Plant microbe symbiosis

Question 1

Define: symbiosis

Answer 1

= prolonged living together of organisms that is beneficial for at least one of them- relationships can change depending on envrionment and other factors

Question 2

Name + define 3 examples of a symbiotic relationship

Answer 2

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

Question 3

Describe the leaf as a habitat for microorganisms

Answer 3

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

Question 4

Describe roots as a habitat for microorganisms

Answer 4

• No UV light as underground
• Moist
• High surface area due to root hairs

= good environment for microorganims to grow and colonise

Question 5

Where are the most relationships between organisms found on a plant?

Answer 5

Roots

Question 6

Name some examples of microorganisms that interact with plants

Answer 6

Prokaryotes = bacteria + archaea

Eukaryotes = fungi, oomycetes, algae, protozoa

Question 7

How does the structure of a root make a good environment for microorganisms?

Answer 7

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

Question 8

Describe a mutualistic relationship between bacteria and a plant

Answer 8

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

Question 9

What is the group of microorganims that provide nitrogen to plants known as and what type of plants do they form a relationship with?

Answer 9

Rhizobia form relationships with leguminous plants

Question 10

What are the characteristics of leguminous plants?

Answer 10

• 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

Question 11

Where are nitrogen fixation bacteria found in plants and who produces them?

Answer 11

Root nodules produced by the bacteria

Question 12

What colour are root nodules and why is this?

Answer 12

Have slight pinkish, reddish tinge because of nitrogenase enzyme requires iron as a cofactor- so nodules contain unusually high concentrations of iron

Question 13

What is found inside a root nodule?

Answer 13

Packed with microorganisms known as bacteriodes

Question 14

What are bacteroides?

Answer 14

= Rhizobia when inside root nodule

Question 15

How are leguminous plants beneficial?

Answer 15

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

Question 16

How much nitrogen is fixed by legumes per year?

Answer 16

3.5 x 10^7 tons

Question 17

Are there other non-legumes that take part in nitrogen-fixation and if yes how is this relationship different?
+ example?

Answer 17

Yes, but do not produce nodules instead produce outgrowths on roots

e.g. birch trees form interactions with actinomycetes = gram + bacteria

Question 18

Describe a mutualistic relationship between a plant and a fungus

Answer 18

Known as mycorrhizae:

1. 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
2. The fungus gains carbon from plant as it is a primary producer

Question 19

Define: mycorrhizae

Answer 19

= mutualistic relationship between a plant and fungus

Question 20

Are mycorrhizae relationships common?

Answer 20

Extremely common under natural conditions = soil undisturbed

Question 21

When are mycorrhizae relationships uncommon and why?

Answer 21

Agricultural practices- plowing destroys the fungal network associated with plants

Question 22

What is an example of a plant that doesn’t take part in a mycorrhizae relationship?

Answer 22

Crucifers e.g. cabbage, parsley

Question 23

What are the 2 types of mycorrhizae relationships and how are they different?

Answer 23

Endomycorrhiza = most fungal biomass inside the root- not within plant cells but between the cells

Ectomycorrhiza = most fungal biomass outside the root

Question 24

What are the 3 forms of endomycorrhiza relationships and how are they different?

Answer 24

arbuscular = most herbs + others- some parts extend outside of the plant itself
ericaeous = heathland plants
Orchidaceous = orchids

Question 25

What species forms ectomycorrhiza relationships?

Answer 25

woody species- trees

Question 26

What did the evolution of mycorrhiza play a critical role in?

Answer 26

= long established relationship that played a critical role in the colonisation of plants on land

Question 27

Describe an experiment used to demonstrate arbuscular mycorrhizal relationships.

Answer 27

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

Question 28

Describe a parasitic relationship between a bacteria and a plant

Answer 28

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

Question 29

Describe another example of bacterial pathogenesis in plants

Answer 29

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

Question 30

Describe an example of fungal pathogenesis in plants

Answer 30

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

Question 31

Describe an example of fungal pathogenesis in plants involving a vector

Answer 31

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

Question 32

What is the most important oomycete parasite and what does it cause?

Answer 32

= Phytophthora infestans = airbourne instead of soil bourne and infects the leaf
- causes potato blight- particularly the Irish potato famine

Question 33

Why do microbes benefit from living on surfaces and what surfaces do they attach to?

Answer 33

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

Question 34

Define: phyllosphere

Answer 34

= community of organisms that live on leaf surfaces

Question 35

Define: rhizosphere

Answer 35

= microbes living on root surface

Question 36

What organisms grow on root surfaces and how do they grow?

Answer 36

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

Question 37

what are root exudates?

Answer 37

= organic compounds that are released by roots

Question 38

What is the basis of commensalism

Answer 38

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

Question 39

What determines the microbial species that are present on plants and soil?

Answer 39

Blend of different chemicals exuded based on plant phenotype

organisms present in soil

Question 40

Are commensal relationships fluid or specific?