General characteristics of spores

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General characteristics of spores

• By wet weight spores generally contain
• 25 protein
• 20 fat,
• they have a low water content relative to
  vegetative mycelium.
• Cell walls of spores are generally not fibrillar,
  but they are multi-layered and often contain
  melanin.
• Spores contain all normal mycelial organelles.
• Respiratory reserves include
• lipids
• glycogen
• phospholipids
• polysaccharides that can include sugar alcohols
  like Trehalose).

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General characteristics of spores

• Respiration rates in spores are only 1-4 those
  of vegetative mycelium, but the more reserves a
  spore has, the longer it will survive.
• Fungal spores vary in size, shape and colour.
• Fungal spores may be unicellular or
  multicellular.
• For example, conidia produced by Alternaria
  species are multicellular.
• While dormant they exhibit a low rate of
  metabolic activity.
• They vary in the primary functions they serve,
  which may including dispersal to a fresh site or
  host, survival at the same site or increasing
  genetic variation.
• They also vary in the methods by which they are
  formed, released and dispersed.

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Importance of Fungal Spores

• Importance
• dispersal of
• reproduction,
• act as a seed
• Survival
• Type of spores
• Survival spores
• Dissemination spores

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• Survival Spores
• Formed in response to
• Adverse abiotic conditions that can include
  desiccation, high UV, high/low temperatures or
  starvation.
• Biotic factors can also induce sporulation
  including competition, antagonism, and pathogens
  presence. These spores have thick cell walls, and
  lots of reserves.
• Dissemination spores
• Spores that are smaller, with thin cell walls,
  and limited reserves
• will germinate readily when on a suitable
  substrate.
• Formed as part of the active life cycle of the
  fungus.
• often concerned with epidemic spread of a
  pathogenic species from plant to plant, or with
  rapid colonization of a substrate.

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Dormancy

• Definition time gap between sporulation and
  germination (spores do not immediately germinate
  after formation).
• Aka break in the life cycle.
• Characteristics No morphological change seen.
  Metabolic rate is slower than during vegetative
  growth.
• Two types
• endogenous (constitutive)
• exogenous (induced)

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Types of Dormancy

• Exogenous
• Occur because of unfavourable conditions.
• Ex no conducive temperature or enough food.
• Endogenous
• Dependence on metabolic characteristics and
  spores structures.
• Need special conditions to trigger this type of
  dormancy.
• Germination of spores can be stopped even when
  conditions are favourable.

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Endogenous vs Exogenous Dormancy
Table 1.
Endogenous dormancy Exogenous dormancy
Definite launch mechanism Released by autolysis
Small and thin walled spores Large and thick walled
Short survival time Survive for a long time
Germinate readily under suitable conditions Germinate after a specific stimulus or removal of an inhibitor
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Factors influencing dormancy

• Self-stopping spore germination
• Mycostasis
• Nutrient storage
• Wall composition and shape of spores

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1) Self-stopping spore germination

• Dense spore suspension and large number of spores
  stop spores from germinating.
• Examples
• Uromyces phaseoli (fungus causing rot in beans,
  sunflower, corn, snapdragon and groundnuts)
  contain a derivative of cinnamic acid that will
  stop growth of germ tubes.
• Puccinia graminis tritici (cause rot in wheat
  stalks) have germ tubes that grow out of their
  spore wall. Germination of this fungus is stopped
  when a complex protein is formed and closed these
  pores and prevent them from germinating.
• Conidium of Peronospora tabacina become dormant
  when a compound called quiesone will stop its
  protein synthesis.

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2) Mycostasis

• Unsterile soil is able to stop germination of
  fungal spores and this condition is called
  mycostasis or fungistasis.
• 3) Nutrient storage
• Spore has nutrients stored in their vacuoles.
• In the conidium of Penicillium, reserved food are
  kept as polyol.
• Germination can only occur when there is enough
  food.
• Dormant spores have low metabolic rates and this
  is only enough to sustain life.

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4)Wall composition and shape of spores

• Dormant spores are exposed to unfavorable
  conditions.
• Spherical spores give minimal surface exposed to
  the environment.
• Melanine which is a black pigment is a component
  of most spores and not easily degraded by
  microorganisms.
• It therefore gives protection to infection by
  microorganisms and also the penetrating power of
  the suns rays.

