7. VL Lipka

• Chitin deacetylation

  • only aminogroup at chitin molecule —> molecules can’t be detected by PRR —> fungi are camouflaged (example: Uromyces)

• masking of chitin polymers

  • effector AVR4 (from Cladiosporium fluvum, biotrophic fungus) has similarity with chitin binding domain (ChBD) from invertebrates

  • AVR4 binds specifically to chitin present in fungal cell walls in plants (like a shield)

  • protects fungal cell walls from hydrolysis by plant chitinases

• scavenging of chitin fragments

  • ECP6 scavengers all chitin molecules in the apoplast before they can be detected by CERK —> ECP6 has a higher affinity (Kd 280 pM) to (GlcNAc)6 than to CERK1 (44.8 µM)

  • ECP6 binds chitin via LysM domains and sequesteres chitin oligosaccharides

• many oomycete effectors have a highly conserved RxLR-dEER motif

• this motif is a plant uptake signal

• Effectors have a signal peptide (=secretory signal), a RxLR motif (=plant uptake signal) and an effector domain (=target within plant cell)

  • effector domain responsible for targeting different subcellular components of the plant (e.g. vacuole for release of plant’s toxins, membrane for destroying transporter proteins)

1. suppression of plant immune responses at multiple levels

2. are required for developmental/metabolic reprogramming

• bacterial effectors that are delivered into the plant cell via the type III secretion system (T3SS)

• type III effectors target key components of the PAMP triggered immunity (PTI)

  • e.g. some effectors target BIK1 or other receptor-like cytoplasmic kinases that mediate PTI signaling or MAPK

1. BIK1

   • AvrAC functions as a uridine-5’-monophosphate transferase

   • AvrAC adds uridine 5’-monophosphate to and conceals phosphorylation sites in the activation loop of BIK1 —> BIK1 phosphorylation sites are masked, BIK1 is therefore inactive

   • think of spallek ZAR1 (CNL) signaling —> PBL2 is decoy for BIK1

2. MAPK

   • the activation loop must be phosphorylated to activate plant kinases

   • HopAl1 irreversibly removes the phosphate group from the threonine (functions as a phosphothreonine lyase)

   • with inactive MAPK, PAMP triggered immunity is suppressed

3. PRRs (FLS2, BAK1, CERK1)

   • AvrPtoB functions as E3 ubiquitin ligase —> ubiquitinates FLS2, BAK1, CERK1 thus mediating their degradation

• bacterial effectors

• TALs mimick transcription factors and turn on host genes needed by the pathogen

• e.g. Xanthomonas AvrBs3

• TAL effectors bind to a specific TAL-box (=UPA-box) in the plant promoter which leads to the transcription of susceptibility genes and therefore to disease

• bacterial TAL effector

• AvrBs3 has 17,5 repeats of 34 amino acids that can bind DNA in plant promoters

• AvrBs3 modulates plant gene expression and induces cell hypertrophy by mimicking host transcription factors

  • induces several pepper genes, all with a conserved cis element: UPA (UPregulated by Avrbs3) box

  • TALs bind to conserved region of the upa promoter

TALENS (=transcription activator-like effector nuclease)

• can be used for genome editing: between TALs, unspecific dimeric DNA-endonuclease is inserted

• —> homology-dependent repair or erroneous non-homologous end-joining —> genome editing (specific genes can be targeted)