Week 15 L2: Plant-microbe Interactions pt. 2 - PATHOGEN LIFESTYLE Flashcards
What did Louis Pasteur and Robert Koch discover?
animal diseases were caused by microbes, and the germ theory was born
Can the Koch’s postulates be applied to plats?
YES
What are Koch’s postulates?
•The microbe is always associated and isolated
from the patient with the disease
•The microbe must be grown in pure culture
•The microbe can be injected or inoculated into an
animal (plant) and cause disease
•The microbe can be re-isolated in pure culture
What research was carried out to focus on the prevention and control of disease outbreask?
chemical warfare
Better Hygiene
Genetics of disease resistance
What family of organisms associate with plants intracellularly?
viruses and prokaryotes
What family of organisms associate with the plant extracellularly?
prokaryotes and eukaryotes
What are examples of eukaryotes?
Fungi Oomycetes Nematodes Insects Plants!
Can plants infect plants?
YES, parasitic plants
What are mutualistic symbionts?
symbioses that are beneficial to both (or all) symbiotic partners; they are intricate, complex and variable in their
What are examples of mutualistic symbionts?
Mycorrhizae (fungi)
Rhizobium (bacteria)
How does Mycorrhizae work?
ecto or endo
What is the difference in endo and ecto invasion?
endo - fungus invades the plants cells directly
ecto - fungus invades around the plant cells.
What does ecto Mycorrhizae invasion cause?
a Hartig net structure.
work intercellularly
What does endo Mycorrhizae invasion cause?
Arbuscules, finger-like protrusions inside the cell. maintained within cellular membrane.
What are Rhizobium responsible for?
Nitrogen fixation
How do many bacteria attach to the plant?
by producing biofilms to stick to plant tissue.
Xylella fastidiosa in a xylem vessel
What do some pathogens use to stick to the plant?
polysaccharides.
What are a few ways in which pathogens are able to penetrate or circumvent physical barriers?
- Melanized appressoria build up high pressure to
puncture the cell wall - Some pathogens enter through stomata and grow
extracellularly - Some pathogens produce non-melanized but effective
appressoria
What are the pathogen lifestyles?
Biotrophic
Necrotrophic
Hemi-biotrophic
facultative
obligate
What is Biotrophic?
– feed on living plant tissue, not causing cell death
example=Hyaloperonospora arabidopsidis (oomycete)
What is Necrotrophic?
kill plant cells and then feed
example=Botrytis cinerea (fungus)
What is Hemibiotrophic?
initially biotrophic and then become necrotrophic
example=Pseudomonas syringae (bacteria)
What does obligate mean?
can only grow on their specific living host
What does facultative mean?
pathogens can attack living plant cells but can also
grow by themselves, e.g. on artificial medium
What is an example of obligate growth?
Hyaloperonospora arabidopsidis
What are features of biotrophic pathogen?
•“Pretend harmony” •Fewer cell wall-degrading enzymes than non-biotrophs •Evade detection and avoid elicitation of defence responses
Lots of effector cell released
What does ‘pretend harmony’ mean?
Looks like there’s not a lot of damage taking place but there is.
Lots of effector cells produced to suppress immune response
What are features of the necrotrophs?
- “Smash and grab”
* Produce toxins and cell wall-degrading enzymes
What does ‘smash and grab’ of necrotrophs mean?
Cause cell death and cause a lot of damage.
What is haustoria?
feeding structures
What lifestyle of pathogen makes haustoria?
fungal and oomycete hemi-biotrophs
specialized for nutrient and signal exchange
What is the role of hyphae?
parasitic fungi are specialized for nutrient absorption within a specific host.
What is the link between haustoria and Hyphae?
The hyphae is the branch which absorbs nutrients and the haustoria penetrate cells and release effector cells.
Does the haustoria penetrate the plant cells membrane?
no, there is space between the haustorium and the plant cell plasma membrane
What is the hemibiotroph’s lifestyle?
e.g. phytophthora infestans
invade with appressoria and intracellular haustoria,=.
The infection also causes necrosis of plant tissues.
The plant releases sporangia which develops to a sporangium and eventually a appressorium where the cycle starts again
What phase does the haustoria maintain?
biotrophic phase
immune suppression
What are the sporangia?
reproductive organs which can start new cycle of infection.
What effectors are the effectors secreted in biotophy vs necrotrophy?
bio - immune suppressors effectors
nec - cell death promoting effectors (necrosis-inducing)
What do necrotrophic pathogens thrive on?
cell-death environment
Why are there not many resistance proteins associated with the nectrophs?
as they thrive in a cell-death environment
What does DAMPS stand for?
Damage-Associated Molecular Patterns (DAMPs).
What is DAMPS?
When the necrotroph produce cell wall degrading enzymes, the damaged cell wall cells have DAMPS which can be detected by the plant and initiate an immune response. Detect their own damage
What are the 2 types of necrotrophs?
Broad-sense range
Host-specific
Host-specific nectrotrophs produce toxins, what can these be classed as?
effectors
How can plants become resistant to toxins by host-specific necrotrophs
- mutating the target, not sensitive to toxin
- Desensitisation mechanism - e.g. enzyme which targets toxin and breaks it down.
What is Host-specific necrotrophs?
produce specific toxins necessary for their pathogenicity; sometimes these act through specific R genes, in a counterintuitive manner
What is broad-sense range necrotrophs?
produce cell wall degrading
enzymes (CWDE), necrosis and ethylene-inducing proteins (NEPs), and also can suppress the plant’s
immune response.
Are resistance genes useful with regards to necrotrophic pathogens?
No - binding to R protein, makes the plant susceptible due to recognition by the R protein
How do biotrophs and necrotrophs differ in hormone response?
JA works to defend against necrotrophs
SA works against biotrophs
Do viruses also have effectors?
YES
What mediates plants response to a virus?
siRNAs
What is siRNA?
small interfering RNA
How do plants respond to a viruses?
Viral components can trigger immune responses through PRRs as well as R proteins, leading to cell death and systemic immunity
What are nematodes?
Nematodes are tiny
round worms (0.1 mm
diameter) that are
found ubiquitously
Where in a plant do nematodes target?
roots
stunted roots
What are the 2 classes of nematode?
root-knot nematode and cyst nematode
What happens in the root-knot nematodes?
female feeds on plant and induces formation of giant cells, actively changing what plant cells look like in the roots
What happens in the Cyst nematode?
partially dissolve cells walls between cells to produce a syncytium
How do nematodes generally work?
nematode effectors modify root cells to become specialised feeding grounds.
Active manipulation by release of effector cells.
What do biotrophs do to keep the plant alive so it can keep feeding?
biotrophic interaction, it still reduces crop yield but keeps it alive so it can feed
What are types of herbivore insects?
– Chewers – Sap-sucking – Gall-inducing – Boring and Mining – Seed predation
e.g. aphids
