MCB3024S
Plant Immunity
The plant-pathogen arms race: Evolution of the plant immune system
- the pathogen has to constantly invent new ways to attack the host and the host has to constantly invent new mechanisms to
protect itself against pathogens
Plants come under attack from a wide range of pathogens
- broadly split into 3 groups: bacterial pathogens, fungal pathogens and viral pathogens
Why should we care
- effect human societies, an outbreak potato late blight, caused by Phytophthora infestants, caused the Irish Great Famine
- graph shows population of Ireland, see a spike and then a catastrophic decline
- cause was a plant pathogen which was responsible for the great Irish famine
- brining in potato brought in the pathogen and the Irish potato crop was down by 80% for 7 years
- 1 million people died of diseases related to starvation and 1 million immigrated
Closer to home
- infected with a fungus called stemrus fungus
- strain has evolved 20 years ago in uganda and the fungus destroys all main weeds
- pathogen causes massive yield losses
What turns an interaction into disease?
- while many pathogens are around, most plants are not sick
- plants are exposed to countless microbes but very few of these interactions lead to disease
- disease comes down to the pathogen, the host and the environment
- pathogen: can only infect if it is able to suppress the immune response it triggers and evade
- some pathogens require a humid environment so environment can tip balance
- host: can the plant detect and respond to the pathogen
Pathogens adopt different strategies
- innate immunity: relies on a thick set of receptors and are born with
detection molecules, plants only have innate immunity
- difference between plant and mammalian immunity: immunity is
cell autonomous and there is no adaptive immunity in plants (no
antibodies or ability to generate new recognition molecules)
- PAMP-triggered immunity, effector-triggered immunity, RNA
interference and systemic-acquired resistance (where an initial
infection it survived triggers a stronger response the next time,
requires mobile signals)
PTI and ETI
- PTI and ETI function mainly against bacterial and fungal pathogens and RNAi against viral pathogens
- all rely on the detection of non-self
- Pseudomonas syringae: ground-negative bacteria, flagella to move around, Hrp pilus important in pathogenesis
- Arabidopsis thaliana: the plant model
- systems used because both are attractive for molecular biology, pathogen allows genes
to be easily knocked and can add in genes, the plant can be made to express genes and
ability to knockout genes of interest using C~RISPR technology to target genes
- advantage to knockout of genes and overexpress genes allows to test function (in
virulence and defense for example)
- plant also has a very short generation time (6 weeks) which allows various studies
requiring generation, attractive as short time between generations
- genetically attractive ti test gene function
, The Pst infection cycle
- bacteria arrives on leaf surface, leaves are covered by waxy layer
so bacteria cannot enter
- bacteria enters through stomata which are openings on leaf surface
(for gaseous exchange)
- bacteria once in grows between the cells in the intercellular spaces
(apoplast)
- as they proliferate, they take resources away from the plant leading
to the development of disease
- overtime, the plant will die once sufficient resources are taken
away and the bacteria disperse from the dead plant to a new one
Ways to infect
- use pressure infiltration and spraying
- pressure infiltration: use a syringe full of bacterial cell culture,
forces bacteria into the leaves, forced through the stomata (ad: can
put a controlled amount of bacteria into the leaf with a known cell density and volume, good for experiments with
quantitively data, disad: not natural as forced into plant)
- spraying: bacteria culture in bottle is s prayed onto leaf surface (ad: mimics the natiral infection and pathogen has to enter
plant on its own, downside is cannot control how many bacteria enter the plant and on the leaf, cannot ensure each plant is
infected with same amount of bacteria)
- take leaf disc of known area, put into magnesium chloride of known
volume, dry and do serial dilution and count number of colonies
- graph shows colony forming units (number of colonies per cm leaf
surface) and days after infection
- day 1: phenotype same but bacteria is 100 fold increase
- day 2: plant still healthy but bacteria has increased another 100 fold
- day 3: 10 fold increase but disease phenotype
- point is that the symptoms developed lag the amount of bacteria
present in the plant and often what a plant looks like is not a good
representation for the amount of bacteria present
- low tech way to measure
PAMP-triggered immunity (PTI)
- PAMP is a pathogen associated molecular pattern
- have 3 distinct characteristics: molecules not found in plants but are found in potential pathogens
- firstly, are highly conserved in terms of the sequence or structure across wide classes
- are widely distributed (a whole range of pathogens have them)
- thirdly, are essential in some way or other to the pathogen (pathogen can lose the thing it is being detected by so important
as essential for survival so cannot lose it)
- plants have a detection system to detect these potential pathogens and these properties make them good targets for the
plant immune system
- flagellin (protein that makes up the flagellum, every mobile bacteria)
- lipopolysaccharide (carb compound found in every gram-neg bacteria, broad range)
- EF-Tu is involved in protein synthesis and is found inside the bacterial cell unlike the other components
- induction of PTI is dependent on recognition of non-host molecules by the plant
Fkg22: the first PAMP
- knew that if you have plant cells in media and you add pseudomonas bacteria to the
culture, the cells may have a response and the changes in iron flux across the plasma
membrane and the effect of that is that the pH of the media increases (media alkalization)
- want to know if it was a bacteria or plant driven response: same test with dead bacteria into
the media and see no response, concludes the bacteria is driving the response and if a
response is seen, the plants is driving the response
- addition of dead bacteria and still had response, when dead bacteria added, still get a rapid
pH change of the media so plants recognize the presence of pseudomonas and plants respond
to the presence
- 3 classes of molecules may be potential recognition component (DNA, RNA or protein)
- if DNA: add DNase to bacteria, if RNA: add RNase, if protein: add protease
- repeated the experiment with the dead experiment and treated the bacteria with either
DNase, RNAse or protease, when treated with DNA, no change and got a change in pH so
DNA not important, same result for RNA
