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 re-classified 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?