Symbioses: (including mycorrhizae, Nfixing, endophytes)
Questions
• Discussions:
– Tatiana: posting paper and questions for this week
– Maria M: will be picking a paper to post for next
week
• Paper due in 1 week
• Any questions?
Symbioses
• Who are they and what do they do?
– Mycorrhizal fungi (http://mycorrhizas.info),
Endophytes, and Nitrogen-fixing bacteria
Ferns
Gymnosperms
Angiosperms
Fungi
Mycorrhizae
• Define: Root-fungi association that is not
pathogenic
– A fungus that invades the root (cortex)
– Roots typically have few to no root hairs, but
fungus has better uptake
• Mycelia are small in diameter
• Excretes organic acids
• Two main types: EM and VAM
Distribution
• Most plants have mycorrhizal associations
– VAM is more common and ancestral ~80% of
species
• First records from 400 mya with early bryophytes
• Fossils show association since plants first colonized land
– EM found in ~10% of species in specific clades
(e.g., Pinaceae, Fagales, Malvales, Cyperaceae,
Caryophyllales)
Distribution
• Most plants have mycorrhizal associations
(with other types evolving during Cretaceous)
– Ericaceae (complex with VAM ancestral)
– Orchid (single lineage)
– Myco-heterotrophic plants with Exploitative:
primitive, eudicots (e.g., Ericaceae) and monocots
(e.g., Orchidaceae)
– Nonmycorrhizal found in disturbed habitats with
extreme conditions
Mycorrhizae
• Fungus:
– Accesses water and minerals from the soil and
decaying material and provides them in a form the
plants can use (especially P)
– Protects plants against pathogenic fungi
• Plant:
– Provides sugars, amino acids, and other organic
materials
• Dependent on coordinated growth of root and
hyphae
EM: Cortical and Epidermal
• Ectomycorrhizal (EM): Basidiomycota,
Ascomycota, Zygomycota; Gymnosperm &
Angiosperm
– Covers the root tip with a dense hyphal mantel
– Hartig net: Hyphae invades intercellular spaces
– Roots are short, branched and look swollen
– Root hairs growth is suppressed
EM: Monotropoid
• Basidiomycota; Myco-heterotrophic ericoid
plants
EM: Arubtoid
• Basidiomycota; Ericaceae
VAM: A number of types
• Endomycorrhizal/Vesicular arbuscular
mycorrhizal (VAM): Glomeromycota; Most
plants
– The mantle is less obvious
– Fungal hyphae invade intracellular spaces. They
have arubscules (exchange) and vesicles (storage)
Orchid
• Basidiomycota; Orchidaceae (root, stem,
exploitative - myco-heterotrophic)
Ericoid
• Ascomycota; Ericaceae
But…
• Fungi can be cheaters (pathogenic, parasitic,
or saprophytic)
Myco-heterotrophic
• And so can plants
– Orchidaceae (germination!)
– Ericaceae
Tripartite: A fungus, an
underground orchid
(Rhizanthella gardneri), and a
Melaleuca (Melaleuca
uncinata)
Endophytes
• Distribution:
– Most plants
• Especially studied in Poaceae
• Found in shoots, roots and rhizomes
– Fungi: differ in type of host, where they colonize,
how they are transmitted, and fitness benefits to
plant
• Clavicipitaceous (class 1); grasses
• Nonclavicipitaceous (class 1, 2, and 3); vascular and non
vascular plants
Endophytes
• Poorly understood,
– Protect plants against pathogens, growth
enhancers, and tolerance of drought, pH, salinity,
and temperature
• Transmitted vertically (parent to offspring) via
fungal hyphae or horizontally (among
unrelated individuals) via spores
• Live completely within host (spores produced
on host senescence)
• Important endophytic chemical?
Nitrogen-fixing bacteria
• Where is bulk of N?
• Distribution:
– Plants: Found in 4 orders in the subclade of the
rosids I (Fabales, Rosales, Cucurbitales, Fagales)
– Bacteria: Frankia and Rhizobium are often
involved
– Also ferns, cycads, and Gunnera with
cyanobacteria associates
Nitrogen-fixing bacteria
• Plant roots form nodules where bacteria are
located
• N2 + 6 H -> 2 NH3 (anaerobic)
Evolution of mixotrophy
Selosse
Tedersoo
Question
• Can undestory green plants obtain some of
their carbon from overstory green plants via
fungal networks?
– Orchids: Shown already that mixotrophy can
evolve near mycoheterotrophic taxa
– Ericaceae: Can this model be used to predict
mixotrophy taxa?
Evolution of mixotrophy
Ferns
Gymnosperms
Angiosperms
Evolution of mixotrophy
Question
• Can undestory green plant obtain some of
their carbon from overstory green plants via
fungal networks?
– Orchids: Shown already that mixotrophy can
evolve from mycoheterotrophic taxa
– Ericaceae: Can this model be used to predict
mixotrophy evolution?
• Used δ13C to show that Pyroleae have C signatures
suggesting some C acquisition from fungi
• Suggest 2 adaptations: vernal photosynthesis & fungal
C exploitation