Zygomycetes II: Endogonales & Glomales
Endogonales & Glomales
- coenocytic hyphae
- production of azygospores
- lack fruiting structure of Basidiomycetes & Ascomycetes
- no flagellated state in life cycle
- saprophytes and endomycorrhizae or vesicular-arbuscular mycorrhizae (VAM)
Evolutionary Importance of mycorrhizae
- success of land plants; land plants colonized land approx. 400 MYA
- fossils of Devonian land plants contain VAM-like fungi
- 90% of all plant species are members of families that are characterized as mycorrhizal
- Proteaceae - only nonmycorrhizal woody plant
- some herbaceous families nonmycorrhizal
VAM vs. Ectomycorrhizae
- penetration of cortex cells
- VAM do & ectomycorrhizae do not
- estimated that 300,000 plant species to have VAM; only 130 spp. fungi participate
- est. 2000 plant species have ectomycorrhiza; 5000 fungi participate
- taxonomic distribution
- VAM restricted to 6 genera in Glomales
- ectomycorrhiza in 73 genera in 9 orders (Zygos, Basids, Ascos)
- host distribution
- ectomycorrhiza mostly trees
- Gymnosperms - e.g., Pinaceae & Cupressaceae
- Angiosperms - e.g., Fagaceae, Betulaceae, Salicaceae, Dipterocarpoideae, Myrtaceae
- VAM: woody & herbaceous plants
- 90% of vascular plants normally assoc./w VAM fungi
- easier to focus on families that lack VAM fungi
- carbohydrates (=photosynthates, monosaccharides) that are converted to trehalose, mannitol, glycogen
- necessary cofactor for spore germination
- hyphae extend into the soil away from roots and greatly increase the potential for the absorption of water and the uptake of phosphorus
- currently debated to what extent, if any, fungi increase nitrogen uptake by host plant
- suggested that hyphae of VAM fungi may absorb and transfer metabolities from other fungi, bacteria, actinomycetes, algae and cyanobacteria in the rhizosphere to their associated plants (Linderman, 1992)
- endomycorrhizal associations may contribute to the resistance to certain root pathogens including various fungi and nematodes by production of antibiotic substances
Endogonales and Glomales
Endogonales (See p. 152)
- Formerly, this order included forms that were both saprobic and VAM
& ectomycorrhizal. The majority of VAM taxa are now
classified in the order Glomales.
- The Endogonales contains a single family, the Endogonaceae, with
only the genera Endogone and Sclerogone.
- The sporocarps are subterranean and contain only azygospores;
sporangia are unknown.
- Species of both genera can be found in soil, on rotting wood,
sphagnum or other plant materials, either as saprobes or ectomycorrhizal
Glomales (See p. 153)
- often referred to as "VAM fungi" because they form so-called
mycorrhizae, a form of endomycorrhizae.
- estimated that vesicular-arbuscular mycorrhizae can be found in
about 70% of all plant families
- form mycorrhizal relationships with most agronomically important
angiosperms, some gymnosperms, as well as certain bryophytes and
pteriodophytes and even a few algae
- VAM fungi do not significantly alter the external morphology of the
roots of their higher plant partners
- produce neither a mantle nor a Hartig network as in ectomycorrhizae
- hyphae grow both between and into the cortical cells by penetrating
the wall and causing invagination of the plasma membrane.
- two types of unique structures; arbuscules & vesicles
- highly branched haustorium-like structures that extend through the
host cell wall and greatly invaginating the host cell plasma membrane.
- create a large surface area between the fungus and the host cell
- involved in the bidirectional transfer of metabolites and nutrients
between the two mycorrhizal partners.
- remain alive only for a few days before disintegrating and being
digested by the cells of the plant.
- in a healthy mycorrhizal relationship there is a continuous sequence
of development and disintegration of arbuscules
- terminal swellings called vesicles
- not formed by all species of Glomales
- formed either between or within host cell walls
- thought to function as energy stores for use by the fungus when the supply of host metabolites is low.
General characteristics of Glomales
- Glomales contains three families and six endomycorrhizal genera; all formerly included in the Endogonales.
- Zygospores are not confirmed in any member of the Glomales
- Gerdemann and Trappe (1974) monograph containing the foundations for the taxonomy of the Glomales
- somatic hyphae and arbuscles and vesicles (if present), are similar in all taxa.
- spores provide the only useful criteria for differentiation of species.
- methods of spore germination also have been used in delimiting certain genera
- germ tubes may emerge directly through the spore wall, grow out through hyphal attachment regions or emerge through special regions referred to as "germ shields" (Siquiera et al., 1985).
- Gigaspora and Scutellospora
- only arbuscules in the roots of their mycorrhizal partners
- auxiliary cells are produced in the soil along with structures
called "azygospores". The exact nature of these spores has not been
dogma maintains that these fungi reproduce only by asexual means. The
term azygospore was used for spores of Gigaspora
that are produced on a subtending hypha that resembles the single
subtending hypha of gametangia of Endogone.
Acaulosporaceae and Glomaceae
- produce both vesicles and arbuscules in roots, but do not form auxiliary cells.
- sporulation is by "chlamydospores."
- Acaulosporaand Entrophospora
- produce "chlamydospores" in the soil either singly or in sporocarps.
- spores of both these genera arise from a hypha that subtends a swollen, sac-like structure referred to as the "sporiferous saccule."
Aculospora the spore forms laterally on the subtending hypha
Entrophospora spore develops within the neck of the hypha
- Glomus and Sclerocystis
- "chlamydospores" are borne apically on fertile hyphae.
- spores of most taxa of Glomales are formed singly in the soil, but some species of Glomus and Sclerocystis formed in sporocarps