Lecture 25: Competition in the Rhizosphere Flashcards
What is the rhizosphere?
Soil surrounding the roots
Whats the rhizoplane?
the surface of the roots
What is the endosphere?
the interstitial space between cells of the roots
What are the different interactions between different organisms united by surrounding soil?
Soil is the environment for interaction between:
1. Pathogens
a. Harmful to plant
2. The plant(s)
3. Biocontrol PGPRs
a. Symbiotes: integrates, sometimes
irreversible relationships
b. Commensals: independent colonisers;
don’t really add or take away anything
What are root exudates?
- Amino acids
- Organic acids
- Putrescine
- Sugars
- Vitamins etc
Plants secrete 20% of the fixed carbon they produce into the soil. This allows for a high biomass in the rhizosphere, causing bacteria to chemotax to the roots.
Plants do this to feed the bacteria
What happens when bacteria chemotax to the exudates from the roots
- attach and form a microcolony
- Colony enlarges, forming a
exopolysaccharide biofilm around it
How is rhizosphere colonisation possible?
- Various forms of attachment and motility
systems in different bacteria - EPS (exopolysaccharides) and fimbriae
a. Allow bacteria to attach to surfaces - Flagella - allow chemotaxis
- Type 4 pili - allow bacteria to move
around on the rhizoplane
How can bacteria behave hostile to each other?
Through enzyme secretory pathways
- Type 2: secrete toxins to kill fungi, insects
and other bacteria - Type 3: effectors
What are the different enzymes and their functions secreted by bacteria?
- Chitinases
a. Prod. by certain Pseudomonas spp.
b. Hydrolyse fungal cell (by
hydrolysing chitin) walls - biocontrol - Pectate lyase
a. Digest plant material (e.g., potatoes)
b. allows bacteria to live off decaying
material - AHL Lactonases
a. Degrade QS molecules (what
bacteria to know their position and
surrounding bacterial population
b. Interferes with Quorum sensing of
other bacteria - changing their
behaviour - Bacteriocins
a. Diverse, narrow-spectrum
proteinaceous toxins
b. Allows bacteria to kill each other to
fight for rhizosphere space
c. There is often immunity genes
against these
How can secretion systems create virulence and cytotoxicity?
Type 3 secretory effectors
Affects the plant/fungal cells; e.g., by toying with their immune system
What are the different plant pathogenesis
- Erwinia amylovora - cause fire blight
- Xanthomonas campestris - cause
black rot/leaf spot
a. excessive secretion of type 2 enzymes - Steptomyces scabies (common scab)
a. plant immune system trying to fight
off bacteria
Why are plants able to survive the different pathogensis’ caused by bacteria?
Biocontrol PGPR bacteria
- Suppress pathogenic plant bacteria
- Contribute to:
a. biofertilisation
b. root growth stimulation
c. Plant stress control
d. Pathogen antagonism
What are the different ways biocontrol PGPR bacteria contribute to plant growth promotion and biocontrol?
- Biofertilisation
a. Plants need various things. E.g.,
nitrogen, phosphorus, etc. which
they may not be very good at
acquiring themselves
b. Rhizobium - ‘symbiotic’ nitrogen
fixation
c. Azospirillum, Azotobacter -
free-living N2 fixers (nitrogen
bio-fertilisers)
d. Phosphate solubilisation - inorganic
phosphate to organic acids - Root growth stimulation
- Plant stress control
- Pathogen antagonism
How do bacteria encourage plant growth?
- Hormones
- Auxins - plant growth hormones,
promote growth and development
a. Indole-3-acetic acid (IAA) - Bigger plant = more exudate!!!!
- Azotobacter, Pseudomonas sp.
a. Encode genes for production and
degradation of IAA
b. also produce other plant growth
factors - “Gibberellins” and “Cytokinins”
How do volatiles and cofactors affect plant growth
- Bacillus, Enterobacter, certain
Pseudomonas sp
a. Prod enzymes for synthesis of “2-3-
butanediol acetoin” (plant growth
promoting metabolites)
b. (“Volatile”) - Pyrroloquinoline quinone (PQQ)
a. Antioxidant
b. Antifungal activity
c. ISR induction
d. (“Cofactor”)
How do ACC deaminase control plant stress responses?
“Aminocyclopropane-1-carboxylic acid” (ACC) ethylene precursor
- Stress -> ethylene production
- = Inhibition of root elongation,
accelerated abscission, aging and
senescence - ACC deaminase turns the ACC into
NH3 and alpha-ketobutyrate - ACC deaminase protects against:
a. Polyaromatic hydrocarbons
b. Phytopathogenic bacteria
c. Heavy metals (e.g., Ca2+, Ni2+)
d. Drought
e. Salt
How do the biocontrol molecules produced by Pseudomonas aid pathogen antagonism
- Siderophores
a. Pyoverdin
b. Pyochelin - Phenazines
a. Phenazine-1-carboxylate
b. Phenazin-1-carboxamide
c. Pyocyanin - Antibiotics
a. Hydrogen cyanide
b. 2-4-DAPG
c. Pyoluteorin
d. Pyrrolnitrin - Cyclic lipopeptides
a. Viscosinamide
What do siderophores do?
- Produce by many bacteria
- Iron/other metal-binding small (500-
1000Da) molecules - Facilitate metal ion scavenging
- Once secreted, bind to a metal, and then
is re-up-taken by a different bacteria - E.g., Pyoverdin:
a. Fe3+ scavenger
b. Can inhibit pathogen growth via iron
limitation - E.g., Pyochelin
a. Weak Fe3+, strong Cu2+ & Zn2+
chelator
b. Can function as antifungal antibiotic
What do cyclic lipopeptides do?
- Function as powerful surfactants
- Antibiotic activity due to membrane
solubilisation - Allow bacteria to move across plant
surface - Without these genes (when KO) they
dont move
What do phenazines do?
E.g., Pyocyanin & Phenazine-1-carboxamide
- Inhibit electron transport in pathogenic
species - Catalyse formation of hydroxyl radicals in
combination with ferripyochein - damage
lipids and other molecules - Contribute to oxygen shuttling in
microoxic environments - May contribute to iron mobilization in soil
there are many antibiotic properties
What is iChip
Raises percentage of culturable soil bacteria from <1% to ~50%
- Bacteria can get in but not out and then
be cultured on soil
How is the rhizosphere diverse?
> 30,000 different species of bacteria
Even the same species can have HUGE diversity between them
Do different things previously discussed
What are Arbuscular mycorrhizal fungi (AMF)
Symbiosis of plants with AMF occur in more than 90% of plant species
For the plant:
- improved phosphate uptake
- improved nitrogen uptake
For the fungus:
1. Carbon - hexose sugars
