Symbiogenesis and the Origin of Land Plants
Basic pattern of diversification of terrestrial plants and fungi
The fossil record
For most of the history of life, diversity was
dominated by marine forms
Only in last 500 million years has terrestrial
biodiversity exploded – appeared in Silurian,
established in Devonian
Fossil data; terrestrial plants were part of this explosion
Comparison of fossil and molecular data
Molecular clock data – Delwiche and Timme, 2011
Consistent with fossil record
Vascular plants arose the Silurian period (400 mya)
Main basal plant lineages diversified over a period of 100 mya
The earliest vascular plants:
No roots, leaves, fruits, flowers
Cooksonia and Aglaophyton major – branching structures, ~440 myrs
Challenges faced by early plants in order to occupy terrestrial environments
Dessication
Strong fluctuations in temperature
Radiation (UV rays)
Obtaining water
Obtaining nutrients (eg. nitrogen and phosphorus) – reduced access to water
suggests that access to mineral nutrition is impaired
Detoxification of early soils (ie. heavy metals)
Land plant diversity is complex and arose rapidly over a relatively short period
about 450 million years ago
,Complexity derived from several key structures
Vascular system Cell Wall Lignin
(protostele)
Allows circulation of - New sugar polymer structures - Structural function –
water = protection - Protects cell from dessication, allows large plant
against dessication osmotic stress and invaders function
- Waxy epidermis, cutin also - Allows development of
protects against dessication biological structures to
- Suberin also a waxy substance maximise phototrophy
for waterproofing (stems, trunks, leaves)
What enabled these rapid changes?
1. Hypothesis: Terrestrial plant origins and
evolutionary radiation of land plants only
possible because of mutualistic association
between “semi-aquatic” ancestral alga and an
aquatic fungus – an oomycete (Pirozynski and
Mallock, 1975)
- Biological bottleneck – a particular innovation
led to rapid radiation of forms (similar to mitochondria and rise of eukaryotic cell)
- Mycotrophism, idea brought up to date by Selosse and Tacon, 1998
“Oomycete” really means “model” fungus
Once thought to be an ancient derived fungal form but it is
actually more closely related to seaweed – it is not a fungus
Hypothesis is really referring an early “model” fungus
Trend to asuumue simple/different means anciently derived
- Possible but often turns out not to be the case as these
types of conclusion assume evolutionary stasis in deeply
derived branches
- Placing oomycetes is an example of this
Symbiogenesis
, The fungus provides The plant provides
Alga lacks heterotrophic ability to extract Phototrophs good at gas exchange and
nutrients (ie. array of osmotrophic photon utilisation
functions) - Mutualistic association between plants
- Fungi association therefore provides and fungi is well suited for exploiting
access to nitrogen and phosphorus interface between soil and atmosphere
- Nutrients likely to be available in low - Fungi in poor underdeveloped soil
quantities in paleozoic soil environments lacked access to high
- Mycelium and hyphae growth adapted concentrations of fixed carbon
to 3D exploration for food and water - Eukaryotic algae make mobile
(digest/feed as they grow) carbohydrates – (sometimes called
- Huge “weathering potential” allowing polyols) as a product of plastid based
access to non-soluble mineral elements photosynthesis
- Saptrophic ability allowing further - Sugars can be polymerised and
access to nutrients and compensation transported and hence easily
for low availability in some soils exchanged with fungi (eg. starch and
sucrose)
Individually neither can cope with dessication
Invidually neither can provide their nutrient requirements
- Encouraging conglomeration
- Favouring co-operative interactions
- Both not capable of phagotrophy (process large material intracellularly),
encouraging an osmotrophic interactions
- Leading to 3D growth (eg. roots and mycelial networks)
Osmotrophic phenotypes lead to public goods games:
collaboration/cooperation manifests in plant/fungi
symbiosis
Two cells interact extracellularly, can end up
cooperating
Fungus taking up phosphate, nitrogen (as urea,
nitrates, ammonium ions) from soil converting it
into other substances which are secreted into the osmotrophic environment and
