Symbiosis Initiation and Consequences in Mycorrhizae Flashcards
What 3 aspects are involved in the disease triangle?
Severity of the environment
Pathogen: virulence, abundance
Host susceptibility
What is non-host resistance?
Most plants are resistant to more pathogens, most of the time
Resistance occurring in all genotypes of a plant species to all genotypes of a pathogen species
What is basal resistance (innate immunity)?
Most plants have the ability to limit the effects of any infection; the basal level of resistance is based on the ability to recognize non-self cues
Defense that plant species mount against unadapted microbial intruders
What are PAMPs and what recognize them?
Pathogen-associated molecular patterns
Can come from a wide range of microbial molecules, including glycans and lipopolysaccharides
They are recognized by PRRs
What do PRRs do?
They lead to pattern-triggered immunity - phytoalexins, and other secondary metabolites, proteinase inhibitors
What is the first level of innate host immunity?
Pattern-triggered immunity
What do pathogens do?
Deploy effector proteins
What are effectors?
Any secreted regulatory molecules
What are effectors often matched by?
Host receptor-like proteins encoded by R (resistance) genes
What do R genes usually contain?
A nucleotide-binding domain and a leucine-rich repeat
What does the secretion of effectors that suppress the plant’s basal immunity cause?
Effector-triggered susceptibility
What is a second-level immune response by the host?
Effector-triggered immunity
More robust than PTI
What are some examples of ETI?
Secretion of phytohormones and oxidative substances
Deposition of callose
This causes a hypersensitive response
What does the hypersensitive response do?
Restricts pathogen spread/ingress
What is Rhizobium?
A nitrogen-fixing, non-photosynthetic bacterium
How does Rhizobia get there?
Historically they were thought to penetrate root cells by cell wall degradation
Similarly, AMF was assumed to open root apoplast with cell wall-degrading enzyme
How does rhizobial symbiosis initiate?
Two membrane-bound receptors bind nod factors, interact with SYMRK receptor kinase
SYMRK activates the synthesis of mevalonate
Mevalonate transport into the nucleus triggers calcium spiking
Calcium-dependent kinase activates gene expression for symbiosis
How are AM and EM fungi different?
Em fungi stay in the apoplastic space while AM fungi penetrate the root hair cells
What is mRNA an indication of?
Gene expression
What are MiSSPs?
Mycorrhiza-induced small secreted proteins
8-28% of symbiosis-induced genes encode candidate secreted effectors - these are called MiSSPs
What does the affector affect?
Alters the physiological status of the plant host such that symbiosis is favored
How do MiSSPs work?
When jasmonic acid accumulates, a protein called JAZ is degraded
MiSSP7 prevents JAZ degradation which suppresses the plant immune response
What does JAZ do?
It keeps specific transcription factors downregulated
When it’s degraded, they can activate, enabling stress responses
What source of carbon do fungi most often use?
Monosaccharides such as glucose and fructose
Are monosaccharides usually freely available?
Nope
What are many fungi uniquely equipped to do?
Cleave monosaccharides out of polysaccharides
What are the 3 main cellulases?
Endoglucanases
Cellobiohydrolases
Beta-glucosidases
What are endoglucanases?
Endo-acting enzymes which break up cellulose into progressively smaller fragments
Size = 15-60 kDa
What are cellobiohydrolases?
Exo-acting which reduce cellulose to cellobiose disaccharides
Size = 50-60 kDa
What are beta-glucosidases?
Cleaves cellobiose into two glucose monomers
What is lignocellulose?
It is embedded in lignin, a hydrophobic, phenolic biopolymer
How much cellulose does wood contain?
40-45%
What is selective delignification in white rot fungi?
They remove the lignin first and mostly leave the cellulose exposed also for use by other organisms
What are CAZymes?
Glycoside hydrolases which cause the hydrolysis of glycosidic bonds
What are absent in AM fungi?
Important glycoside hydrolases