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Types of Dispersal

• 1) Usage Of Energy
• Passive Dispersal of Spores No Own Energy Use
  Environment.
• Active Dispersal of Spores Use Energy From
  Fungi.
• 2) Characteristics of Spores
• Dry Spores
• Slimy Spores
• Wet Spores

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Passive Easily Wetted Spores

• Stalked Spore Drop
• Stalk is 50µm 1 mm long.
• Transported by insects.
• Carried by rain water. Can also transport dry and
  slimy spores.
• Examples Sporangia of Dictyostellum and Mucor
  Perithecia of Ceratocystis Conidiophore of
  Cephalosporum and Graphium.

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Stalked spores

• Sporangium of Mucor

Synemmata of Graphium spp. Synemmata are bundles
of erect hyphae and conidiogenous cells bearing
conidia.
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Passive Easily Wetted Spores

• 2) Rain Splash / Drip Splash / Splash Cup
• Rain wet fungi and slime are diluted. Water
  droplets carry spores far away.
• Peridioles are specific names for splash cups and
  spore sacs in some fungi. Peridioles which are
  birds nest-like are found in Basidiomycetes.
• Spores can be transported 1 m away through this
  method.

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Rain splash / drip splash / splash cup

• Peridioles of birds nest fungi

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via air and water

• Earthstars release their spores in the same
  way as puffballs. Amongst the other puffball
  relatives the tough skin of Scleroderma splits to
  expose the spores to wind and water, allowing the
  spores to be washed or blown away.

Scleroderma
Earthstar
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Via animals

• Truffles Stickhorn

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Passive Dry Spores

• Dry spores dispersed from spore sacs by -
• Mechanical Action
• Stream of air form when animal knocks on
  vegetation and small spores are released.
• When hot air rises and cold air settles. Hot air
  carry spores.
• Rain can also break spore sacs and release
  spores.
• Example Podotaxis have spore sacs containing
  elaters that change to changes in moisture and
  thereby further dispersing spores.

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Passive Dry Spores

• 2) Electrostatic Repulsion
• There are different charges in fungi and leaves.
• The different charges are results of changes in
  moisture and infra red rays.
• If charges are similar for example spores and
  their spore sacs, they repel each other. Spores
  are then released.

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Active Dispersal

• Turgid Cells Break
• When asci mature, ascospores are pushed out of
  asci. There is hydrostatic pressure in asci and
  ascospores are thrown a few mm away. This will
  depend on size of ascospores.
• Example Neurospora, Pyronema, Sordaria,
  Claviceps (ASCOMYCETES).
• Some spores are released with strong vibration.
• Example Nigrospora (DEUTEROMYCETES), Pilobus
  (ZYGOMYCETES).

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Active Dispersal

• 2) Change in Cell Shape
• Spores can be dispersed if shape of sacs changed
  quickly.
• Aeciopores of Puccinia are polyhedral in shape (7
  faces) absorb water and become spherical during
  the change force is produced and throw spores out
  of sac.
• Example Spores of Puccinia.

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Budding Process

• Yeasts buds initiated when mother cells attain a
  critical cell size at a time coinciding with the
  onset of DNA synthesis
• Followed by localized weakening of the cell wall
  together in tension exerted by turgor pressure ?
  allow extrusion of cytoplasm in an area bounded
  by new cell wall material synthesized by enzymes
  (glucan chitin synthases).
• Chitin, a polymer of N-acetylglucosamine, forms a
  ring at the junction between the mother cell and
  the bud.

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Budding Process (cont)

• The chitin ring will eventually form the
  characteristics bud scar after cell division.
• Once new daughter bud has initiated, cell surface
  growth during the remainder of the cell division
  cycles is restricted to the bud (mother cell wall
  does not grow very much during budding.
• mother and daughter bud are contiguous during
  bud development)
• Once mitosis is complete and the bud nucleus and
  other organelles (ex mitochondria) have migrated
  into the bud, cytokinesis ensues and a septum is
  formed between mother and daughter.

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Budding Process (cont)

• A ring of proteins called septins are involved in
  positioning cell division. These septins are
  encircle the neck between mother and daughter for
  the duration of cell cycle.