taken up by the plant cell
Basic pattern of diversification of terrestrial plants and fungi
The fossil record
For most of the history of life, diversity was
dominated by marine forms
Only in last 500 million years has terrestrial
biodiversity exploded – appeared in Silurian,
established in Devonian
Fossil data; terrestrial plants were part of this explosion
Comparison of fossil and molecular data
Molecular clock data – Delwiche and Timme, 2011
Consistent with fossil record
Vascular plants arose the Silurian period (400 mya)
Main basal plant lineages diversified over a period of 100 mya
The earliest vascular plants:
No roots, leaves, fruits, flowers
Cooksonia and Aglaophyton major – branching structures, ~440 myrs
Challenges faced by early plants in order to occupy terrestrial environments
Dessication
Strong fluctuations in temperature
Radiation (UV rays)
Obtaining water
Obtaining nutrients (eg. nitrogen and phosphorus) – reduced access to water
suggests that access to mineral nutrition is impaired
Detoxification of early soils (ie. heavy metals)
Land plant diversity is complex and arose rapidly over a relatively short period
about 450 million years ago
,Complexity derived from several key structures
Vascular system Cell Wall Lignin
(protostele)
Allows circulation of - New sugar polymer structures - Structural function –
water = protection - Protects cell from dessication, allows large plant
against dessication osmotic stress and invaders function
- Waxy epidermis, cutin also - Allows development of
protects against dessication biological structures to
- Suberin also a waxy substance maximise phototrophy
for waterproofing (stems, trunks, leaves)
What enabled these rapid changes?
1. Hypothesis: Terrestrial plant origins and
evolutionary radiation of land plants only
possible because of mutualistic association
between “semi-aquatic” ancestral alga and an
aquatic fungus – an oomycete (Pirozynski and
Mallock, 1975)
- Biological bottleneck – a particular innovation
led to rapid radiation of forms (similar to mitochondria and rise of eukaryotic cell)
- Mycotrophism, idea brought up to date by Selosse and Tacon, 1998
“Oomycete” really means “model” fungus
Once thought to be an ancient derived fungal form but it is
actually more closely related to seaweed – it is not a fungus
Hypothesis is really referring an early “model” fungus
Trend to asuumue simple/different means anciently derived
- Possible but often turns out not to be the case as these
types of conclusion assume evolutionary stasis in deeply
derived branches
- Placing oomycetes is an example of this
Symbiogenesis
, The fungus provides The plant provides
Alga lacks heterotrophic ability to extract Phototrophs good at gas exchange and
nutrients (ie. array of osmotrophic photon utilisation
functions) - Mutualistic association between plants
- Fungi association therefore provides and fungi is well suited for exploiting
access to nitrogen and phosphorus interface between soil and atmosphere
- Nutrients likely to be available in low - Fungi in poor underdeveloped soil
quantities in paleozoic soil environments lacked access to high
- Mycelium and hyphae growth adapted concentrations of fixed carbon
to 3D exploration for food and water - Eukaryotic algae make mobile
(digest/feed as they grow) carbohydrates – (sometimes called
- Huge “weathering potential” allowing polyols) as a product of plastid based
access to non-soluble mineral elements photosynthesis
- Saptrophic ability allowing further - Sugars can be polymerised and
access to nutrients and compensation transported and hence easily
for low availability in some soils exchanged with fungi (eg. starch and
sucrose)
Individually neither can cope with dessication
Invidually neither can provide their nutrient requirements
- Encouraging conglomeration
- Favouring co-operative interactions
- Both not capable of phagotrophy (process large material intracellularly),
encouraging an osmotrophic interactions
- Leading to 3D growth (eg. roots and mycelial networks)
Osmotrophic phenotypes lead to public goods games:
collaboration/cooperation manifests in plant/fungi
symbiosis
Two cells interact extracellularly, can end up
cooperating
Fungus taking up phosphate, nitrogen (as urea,
nitrates, ammonium ions) from soil converting it
into other substances which are secreted into the osmotrophic environment and
taken up by the plant cell
