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Fungal & Oomycete Pathogenesis

Fungal & Oomycete Pathogenesis. According to Agrios, >8,000 species of fungi can cause plant disease. Latijnhouwers et al. 2003. Trends in Microbiology 11:462-469. Powdery mildew can suppress senescence. Can biotrophs help to keep their host alive?.

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Fungal & Oomycete Pathogenesis

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  1. Fungal & Oomycete Pathogenesis • According to Agrios, >8,000 species of fungi can cause plant disease Latijnhouwers et al. 2003. Trends in Microbiology 11:462-469

  2. Powdery mildew can suppress senescence Can biotrophs help to keep their host alive? Schulze-Lefert and Vogel. 2000, Trends in Plant Sci. 5:343-348

  3. Question: If you were a typical fungus, what key activities/functions might you need in order to infect a susceptible host?

  4. General mechanisms involved in pathogenesis • - Mechanical forces • Formation of appressoria and penetration of the host cuticle and cell wall • - Chemical weapons • Enzymes: cutinases, pectinases, cellulases, hemicellulases, ligninases • proteinases, amylases, lipases • Toxins: non-host specific, host specific • - Growth regulators • auxins, gibberellins, cytokinins, ethylene, abscisic acid • - Polysaccharides • See chapter 3 of “Plant Pathology” by Agrios.

  5. Plants present several barriers to pathogen infection: Cuticle Cell wall Suberin layers Seed coat from Agrios

  6. How do fungal pathogens breach the physical barriers of plants? Direct penetration (physical and/or biochemical means) Go through natural openings Wounds Insect vector from Agrios, Plant Pathology, 3rd ed.

  7. Examples of appressoria and haustoria of fungi and Oomycetes • Features of appressoria of fungi: • - Adhere to plant surface • Melanization • High turgor pressures • Form in response to appropriate signals Magnaporthe grisea, 3M glycerol, 8 MPa/1160 psi Latijnhouwers et al. 2003. Trends in Microbiology 11:462-469

  8. Signaling pathway leading to appressorium development in rice blast fungus Caracuel-Rios & Talbot. 2007. Current Opinion in Microbiology 2007, 10:339–345

  9. g a b G-protein coupled receptors Are called that because they are always coupled to a heterotrimeric G protein

  10. G-proteins - bind to and hydrolize GTP • Heterotrimeric a, b, and gsubunits; a binds GTP • Small e.g. ras, rac, rho ©2002 Lee Bardwell

  11. Heterotrimeric G protein cycle g b GEF (ligand-bound receptor) g b a a “OFF” “ON” GAP (RGS) g b ©2002 Lee Bardwell

  12. What does Ga do next ? a   Target protein ©2002 Lee Bardwell

  13. ATP Adenylate Cyclase cAMP ©2002 Lee Bardwell

  14. cAMP-dependent protein kinase

  15. Disease of corn caused by Ustilago species (smut) Galls are associated with high levels of auxin Ustilago zeae Bölker (2001) Microbiology. 147:1395-1401

  16. Dimorphous lifecycle of U. maydis Bölker (2001) Microbiology. 147:1395-1401

  17. Signaling in Ustilago development and pathogenicity

  18. Haustoria form contact between fungus and host Panstruga. 2003. Curr. Op. Plant Biol. 6:320-326

  19. Secretion of effectors into intercellular and intracellular spaces Kamoun. 2006. Ann. Rev. Phytopath. 44:41-60

  20. The RXLR motif present in oomycete effector proteins Rehmany et al. 2005. Plant Cell 17:1839–1850,

  21. RXLR is a conserved host targeting signal Bhattacharjee et al. 2006. PLoS Pathog 2(5): e50

  22. RXLR is followed by sequences enriched for amino acids E, D, & R required for host targeting Bhattacharjee et al. 2006. PLoS Pathog 2(5): e50

  23. Avr3a triggers HR in plants expressing R3a resistance gene Bos et al. 2006. Plant J. 48:165–176

  24. AVR3a suppresses INF1-elicited cell death in N. benthamiana Avr3a is a cell death suppressor

  25. Different amino acids affect R3a HR and suppression of cell death Bos et al. 2009. MPMI. 22:269-81

  26. P. sojae races vary in sequence and expression of Avr3a

  27. Copy number variation in P. sojae races at Avr3a locus

  28. Cloning flax rust Avirulence genes Dodds et al. 2004. Plant Cell. 16:755–768

  29. AvrL567 is recognized by R genes in planta and expressed in haustoria

  30. Kamoun. 2007. Curr. Opin. Plant Biol. 2007, 10:358–365

  31. Summary Fungi and oomycetes have a variety of different lifestyles Common themes in fungal signal transduction controlling development and pathogenicity - G-protein coupled receptors and heterotrimeric G proteins - MAP kinase signaling cascades - cAMP is an important second messenger Oomycetes and fungi can form appressoria and haustoria Oomycetes and fungi can secrete a variety of effector proteins into intracellular and intercellular spaces RXLR & dEER motifs are found in oomycete and plasmodium virulence proteins

  32. Images of appressoria Magnaporthe grisea, 3M glycerol, 8 MPa/1160 psi Colletotricum coccodes Rust germling

  33. Growth of Ustilago maydis in infected maize leaves

  34. 2nd messengers - small molecules that transmit the signal by binding to protein targets, typically protein kinases • Cyclic AMP (cAMP) and cGMP • Calcium • Diacylglycerol (DAG) Released from phosphatidyl inositol biphosphate (PIP2) by Phospholipase C, activates protein kinase C • Inositol triphosphate (IP3) Released from PIP2 by Phospholipase C; opens a calcium channel in the ER • PI-3-phosphates Inositol derivatives phosphorylated on the 3 position of the inositol ring ©2002 Lee Bardwell

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