb moisture and certain critical minerals not readily
accessible to the alga. Since lichens are the first invaders of volcanic
islands after they have cooled, obtaining minerals and water in such a
harsh environment would be difficult for the alga alone. Lichens reproduce both sexually and asexually.
Asexually they produce isidia
(columnar outgrowth) and soredia
(a few algal cells surrounded by mycelium). Isidia and soredia contain
both fungal and algal partners,
and when blown to a new area they are ready to “set up housekeeping. " They
have various ways of reproducing sexually, depending on the type of
mycobiont present. Most lichen fungi are Ascomycetes, including those
that produce apothecia (discomycetes), perithecia (pyrenomycetes), and
pseudothecia (loculoascomycetes). Almost half of the earth’s fungi are
Ascomycetes and almost half of these live with algae as lichens. There
are a few with a basidiomycete mycobiont (Hale, 1983. The biology of
lichens. Edward Arnold, London). Most lichens also have rhizomorph
(rootlike) structures that attach them to their substrate.
Lichens are extremely slow growing, expanding from 1 to 2 mm per year.
They have proven to be good monitors for atmospheric pollution
and for years their population has decreased in industrial areas of the
world. Some have industrial uses, such as litmus
paper used to measure pH levels. 3. Symbiosis and Evolution of Land Plants Pirozynski and Malloch (1975, BioSystems 6:153-64)
hypothesized that terrestrial plants are the product of an ancient and
continuing symbiosis of a semi-aquatic ancestral green alga and aquatic
fungus, and that the futher colonization of land, and the evolution of
plants thereon, was made possible only through such mutualistic
partnerships. It is generally believed that vascular plants arose from
green algae. There is considerable fossil evidence that mycorrhizae were
present in the Precambrian age. Some of the earliest known land plants
were associated with fungi, some of these indistinguishable from
modern-day vesicular-arbuscular mycorrhizae. Research data shows that VAM
are instrumental in securing water, phosphorus, and other minerals from
the soil, prevent desiccation, buffer the plant from toxic materials in
the soil, and help the plant to maintain a resistance to pathogenic
organisms. They provide a “sump” for metabolites of the plant that may
become toxic, as well as storage of photosynthates for use by the plant
during “lean” times. The early invasion of our planet, a planet on
which there was little fresh water, little organic material, and mineral
levels that might have been toxic to plants, perhaps was made possible by
this unique partnership we refer to as mycorrhizal or endophyte
associations.
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