Plant microbe symbiosis Flashcards
Define: symbiosis
= prolonged living together of organisms that is beneficial for at least one of them- relationships can change depending on envrionment and other factors
Name + define 3 examples of a symbiotic relationship
Parasitism/pathogen = host is harmed but pathogen/parasite benefits
Commensalism = one organism derives a benefit and one organism neither is harmed or benefits
Mutalism = both organisms derive a benefit
Describe the leaf as a habitat for microorganisms
Upperside of leaf:
- Exposed to high UV levels = difficult for organisms to withstand
- Covered in waxy cuticle made from hydrophobic components to prevent water loss = dry surface and difficult to penetrate
Underside of leaf:
- Stomata = trapdoors for microorganims to enter leaf
Describe roots as a habitat for microorganisms
- No UV light as underground
- Moist
- High surface area due to root hairs
= good environment for microorganims to grow and colonise
Where are the most relationships between organisms found on a plant?
Roots
Name some examples of microorganisms that interact with plants
Prokaryotes = bacteria + archaea
Eukaryotes = fungi, oomycetes, algae, protozoa
How does the structure of a root make a good environment for microorganisms?
Provides a range of different surfaces for microbes to interact with:
- root cap = protects apical meristem as root moves through soil
- Zone of elongation behind apical meristem
- Zone of differentiation- range of root hairs that extend out into soil and provides a high surface area for attachment and growth for many different microorganisms
Describe a mutualistic relationship between bacteria and a plant
Nitrogen fixation from the atmosphere:
- bacteria have nitrogenase gene which is able to fix nitrogen in atmosphere
- The nitrogen is fixed as ammonia in bacterial cells
- This ammonia is provided to plant in nitrogen form which can be readily be used
What is the group of microorganims that provide nitrogen to plants known as and what type of plants do they form a relationship with?
Rhizobia form relationships with leguminous plants
What are the characteristics of leguminous plants?
- Often pioneer plants = 1st plants to colonise poor soils with low nutrients
- Woody legumes important in tropical soils = effective nitrogen scavengers and important for food production
Where are nitrogen fixation bacteria found in plants and who produces them?
Root nodules produced by the bacteria
What colour are root nodules and why is this?
Have slight pinkish, reddish tinge because of nitrogenase enzyme requires iron as a cofactor- so nodules contain unusually high concentrations of iron
What is found inside a root nodule?
Packed with microorganisms known as bacteriodes
What are bacteroides?
= Rhizobia when inside root nodule
How are leguminous plants beneficial?
Humans have exploited mutalistic relationship for agricultural productivity for a long time:
- crop rotations would include a season of legumes, not only for food but for the nitrogen fixed in the soil = provides long-term fertility for soil
- Grain legumes provide a main source of protein
How much nitrogen is fixed by legumes per year?
3.5 x 10^7 tons
Are there other non-legumes that take part in nitrogen-fixation and if yes how is this relationship different?
+ example?
Yes, but do not produce nodules instead produce outgrowths on roots
e.g. birch trees form interactions with actinomycetes = gram + bacteria
Describe a mutualistic relationship between a plant and a fungus
Known as mycorrhizae:
- Involves a fungus connecting to a root system and extends the space in which a root can explore within the soil by creating a fungal mycelium- this grows out of root and forms network of connected cells which all connect directly back to roots and acquires a range of different nutrients- mostly phosphorus
- The fungus gains carbon from plant as it is a primary producer
Define: mycorrhizae
= mutualistic relationship between a plant and fungus
Are mycorrhizae relationships common?
Extremely common under natural conditions = soil undisturbed
When are mycorrhizae relationships uncommon and why?
Agricultural practices- plowing destroys the fungal network associated with plants
What is an example of a plant that doesn’t take part in a mycorrhizae relationship?
Crucifers e.g. cabbage, parsley
What are the 2 types of mycorrhizae relationships and how are they different?
Endomycorrhiza = most fungal biomass inside the root- not within plant cells but between the cells
Ectomycorrhiza = most fungal biomass outside the root
What are the 3 forms of endomycorrhiza relationships and how are they different?
arbuscular = most herbs + others- some parts extend outside of the plant itself
ericaeous = heathland plants
Orchidaceous = orchids
What species forms ectomycorrhiza relationships?
woody species- trees
What did the evolution of mycorrhiza play a critical role in?
= long established relationship that played a critical role in the colonisation of plants on land
Describe an experiment used to demonstrate arbuscular mycorrhizal relationships.
Plantain + grass in different pots:
1. Plantain has ability to form mycorrhizal relationship in soil
2. Plantain + grass = competitive environment and no fungi in soil = plantain struggles to grow and is outcompeted by grass
3. Plantain + arbuscular fungi + grass = plantain more effectively competing with grass to gain nutrients from soil
Describe a parasitic relationship between a bacteria and a plant
Pseudomonas syringae = gram negative, flagellated cell that invades wounds in shoots and leaves
- Normally only way into plant is via stomata but these have evolved to exploit damage in leaf surface
wide range of pathovars of pseudomonas syringae which have their own target of plant species to invade and parasitise
also saprophytic forms used for agricultural benefit for biocontrol of pests
Outcome of infection = induces frost damage due to ice nucleation gene and non-biological anatagonism
Consequences = cell necrosis
Describe another example of bacterial pathogenesis in plants
Gram negative bacteria called erwinia- has wide range of pathovars which invade many different plants:
- potato blackleg- site of infection in stem
- soft rot- infect tubers of potatoes
pathovars are highly specific to the disease that they cause
Describe an example of fungal pathogenesis in plants
Rhizoctonia solani occurs in soil and has wide host range + causes variety of diseases:
- damping off = seedlings rot at stem base and they fall over die
- Potatoes = black scurf and stem canker
- Cereals = bare patches = problem for agricultural productivity
Describe an example of fungal pathogenesis in plants involving a vector
Microbotryum lychnidis-dioicae produces anther-smut of white campion (plant) = specialised narrow host range
Has an insect vector
fungal infestation = anthers and main body change colour (brownish)- affects fertilisation and in turn production of next gen of plants
What is the most important oomycete parasite and what does it cause?
= Phytophthora infestans = airbourne instead of soil bourne and infects the leaf
- causes potato blight- particularly the Irish potato famine
Why do microbes benefit from living on surfaces and what surfaces do they attach to?
they’re so small, they operate at very low reynolds numbers = water around them very viscous so they find it difficult to move through
More effective for microbes to attach to surface and allow water + nutrients to flow over them
attach to plants particularly roots but also leaves which provide safe effective surfaces for microbes to grow on
Define: phyllosphere
= community of organisms that live on leaf surfaces
Define: rhizosphere
= microbes living on root surface
What organisms grow on root surfaces and how do they grow?
Dominated by very specific groups of microorganisms that exploit surfaces- capable of fast growth on very simple organic compounds
- use root exudates that are released through root surface
what are root exudates?
= organic compounds that are released by roots
What is the basis of commensalism
microorganisms utilise resources from host, but do not exacerbate production
but some members can produce bioactive compounds like:
- Plant growth regulators (hormones)
- can outcompete pathogens so protect plant from pathogenic attack
What determines the microbial species that are present on plants and soil?
Blend of different chemicals exuded based on plant phenotype
organisms present in soil
Are commensal relationships fluid or specific?
