BB30129: Symbiotic interactions
Symbiosis = a mutually beneficial interaction between 2 organisms
living in close physical association
Rhizobia symbiosis (plant-bacteria): new organs (small nodules)
formed by roots of infected plants to house rhizobia bacteria
Mycorrhiza symbiosis (plant-fungi): fungal mycelium wraps around
plant roots and integrates into plant cells (but no new organ formation
involved)
Rhizobial interactions
Symbiotic nitrogen fixation
As discussed, nitrogen is a key element required for plant growth (to
create amino acids, etc.). Nitrogen makes up 80% of the
atmosphere, but plants cannot utilise atmospheric nitrogen.
Biological nitrogen fixation is the reduction of atmospheric
nitrogen gas (N2) to ammonium ions (NH4+) by the enzyme
nitrogenase, with reducing power provided by NAPH/ferredoxin.
This reaction is one of the most energy-expensive processes in
nature – nitrogenase consumes 16 moles of ATP for every mole of
nitrogen reduced to NH3 (ammonium). This is due to the strong triple
bond in atmospheric nitrogen (N2) which requires considerable
energy to dissociate.
Plant genomes do not contain genes encoding this nitrogenase
enzyme, which occurs only in certain bacteria
Some soil microorganisms can perform nitrogen fixation - convert
atmospheric nitrogen (N) into ammonia, which can then be taken up
by plants
Plants engaging in this symbiotic interaction have access to more
fixed nitrogen and so outcompete those plants that lack symbiosis –
have increased growth & crop yield
Symbiosis occurs in root nodules that house N-fixing bacteria
However, this interaction is expensive for plants – they expend 12-
17g of carbohydrate energy per g nitrogen fixed. This means that
nodule formation is suppressed under N-rich conditions – if sufficient
nitrate is available in the soil, this symbiosis does not occur.
Production of inorganic fertilisers:
The industrial Haber-Bosch process uses Fe to catalyse dissociation
of N2 to atomic nitrogen on the catalyst surface, and reaction with H2
(reduction) to form ammonia
This reaction runs at 450 degrees and 500 atmospheres pressure,
consuming a huge amount of energy
, Designing a nitrogenase-like system would consume less energy
Also potential to transfer N-fixing symbioses to crops (through
genetic engineering or infection of plants)– ability to engage in this
interaction is restricted to a limited number of plants.
Types of symbiotic nitrogen fixating bacteria
Legume-rhizobial symbiosis:
The rhizobial symbionts fix atmospheric nitrogen & release
ammonia to the plant
In exchange, plant metabolism in nodules generates organic acids
that provide carbon substrates to rhizobia bacteria.
It also provides carbon skeletons for N-transporting compounds
that transfer fixed N to the rest of the plant.
N-fixation is most useful to bacteria when they are symbiotic – few
bacteria fix N when free-living in the soil
Similarly, N-fixation is most useful to N-starved plants – nodule
formation is down-regulated in plants with adequate nitrate or
ammonia
Co-evolution of symbioses:
Symbiosis is widespread in legumes, and different genera of
bacteria may nodulate a single host plant species
Co-evolution of plants and bacteria is the result of positive selection
pressure for plants to engage in symbiosis outweighing negative
selection pressure:
Positive selection pressure: host plant allowing rhizobial infection
may benefit from enhanced N uptake.
Negative selection pressure: allowing invasion of non-fixing
microbes in addition to rhizobia bacteria could lead to pathogenic
Symbiosis = a mutually beneficial interaction between 2 organisms
living in close physical association
Rhizobia symbiosis (plant-bacteria): new organs (small nodules)
formed by roots of infected plants to house rhizobia bacteria
Mycorrhiza symbiosis (plant-fungi): fungal mycelium wraps around
plant roots and integrates into plant cells (but no new organ formation
involved)
Rhizobial interactions
Symbiotic nitrogen fixation
As discussed, nitrogen is a key element required for plant growth (to
create amino acids, etc.). Nitrogen makes up 80% of the
atmosphere, but plants cannot utilise atmospheric nitrogen.
Biological nitrogen fixation is the reduction of atmospheric
nitrogen gas (N2) to ammonium ions (NH4+) by the enzyme
nitrogenase, with reducing power provided by NAPH/ferredoxin.
This reaction is one of the most energy-expensive processes in
nature – nitrogenase consumes 16 moles of ATP for every mole of
nitrogen reduced to NH3 (ammonium). This is due to the strong triple
bond in atmospheric nitrogen (N2) which requires considerable
energy to dissociate.
Plant genomes do not contain genes encoding this nitrogenase
enzyme, which occurs only in certain bacteria
Some soil microorganisms can perform nitrogen fixation - convert
atmospheric nitrogen (N) into ammonia, which can then be taken up
by plants
Plants engaging in this symbiotic interaction have access to more
fixed nitrogen and so outcompete those plants that lack symbiosis –
have increased growth & crop yield
Symbiosis occurs in root nodules that house N-fixing bacteria
However, this interaction is expensive for plants – they expend 12-
17g of carbohydrate energy per g nitrogen fixed. This means that
nodule formation is suppressed under N-rich conditions – if sufficient
nitrate is available in the soil, this symbiosis does not occur.
Production of inorganic fertilisers:
The industrial Haber-Bosch process uses Fe to catalyse dissociation
of N2 to atomic nitrogen on the catalyst surface, and reaction with H2
(reduction) to form ammonia
This reaction runs at 450 degrees and 500 atmospheres pressure,
consuming a huge amount of energy
, Designing a nitrogenase-like system would consume less energy
Also potential to transfer N-fixing symbioses to crops (through
genetic engineering or infection of plants)– ability to engage in this
interaction is restricted to a limited number of plants.
Types of symbiotic nitrogen fixating bacteria
Legume-rhizobial symbiosis:
The rhizobial symbionts fix atmospheric nitrogen & release
ammonia to the plant
In exchange, plant metabolism in nodules generates organic acids
that provide carbon substrates to rhizobia bacteria.
It also provides carbon skeletons for N-transporting compounds
that transfer fixed N to the rest of the plant.
N-fixation is most useful to bacteria when they are symbiotic – few
bacteria fix N when free-living in the soil
Similarly, N-fixation is most useful to N-starved plants – nodule
formation is down-regulated in plants with adequate nitrate or
ammonia
Co-evolution of symbioses:
Symbiosis is widespread in legumes, and different genera of
bacteria may nodulate a single host plant species
Co-evolution of plants and bacteria is the result of positive selection
pressure for plants to engage in symbiosis outweighing negative
selection pressure:
Positive selection pressure: host plant allowing rhizobial infection
may benefit from enhanced N uptake.
Negative selection pressure: allowing invasion of non-fixing
microbes in addition to rhizobia bacteria could lead to pathogenic
