Lecture 12: Plant-Microbes Flashcards
How do microbes interact with plant roots?
- Enter root cells and act symbiotically or pathogenically
- Microbes are affected by plant root changes in the physical and chemical properties of the soil
What is the above ground portion of a plant called?
Phyllosphere
What is the rhizosphere?
The region of soil directly impacted by plant roots
Define endorhizosphere, rhizoplane, ectorhizosphere, and the rhizosphere effect.
- Endorhizosphere: epidermal and cortical cells
- Rhizoplane: colonization of the root surface
- Ectorhizosphere: soil surrounding the root surface
- Rhizosphere effect: 100x more microbes in that area
What is rhizodeposition?
Release of organics from roots
What is root exudate?
Organic compounds that improve growth
In what stage of plant development might bacterial colonization be established?
Bacteria will spread along the root as it growth downward and laterally before germination
What role does chemotaxis have in bacterial colonization of plant roots?
What are the the benefits for plants in microbe-root associations?
- Nutrient recycling
- Vitamins, amino acids
- Hormones
- Exclusion of pathogens
- Exclusion of other plants by allelopathic substances
The R/S ratio is an indicator of _____.
soil health
What can you say about species evenness?
- Dry soils: able to host even distribution of species, including rare species
- Why rare species? Dry environments are hard to survive in, so might foster growth of extremophiles, etc.
What gas makes water acidic?
CO2
Who respires in the rhizosphere?
What is ethylene?
A microbially produced plant growth regulator
- Low levels beneficial
- High concentrations inhibitory
What are precursors to ethylene?
- Sugars
- Alcohols
- Organic acids
- Amino acids
What is a phytotoxin? Give an example.
Compounds produced by plant pathogens to damage the plant cells
- Ex. cyanide
What is the purpose of siderophores?
Used by bacteria to take up iron
Where are hotspots of antibiotic resistance genes in soil microbes?
ARGs peaked in high-latitude, cold boreal forests
- Contained many ARGs of diverse types
What is glomalin?
A soil stabilizer
What is the end goal of the nitrogen fixation?
To release nitrate (NO3+) and ammonium (NH4+) for plants to use
What is the term for nitrogen-fixing associations?
Diazotrophy
What types of plants and bacteria form root-nodulating associations?
- Plants: legumes
- Bacterial genera: Rhizobium, Bradyrhizobium, Sinorhizobium, Mesorhizobium, Azorhizobium
What genes control root-nodulating associations?
nod and nif genes
Where on the plant are most nodules located?
Roots
What are the stages of nodulation?
- Plant root hair releases amino acids and flavonoids
- Rhizobium recognition chemotaxis and attachment
- nod factors released by bacteria causes curling of root hair
- Rhizobium enzymatically enter root hair cells, but not cytoplasm
- Root cell membrane invaginates forming an infection thread to enter root
- Root cortex cells form deformation layer called a nodule
- Continued cell division leads to a bacteroid
- Bacteroids produce nitrogenase for N2 fixation (can’t tolerate O2)
What are mycorrhizae?
Mutualistic plant fungi
What nutrients are exchanged in mycorrhizal associations?
- Carbon to fungi
- Nitrogen and phosphorus to host
What are endomycorrhizae?
Layer of hyphae that penetrate cell wall but not plasma membrane
- In about 85% of plant species
- Aka arbuscular
What are ectomycorrhizae?
Layer of hyphae around roots (not in cells)
- In about 10% of plant species (typically woody plants)
What types of organisms are the most common plant pathogens?
- Fungi
- Viruses
- Bacteria
What are often the vectors of plant pathogens?
insects
What are aggregates? Why are they important?
Soil particles held together to form a larger mass
- Allow air to penetrate between the particles –> allows more stuff to colonize
Which of the following is true about nodulating symbioses?
A. They are important for carbon fixation for the plant
B. They involve physical transformations of the plant but not the bacteria
C. They involve chemical communications between host and plant
D. Both B & C
E. All of the above
