PAMP-triggered immunity (PTI)
• Recognition of danger signals
- Distinguish self or damaged self versus non-self – fundamental to any immune system
- PAMP or MAMP – pathogen/microbe-associated molecular pattern
- DAMP – damage-associated molecular pattern
• PRR – Pattern recognition receptors (receptor-like proteins/kinases)
- Membrane receptors
- Ligand-receptor interaction
- Initiate signal transduction cascade
• Conserved features across plants, insects, vertebrates
• Adapted pathogens use effectors to suppress
• PTI often considered weaker than ETI, but is it really?
The zigzag model for plant pathogen interactions
Dangl and Jones. 2006. Nature 444:323-329
How to distinguish a PAMP from an effector??
• PAMPS can be widely conserved or narrowly conserved
• Invariant or highly constrained sequence
• PAMP is essential for microbial fitness and survival
• Effector specifically contributes to virulence by targeting host (defense) physiology
• Lines are blurry
From the plant immunity point of view, the nature and intrinsic
function of the ligand is not relevant as long as it timely and accurately betrays
the potential microbial invader to the plant surveillance system. As a result of
continuous coevolution between plant and pathogen, a wealth of plant
perception systems for microbe-derived molecules has been shaped that
reliably fulfills roles in mediating the establishment of plant immunity
Thomma et al. (2011) Plant Cell 23:4-15
Dubery et al. 2012. in Self and Nonself. pp79-107
Structure of bacterial lipopolysaccharide and peptidoglycan
Erbs and Newman (2012) Mol. Plant Pathol. 13:95-104
Pattern recognition receptors (PRRs)
• First layer of active defense, extracellular membrane receptors
• PRRs perceive PAMPs by receptor-ligand interactions
• Evolutionarily ancient and some widely occurring in plant kingdom
• Interfamily transfer of PRRs more successful than R genes
• R proteins in contrast are evolutionarily young, with many novel
members
• Lines are blurry, some interactions classified as Avr-R gene may be reclassified as PAMP-PRR
Thomma et al. (2011) Plant Cell 23:4-15
Recognition of bacteria by plant PRRs
Zipfel. 2014. Trends Immunol. 35: 345-351
Recognition of fungi by plant PRRs
Zipfel. 2014. Trends Immunol. 35: 345-351
LysM domain-containing receptors
Antolin-Llovera et al. (2012) Annu. Rev. Phytopathol. 50:451-473
Damage-associated molecular patterns (DAMPs) in plants
Tanaka et al. 2014. Front. Plant Sci. 5:446
Recognition of the infectious self by plant PRRs
Zipfel. 2014. Trends Immunol. 35: 345-351
Dubery et al. 2012. in Self and Nonself. pp79-107
Dubery et al. 2012. in Self and Nonself. pp79-107
Immune receptor complexes
Bӧhm et al. 2014. Curr. Opin. Plant Biol. 20:47-54
Ligand-induced rearrangements of the FLS2 receptor complex
Bӧhm et al. 2014. Curr. Opin. Plant Biol. 20:47-54
Ligand-induced oligomerization of LysM domain immune receptor complexes
Bӧhm et al. 2014. Curr. Opin. Plant Biol. 20:47-54
Negative regulation of PRR complexes
Macho and Zipfel. 2014. Mol. Cell 54:263-272
Inhibition of
flg22 perception
by P. syringae pv.
tomato
Nurnberger and Kemmerling (2009)
Zigzag model incorporating co-evolution between Cladosporium fulvum and tomato
Ecp6 – LysM effector
- competes with CEBiP
- present in all C.f. isolates
- widely occurs in fungi (chitin scavenging important for virulence)
- some tomato genotypes develop HR to Ecp6
Thomma et al. (2011) Plant Cell 23:4-15
PAMP perception
in animals and plants
Nurnberger and Kemmerling (2009)
Comparison of plant and animal
perception of conserved
microbial signatures
Ronald and Beutler (2010) Science
Some outstanding remaining questions
Match more PAMPs to their PRRs
PRR specificity?
do plants activate congruent pathways to different microbes?
How does PTI restrict pathogen growth?
Links between PTI and cell death?
Where does specificity in signaling occur for different microbes?
Can PRRs be pyramided to give broad spectrum immunity?
What molecular events initiate distinct signaling branches?
How is PTI tightly controlled to avoid autoimmune disorders?
Molecular interconnection between ETI and PTI?
How is PTI connected to hormone pathways involved in growth and development?