Mycorrhizae, Soil Fungi and Heavy Metals

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Mycorrhizae, Soil Fungi and Heavy Metals
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Soil particle is negatively charged therefore
binding cations
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Mycorrhizae

• Term mycorrhiza refers to a plant root plus its
  associated symbiotic fungus
• Mutualistic association
• Considered a functionally distinct (and sometimes
  morphologically distinct) organ involved in
  mineral uptake from soil, esp PO4 including
  heavy metals
• Fungus depends on plant host for its supply of
  carbon nutrients and amino acids

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Mycorrhizae

• Low level of phosphorus in most soils is often
  limiting to growth
• Mycorrhizae help plants grow in these poor soils
  so the potential for agriculture is tremendous
• 95 of all vascular plants possess a mycorrhizal
  association
• Very long fossil record

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Several Types of Mycorrhizae

• Several types but grouped into two categories
• Ectotrophic Mycorrhizae - Ectomycorrhizae -
  fungus primarily surrounds roots
• Endotrophic Mycorrhizae - Endomycorrhizae -
  fungus primarily penetrates roots

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Ectotrophic Mycorrhizae

• Sheathing mycorrhizae of forest trees
• Found on both gymnosperm trees (conifers) and
  angiosperm trees 2,000 species of trees
• About 5,000 to 6,000 species of fungi involved in
  mycorrhizae, mainly Basidiomycota but several
  Ascomycota
• Families especially prominent as EM are
  Amanitaceae, Boletaceae, Russulaceae many
  woodland mushrooms
• Trees with EM oaks, birch, beech, willow, pine

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Ectotrophic Mycorrhizae

• Important in uptake of minerals especially PO4
  (also many others)
• Some trees like pine are obligately mycorrhizal
  will grow poorly and become mineral deficient in
  the absence of mycorrhizal
• Infection by fungus leads to a suppression of
  root hairs and altered root morphology

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Role of Ectotrophic Mycorrhizae

• Main role is nutrient uptake
• Increased surface area available for absorption
  (also role of root hairs)
• Minerals move from cytoplasm of fungus to
  apoplast then to cytoplasm of plant cell and
  through endodermis
• Also increased availability of water to plants
  especially in dry environments

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Ectotrophic Mycorrhizae

• Sheathing mycorrhizae
• Fungus forms a tightly packed sheath several
  layers deep
• Roots have club-like appearance because of the
  fungus

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Ectotrophic Mycorrhize
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Hartig Net

• From the sheath, fungus penetrates into the root
  cortex to a depth of one or two cell layers by
  growing between the cells of the cortex to form
  whats called Hartig Net
• Ectotrophic means outside feeding because it is
  growing between the cells not into the cells
• Fungus also grows into the soil from the fungal
  sheath either as single hyphae or mycelial
  strands

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Hartig Net
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Amanita muscaria on pine roots. Ectomycorrhizae
with mycelial strands growing into soil
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Douglas fir seedlings with and without
ectomycorrhizae
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Ectomycorrhizal Inoculum

• Many EM mycobionts can be grown in culture
• Prepared as commercial inocula
• Pisolithus tinctorius used in several projects

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Endotrophic Mycorrhizae

• There are actually several types of endotrophic
  mycorrhizae
• The most common is the vesicular-arbuscular
  mycorrhizae or VA mycorrhizae (VAM) - sometimes
  just called arbuscule mycorrhizae (AM)
• Extremely common on a huge range of herbaceous
  plants and also some trees

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Long Fossil History

• Fossil record dates back about 460 million years
• Evidence in early land plant fossils seems to
  have mycorrhizae of this type
• Recent find of spores similar to those formed by
  these fungi in rocks 460 million years supports
  the idea these helped early plants become
  established on land

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VA Mycorrhizae

• Estimates are that 300,000 plant species have VA
  mycorrhizae - 380 families including ferns,
  fern-allies, gymnosperms, and angiosperms
• These are mainly formed by fungi in the order
  Glomales in the Zygomycota - about 130-150
  species of fungi in about 6 genera

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Glomales

• Soil inhabiting fungi in the Division Zygomycota
  but no zygospores and no sporangia - large
  thick-walled resting spores that are really
  chlamydospores
• Hyphae very efficient at mobilizing phosphorus
  and translocating it to the plant
• Six genera Glomus largest genus - 90 spp
• These fungi will not grow in axenic culture -
  must be associated with plant roots cultivated
  with the roots for inocula

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Glomales

• Resting spores found in most soils but need
  proximity to roots to germinate
• After germination, hyphae grow through soil and
  enter living roots, where they develop arbuscules
  ( sometimes vesicles)
• Roots colonized with VA mycorrhizae are not
  visible to the naked eye - must be stained to see
  the mycorrhizae

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Glomales

• Hyphae grow between cells
• Arbuscules are formed within cells BUT the cell
  membrane is never breached - membrane invaginated
• Arbuscles are where the interchange of nutrients
  occurs with host plants
• Arbuscules last 4 to 15 days than break down
• Vesicles store lipids and may act as resting
  spores - however not all genera form vesicles

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Glomales

• Hyphae extend into the soil 8 cm and obtain
  phosphorus from soil
• Asexual spores are produced in soil also
• Sexual state either extremely rare or absent
• Spores are enormous compared to other fungi --
  spores range from 50 mm to 450 mm
• Spores seem to be resistant to desiccation and
  able to survive for years

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VA Mycorrhizae

• Common on herbaceous plants and trees
• Mycelium branches in cortex of root forming
  arbuscles and vesicles

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VA Mycorrhizae
Arbuscule
Spore
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VAM with many vesicles
VAM with many arbuscles
Glomus versiforme spores
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Plants with and without VAM
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Soil Fungi

• Tremendous diversity of fungi in soil
• Every major taxonomic group included
• Numbers also impressive/gram soil
• True bacteria 106 to 109
• Actinomycetes 105 to 106
• Protozoa 104 to 105
• Algae 101 to 103
• Fungi 104 to 105

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Other estimates

• Total biomass estimates suggest fungi most
  abundant in terms of biomass and physiological
  activity
• In grassland soils fungi 78 to 90 of
  decomposer biomass
• British deciduous forest fungi 89 of total
  living microbial biomass

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Methods of analysis

• Traditional methods of analysis use soil dilution
  plating
• Results show tremendous diversity
• From a single soil sample hundreds of isolates
  with dozens of genera and 50 to 100 species
• HOWEVER these methods favor heavily sporulating
  fungi that are easy to culture several groups
  VAM, basidiomycetes, difficult or impossible to
  culture

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Molecular Methods

• Researchers starting to use 18S rDNA gene for
  trying to understand diversity present in soils

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Soil Fungistasis

• In soils many fungal spores held in a dormant
  state by phenomenon called fungistasis
• Fungistasis microbial induced
• If soil sterilized, spores will germinate
• Possible causes
• Nutrient competition
• Antibiosis (volatile germination inhibitors such
  as ethylene, alcohols, ammonia)
• Interest by agricultural community for pathogens
  suppression