FUNGI AND OTHER ORGANISMS (PLANT)

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MYCOLOGY (MIC 206)

• FUNGI AND OTHER ORGANISMS (PLANT)

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FUNGI AND PLANTS

• Fungi can be
• Facultative symbionts can live freely or within
  plants
• Obligate symbionts can only grow in association
  with plants.
• Hyphae of symbiotic fungi can penetrate plant
  cells, some grow through tissues without entering
  or penetrating plants cells.
• Two way of parasitic relationships of fungi with
  plants
• Biotrophic
• Necrotrophic

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Biotrophic vs Necrotrophic

• Some fungi have phases where they became
  biotrophic or necrotrophic.
• Fungi can penetrate plant tissues through
• aerial surfaces (stems, leaves, flowers, fruits).
• root surfaces (root epidermis).
• wounds (normal process where internal tissues are
  exposed due to breakage of plants surfaces (e.g.
  when fruits and leaves are formed).

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MOA of Plant Pathogen

• Examples of fungi penetrating plant tissues
• Aerial surfaces
• Examples are Cladosporium (leaf), Rhizopus
• stolonifer (fruit), Colleototrichum (fruit
  and leaf),
• Armillaria mellea (wood/ bark).
• Root surfaces
• Fusarium spp. (root), Sclerotinia sclerotonum
  (root).
• Wounds
• Sclerotinia fructigena (wasps or birds break
• epidermis of fruit and introduce conidia of
  fungus).
• Other internal tissues
• Armillaria mellea (tree pathogen and able to
  enter
• living trees but can live as saprotrops
  when trees
• die).

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BIOTROPHIC ASSOCIATIONS
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Biotrophic Associations

• From the Greek
• bios life,
• trophy feeding.
• Live within the plant and obtain nourishment
  without causing cell death to plants.
• Long-term feeding relationship with the living
  cells of their hosts, rather than killing the
  host cells as part of the infection process.

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Characteristics of Biotrophic Associations

• Characteristics
• Do not kill hosts so either parasitic or
  saprophytic.
• Tissues that are invaded remain alive.
• Symbiotic because two organisms live together.
• A relationship between Fungus Root
  Mycorrhiza.

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Construction of Haustaria

• Biotrophic fungi frequently produce haustaria.
• Haustoria specialized hyphae that penetrate the
  cells of other organisms that obtains food from a
  host.
• Fungi grow between the host cells and invade only
  a few of the cells to produce nutrient-absorbing
  structures termed haustoria.

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MOA of Haustaria

• Fungal haustoria will be produced from spores
  that germinate on the surface of plants, on
  leaves or stems.
• The germinating spores may produce a penetration
  peg known as an appressorium and penetrate the
  plants cell wall.
• After penetration, the hyphal tip forms an
  invagination within the cell that becomes the
  haustorium.
• Haustorium is a structure of an extended
  extracellular mycelium, or filaments, of the
  fungus.

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Magnaporthe oryzae Rice Blasts Diseases
appressorium
penetration of leaf
penetration peg
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CLASSIFICATION OF BIOTROPHICS ASSOCIATION

• Two importants groups

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BIOTROPHIC ASSOCIATIONS - RUST FUNGI
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The Rust Fungi

• The infection behaviour of rust fungi is broadly
  similar to that of the powdery mildews.
• Involving nutrient absorption by haustoria to
  support abundant sporulation for epidemic spread.

Close-up of lower surface of the leaf, showing
cup-shaped pustules of aeciospores.
Small lesions on the upper surface of a barberry
leaf, with spermogonia in their centres.
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Penetration and Colonization by Rust Fungi
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Growth of barley rust germ tubes on a barley leaf
replica
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Growth of barley rust germ tubes on a barley leaf
replica
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Bean Rust Uromyces appendiculatus
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Bean Rust Uromyces appendiculatus
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Bean Rust Uromyces appendiculatus
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Black Rot of Grape - Phyllosticta ampelicida
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Phyllosticta ampelicida germinated conidia and
melanized appressoria
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BIOTROPHIC ASSOCIATIONS POWDERY MILDEW FUNGI
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The Powdery Mildew Fungi (Ascomycota)

• The Powdery Mildew Fungi (Ascomycota)
• Powdery mildews produce mycelium (fungal threads)
  that grow only on the surface of the plant.
• They never invade the tissues themselves.
• The fungi feed by sending haustoria, into the
  epidermal (top) cells of the plant.
• Typical forms many powdery mildews, where the
  fungus forms a powdery coating of white spores on
  the leaf surface.

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Powdery Mildew Fungi

• Other common examples in Britain are powdery
  mildew of hawthorn (Podosphaera oxyacanthae),
  gooseberry (Sphaerotheca morsuvae), and cereals
  and grasses (Erysiphe graminis).

Powdery mildew of roses, caused by the fungus
Sphaerotheca pannosa.
Localised lesions of Erysiphe graminis on wheat
leaves.
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NECROTROPHIC ASSOCIATIONS
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Necrotrophic Associations

• Term necro relating to death.
• Necrotrophic Kill cells and absorb nutrients
  from dead cells.
• How?
• ? Frequently secrete enzymes that degrade plant
• components or toxins that kill the
  plants tissue.
• Subsequently live on nutrients from the tissue
  they have killed.

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Characteristics of Necrotrophic Associations

• Characteristics
• Kills host cells.
• Can be recognised by patches of dead, blackened
  tissue.
• Produce toxins which kill plant cells and their
  enzymes to degrade plant constituents so that
  fungus can use them as food.
• Can change to saprophytic or biotrophic in later
  stages.
• Show low specificity to host plants.
• They are easy to culture in the absence of their
  hosts.
• They can survive in the absence of their hosts by
  production of spores or sclerotia in the dead
  tissue.

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Necrotrophic Associations (cont)

• Examples
• Botrytis cinerea cause grey moulds in young
  plants or grapes when conditions are cold and
  damp.
• Grey mould rot causes damage on stored carrot,
  parsnip, mangel, beet, endive, chicory, turnip,
  and many others.

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Necrotrophic Associations (cont)

• Examples
• Phytophthora spp. Cause black pod disease in
  cocoa plant and brown rot disease on orange
  fruit.