IDENTIFICATION OF HOST FACTORS RECRUITED BYPLANT PATHOGENS

Ester BuiatePhysiology of Plant Health and DiseaseInstructor - 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

FungiUstilago 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

GFPRFP

Ustilago maydis and Pep1

SG200Fungal hyphae inside plant

SG200 pep1Mutant was arrested

Ustilago maydis and Pep1

Insertion of RFP on citoplasm and GFP on Pep1A- media culture B – penetration of maize epidermis C,D,E – intracellularly growth

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

Ustilago maydis and Pep1

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

BacteriaPseudomonas 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 HopMhrcC – secretion defective

Pseudomonas syringae

DC3000 – wild type∆CEL – lacks HopMhrcC – secretion defective

Inoculation with a

trafficking inhibitor

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 co-evolution with resistance genes How immune system works New disease management