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IDENTIFICATION OF HOST FACTORS RECRUITED BY PLANT PATHOGENS Ester Buiate Physiology of Plant Health and Disease Instructor - Dr. Aardra Kachroo What are host factors? Gene-for-gene hypothesis For every avirulence (Avr) gene in the pathogen there is a resistance (R) gene in the host The interaction between those proteins leads to activation of host defense system, that stops the pathogen growth Host factor Effector protein The effector proteins Avirulence genes encoded by pathogens that cause infection Effector proteins can suppress host defense responses (Nomura et al.,2006) PAMPs and MAMPs Provides a basal resistance Pathogen-Associated Molecular Patterns Chitin: fungal cell walls Flagellin: bacteria PAMP Recognition Receptors (PRRs) Plant receptors that recognize PAMPs Resistance pathway Jones & Dangl, 2006 Nature How does effectors work? Sctructural roles Haustorial matrix Promote nutrient leakage to the pathogen Supress resistance response: Inhibit PAMPs signaling pathway Cell traffcking Fungi Fungi Ustilago maydis Infects mayze and teosinte Model organism Picture: The American Phytopathological Society Type II Secretory System Can be ATP-dependent Talbot, Nature 2007 Ustilago maydis and Pep1 Gene clusters upregulated during biotrophic development were mutated None was essential for the initial development Search for an effector gene that Carry a secretion signal Produced a novel protein Protein essential during prenetration 1 Ustilago maydis and Pep1 GFP RFP Ustilago maydis and Pep1 SG200 Fungal hyphae inside plant SG200 pep1 Mutant was arrested Ustilago maydis and Pep1 Insertion of RFP on citoplasm and GFP on Pep1 A- media culture B – penetration of maize epidermis C,D,E – intracellularly growth Ustilago maydis and Pep1 Pep1 does not affect saprophytic growth neither appressoria formation Pep1 is essential for successful invasion of maize Plant defense responses are downregulated after penetration Pep1 is a pathogen effector absolutely required for the transition from saprophytic to biotrophic growth Ustilago maydis and Pep1 How Pep1 works? Pep1 could active other effector proteins Pep1 could interfere with plant signalling Virus Tobacco Mosaic Virus Infects tobacco, tomato and other solanaceous First virus discovered Picture: The American Phytopathological Society Tobacco Mosaic Virus Are host dependent Virus RNA are recognized and induce RNA interference defense pathways in the host Study focused on TMV replicase protein and NAC domain protein ATAF2, which is associated with host defense responses and changes in virus accumulation Tobacco Mosaic Virus degradation of ATAF2 GFP expression with TMV Tobacco Mosaic Virus degradation of ATAF2 Reduction of ATAF2 levels Tobacco Mosaic Virus degradation of ATAF2 ATAF2 mRNA levels are high Tobacco Mosaic Virus degradation of ATAF2 Proteasome inhibitor increased ATAF2 level Tobacco Mosaic Virus Interaction between pathogen replicase and host NAC protein ATAF2 Low GFP in infected tissues Virus-directed protein degradation Disrupt host defenses Bacteria Bacteria Pseudomonas syringae Infects a wide range of plants and crops Model organism Tomato bacterial speck Picture: K. Loeffler and A. Collmer, Cornell University Type III secretion system Protein of bacteria that injects effector proteins from the pathogen into the host cell Picture: University of Kansas, Dr. Tang Pseudomonas syringae Mutation in an effector gene (HopM1) No symptoms, low bacterial population Plants with hopM1 complemented the virulence defect They found A. thaliana HopM interactors (AtMIN) AtMIN encondes a protein that is crucial for vesicle trafficking system Pseudomonas syringae DC3000 – wild type ∆CEL – lacks HopM hrcC – secretion defective Pseudomonas syringae Inoculation with a trafficking inhibitor DC3000 – wild type ∆CEL – lacks HopM hrcC – secretion defective Bacteria AtMIN encodes ARF GEF protein, that are crucial for vesicle trafficking system in eukaryotic cells HopM1 probably adapts to that protein, and targets it to the host proteasome system Eliminating a component of vesicle traffic pathway is an effective strategy of avoiding the extracellular cell wall–associated host defense Conclusions Understand effector proteins and their coevolution with resistance genes How immune system works New disease management
