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Biology of FUNgi

Lecture 14Happy Together?

Fungus Plant Symbioses II

Last time - Mycorrhizas

• Glomales.

• Endomycorrhiza and ectomycorrhiza

• Majority of plants were mycorrhizal.

• Mutualism - benefits to plants/fungi?

• Other benefits - soil aggregates/pricedmushrooms.

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Klironomos and coworkers did a cleverexperiment.

They filled plastic tubs with sterilized soil,inoculated with different numbers of AMtaxa.

They then put their tubs outside and let theplant communities establish on their own.

Mycorrhizal diversitycontrols plant productivity

Inoculate with differentdiversities of AM fungi (1-12)

Allow natural seed deposit

This is what they observed:

1) yield was greater if more fungi werepresent

2) plant community composition was, tosome degree, determined by the fungi insoil

– number of AM fungi +

–

+

yield

Mycorrhizal diversitycontrols plant productivity

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Conclusions:

1) Different AM fungi do different thingsin soil (some may be good at P, somewater, other N uptake)

2) Plants respond differently to differenttypes of AM fungi.

– number of AM fungi +

–

+

yield

Mycorrhizal diversitycontrols plant productivity

Plants provide a marvelous habitat for avariety of fungi.

We know that >80% of the plants fromendomycorrhizas.

However, it has recently been brought tolight that a variety of different(ascomycetous?) fungi are present in theplant tissues.

Fungus of the day -Epichloë sp.

Taxonomy: Phylum (subphylum) Ascomycota Order - Clavicipitales Family - Clavicipitaceae

Structures of fungi in the planttissues

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We are aware of different types of fungi inleaf tissues. Some of these may bepathogenic to the plants, some formfunctional mutualisms.

Roots, similarly, host a great number offungi (in addition to mycorrhizas).

Fungus of the day -Epichloë sp.

Leaf tissues host an appreciableamount of fungal hyphae.

Transparent (hyaline) hyphae anddark (melanized) fungal pebblesof plant roots

Perhaps best known association betweenendophytic fungi and plants is the Epichloeand Festuca rubra, the tall fescuesymbiosis.

We assume that the symbiosis betweenEpichloe and Festuca is mutualistic.

Fungus of the day -Epichloë sp.

Early, visible Epichloëcolonization in senescing tissueof Festuca

Colonized leaf tissue

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The non-beneficial Epichloë infectionsoften result in the complete sterility of thehost plant and therefore inability toreproduce and disperse.

Like many other members of Clavicipitales,Epichloë is a parasite of the ovaries andthe mutualisitc endophytic colonizationmay be more of an exception than a rule.

Fungus of the day -Epichloë sp.

Early, visible Epichloëcolonization in senescing tissueof Festuca

Colonized leaf tissue

If the association is mutualistic, what arethe respective benefits for the twoorganisms?

Fungus of the day -Epichloë sp.

CarbonEnergy

Improved fecundityProtection form

herbivores and parasites

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Carbon and energy to the fungus are easy tounderstand.

Improved fecundity and fitness? Possiblyhormonal control of plant performance.

Protection from herbivores and parasites? Let’scontemplate on this one.

Fungus of the day -Epichloë sp.

CarbonEnergy

Improved fecundity and fitnessProtection form

herbivores and parasites

Remember Claviceps purpurea, the causalagent of the ergot?

Epichloë is related to Claviceps.

Claviceps purpurea perithecialascocarps emerging from thesclerotia

Claviceps purpurea ascocarp.

Normal grain and Clavicepspurpurea infected one turned into afungal sclerotium.

Fungus of the day -Epichloë sp.

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Remember Albert Hoffman and his businesstrips with pals.

Like Claviceps, Epichloë contains a goodmixture of alkaloids.

Claviceps had alkaloids which may haveconstricted blood vessels to the point of limbsfalling off. Some alkaloids affected the centralnervous system causing violent convulsions.

The Epichloë endophyte also relies upon thesealkaloids for fending off the grass herbivores.Now, how does that work?

Albert Hofman (1930’s)

Fungus of the day -Epichloë sp.

Let’s take a look at Keith Clay’sexperiment.

Start areas with tall fescue (Festuca rubra)with and without their Epichloë endophyte.

Record plant species diversity and thebiomass.

Results:

1) Diversity was higher without Epichloë

2) Festuca more abundant with Epichloë

– Epichloë +

–

+PlantDiversity

Importance ofendophytes

– Epichloë +

–

+Biomass

Festuca

Other grasses

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Why?1) Diversity was higher without Epichloë

With the endophyte the fescue was not hitby the herbivores and dominated the plantcommunities. Without, fescue was eatenlike the rest of the grass and lost itscompetitive edge. This also explained thegreater abundance of the fescue.

2) Festuca more abundant with Epichloë

– Epichloë +

–

+PlantDiversity

Importance ofendophytes

– Epichloë +

–

+Biomass

Festuca

Other grasses

Take home messages:

The endophyte colonization maypartly control the communitycomposition.

Just like the mycorrhizal fungi do.

Root endophytes have potential forsimilar function in the ecosystems,but this still remains unknown.

– Epichloë +

–

+PlantDiversity

Importance ofendophytes

– Epichloë +

–

+Biomass

Festuca

Other grasses

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Epichloë life cycle is similar tothat of Claviceps.

Conidiospores are produced in anacervulus-like conidiomata andnot aerially dispersed but mixedin a sweet nectary secretion anddispersed by insects.

Fungus of the day -Epichloë sp.

Claviceps purpurea life cycle includedoverwintering sclerotium and production ofperithecia.

Epichloë rarely has an opportunity forluxurious sex. It usually just travels along withthe tall fescue seeds, establishing a newsystemic colonization when the seedgerminates.

Accordingly, it seems that we are observing avery close, interdependent association.

Systemic colonization.

Claviceps purpurea sclerotia andascocarps.

Fungus of the day -Epichloë sp.

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A look forward...

• What is an endophyte?

• Other types of endophytes.

• Endophytes rule the world.

What is an endophyte?

We usually define endophytes as organisms thatintracellularly colonize (plant) host tissues withoutcausing any visible adverse effects.

Symbiotic association, but the definition does not implymutualism. Merely absence of pathogenicity.

Endophytes can also be other microbes, not just fungi.

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Non-mutualistic endophytes and primary saprobes.

Primary saprobes often colonize plant tissues (singlecells), which they use to rest through their latentphase. Once tissue is senescent, and the tissues die, theendophytic saprobe can start utilizing the easilysoluble carbon sources.

Other types of endophytes

As we already established the roots of plantsalso host asymptomatic endophytes.

These are confined, usually, to the root systemsand cause no or minimal systemic infections.

We know, however, that some taxa ofFusarium, can systematically colonize rootsand colonize localized patches of shoot and leaftissues.

Root endophyteOther types ofendophytes

Fusarium, Acremonium,and various Aspergillusspp are commoncolonizers of roots.

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Importance of endophytes

When talking about the Epichloë, we already looked intothe improved fitness and protection form herbivores.

Root endophytes, likewise can have positive effects.These are usually thought to be similar to the benefitsfrom mycorrhizal symbiosis; uptake of nutrients andwater as well as production of growth promotinghormones.

Summary

• Endophyte is a non-harmful micro-organismwithin host cells.

• Foliar endophytes increase fecundity and reduceherbivory. Root endophytes may act likemycorrhizas

• Endophytes may control species composition ofplant communities.