MAP Kinas Signalling

Question 1

PAMP / PRR interactions activate immune signaling via ROS. How does that works?

Answer 1

• Conserved PAMPs flagellin (shown as the peptide Flg22) or chitin bind to their cognate receptors FLS2 or AtCERK1.
• PAMP binding triggers phosphorylation of BIK1, which then activates the NADPH oxidase RBOHD, triggering ROS production

Question 2

Overview: How does the MAP Kinas Signaling works?

Answer 2

PAMP –> Receptor –> MAPKKK –> MAKK –> MAPK (Activation by phosphorylation)
MAPK phosphporilates WRKY (immunity gene expression in the nucleus) = Defense!!!

Question 3

MAP kinase cascades can be activated by…

Answer 3

Flagelling (fgl22), agrobacterium EF-Tu (elf18), additional bacterial PAMP’s leads to stomatoa closture, fungal PAMPs Camalexin production

Question 4

WRKY transcription factors play a central role in induced defense - some general facts

Answer 4

• One of the largest families of transcriptional regulators (ca. 70 WRKYs in Arabidopsis)
• Play a central role in the response to diverse stresses (biotic and abiotic) as well as in development(senescence)
• According to the irstructure, WRKYs are grouped into different classes, they share a common WRKY motif (Trp-Arg-Lys-Tyr), some have zinc fingers
• Bind to the W-Box (C/T)TGAC(T/C), present in the promoter region of many stress responsive genes
• Can interact with other WRKY proteins (interaction network)
• Effect on transcription can be modulated by phosphorylation (e.g. by regulating protein stability or subcellular localization)

Question 5

WRKY transcription factors induce genes of

Answer 5

phytoalexin biosynthesis

• PAD3 catalyzes the final step of camalexin biosynthesis in Arabidopsis
• Expression ofPAD3 is regulated by WRKY33
• Mutants lacking either WRKY33 orPAD3 show increased susceptibility towards certain pathogens(e.g. Botrytis, a necrotroph)

Question 6

Several classes of PR proteins are synthesised during induced defense

What are PR. protens and what else do you know?

Answer 6

• PR-(pathogenesis related) proteins are often secreted into the apoplast
• Several families (PR1; PR2, ets.) whose function is partly unknown

Question 7

Examples for PR proteins

Answer 7

Beta-1,3 glucanase (a tryptophan residue is importnat in the interaction with the glucan substrate=
chitinases types I II III IV V VI VII activities on colloidal chitin may vary over 100 fold

proteinase-inhibitor (impicated in defense agains insects and other herbivores, microorganisims and nematoes

Question 8

So how could microbes become successful pathogens?

Answer 8

1. Suppression or evasion of host immunity
2. Establishment of a niche that favours microbial survival
3. Environmental conditions favoring infection

Question 9

PAMP –triggeredimmunity(PTI)

Answer 9

• PTI comprises a wide array of responses, from cell to organism level, aimed at hampering pathogen replication and disease progression.
• PAMPs elicit a rapid and transient defense response that acts on different levels, e.g.pre-invasive (stomatal closing) and post-invasive (OxidativeBurst H2O2; secretion of antimicrobial compounds, cell wall reinforcement)
• PTI is part of the non-host resistance and in most cases is sufficient toward off potential pathogens.
• PTI is usually symptomless

Question 10

Pseudomonas syringae pv. tomato(DC3000) as a model system to study T3E function

Answer 10

• Pst(DC3000) is the causative agent of bacterial speck disease on tomato,but also infects Arabidopsis and thuse merged as an important modelsystem in molecular phytopathology
• Pstisagram-negative, rod shaped and motile bacterium
• Was the first phytopathogenic bacterium whose genome was sequenced (Buelletal.,2003) ,5763ORFs, ca. 300virulence genes (effectors,transporters,adhesins,toxins)
• Hemibiotroph,i.e.requires living host tissue during the early phase of infection and switches to killing the host in later phases of infection

Question 11

So how could microbes become successful pathogens?

1.Suppression or evasion of host immunity

Answer 11

Adapted pathogens possess virulence factors to suppress host defense
Virulence factors are molecules produced by bacteria, viruses, fungi, and protozoa that add to their effectiveness and enable them to achieve the following:
•colonization of a niche in the host (this includes attachment to cells)
•immuno evasion, evasion of the host’s immune response
•immunosuppression, inhibition of the host’s immune response
•entry into and exit out of cells (if the pathogen is an intracellular one)
•obtain nutrition from the host

Question 12

Examples for virulence factors are:

Answer 12

Examples for virulence factors are:
•Toxins
•Lyticenzymes(cellulases, pectinases, proteinases)
•Hormones (Gibberellins, Auxins)
•Effectors, i.e. proteins that are translocated to the host cell (ortotheapoplast) where they act to suppress immune responses
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Question 13

How to become a successful pathogen? step by step

Answer 13

Step1: Overcome stomatal immunity

Pseudomonas syringae produces the phytotoxin
coronatine to interfere with hormone signaling (SA against biotrophs vs. JA and fälschlicherweise COR Ginst negrotrophs

Coronatine leads to re-opening of stomata
Coronatine mimics JA to antagonize SA responses

Step2: Suppression ofPTI –the role of effector proteins Different mechanism translocate effectors into the host cell (T3SS, haustorium, stylet)

Question 14

Translocation of effector proteins into the host cell
involves a type-III secretion system (T3SS)

T3SS of Yersinia pestis

Answer 14

• Injects effector proteins directly into the host cell
• Present in many gram-negative pathogenic bacteria
• In phytopathogens, components are
encoded by hrp-genes
(hypersensitive response and pathogenicity)
• Composed of ca. 30 proteins
• evolutionarily releated to the flagellum
• Translocated effectors possess numerous, mostly unknown, functions in defense suppression and host cell reprogramming
T3E repertoirs of different bacteria/bacterial strains displayhigh diversity in number and function