MOLECULAR CHARACTERIZATION OF GENE FOR GENE
SYSTEMS IN PLANT- FUNGUS INTERACTION AND THE
APPLICATIONS OF AVIRULENCE GENES IN CONTROL OF
PLANT PATHOGENS
DOCTORAL SEMINAR-II
VINOD UPADHYAY
ID: 440561
INTRODUCTION
NON HOST RESISTANCE :
• Apple trees and tomato pathogens
• Powdery mildew on wheat (Blumeria graminis f. sp. tritici)
and barley
Disease is exception rather than rule
HORIZONTAL RESISTANCE :
• General resistance, quantitative resistance
• Incomplete resistance but durable
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VERTICAL RESISTANCE :
• Race specific , qualitative resistance, differential resistance
• Complete resistance, not durable – high selection presssure
• Follow gene for gene system
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PATHOGEN RACE
PATHOGEN RACE
PLANT VARIETY 1 2 PLANT VARIETY 1 2 3 4
A _ + A _ + + +
B + _ B + _ _ +
C _ + _ +
D + _ + _
Reaction of plants to attacks by various pathogens in relation to resistance of the plant
4Agrios (2005)
GENE FOR GENE HYPOTHESIS
Each gene that confers avirulence (Avr) to the pathogen there is a
corresponding gene in the host that confers resistance (R) to the host
and vice versa – H.H.Flor (1946)5
RESISTANT OR SUSCEPTIBILITY GENES IN THE PLANT
VIRULENCE OR AVIRULENCE GENE IN
PATHOGEN
R (resistant)dominant
r ( susceptibility) recessive
A (avirulent) dominant AR (-) Ar (+)
a ( virulent) recessive Ar (+) ar (+)
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RESISTANCE (R) OR SUSCEPTIBILITY ( r) GENES IN THE PLANT
R1 R2 R1 r2 r1 R2 r1 r2
VIRULENCE (a) OR AVIRULENCE (A)
GENES IN THE PATHOGEN
A1 A2 - - - +
A1 a2 - - + +
a1 A2 - + - +
a1 a2 + + + +
ELICITOR AND RECEPTOR CONCEPT
7Agrios (2005)
R-AVR GENE INTERACTION AS EFFECTOR TRIGGERED IMMUNITY (ETI)
8Jones & Dangl (2006)
OUTCOME OF R- AVR INTERACTION
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FUNCTIONS AND FEATURES OF AVR GENE CODED PROTEINS
Hydrophilic- lacking stretches of hydrophobic amino acids - enable them to be anchored in cell membranes
May produced and localized in pathogen cytoplasm or secreted through membrane pores
If secreted externally – directly acts as elicitors
If localized in cytoplasm of pathogen - acts indirectly as enzyme to produce elicitor molecules
Acting as avirulence factors in elicitor-receptor model (plant defense)
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Contribution towards the virulence of pathogen. eg. AvrBs2 gene of X. campestris pv. Vesicatoria
Avr proteins interact with specific plant proteins (virulence target) -enhances availability of nutrients to pathogen .
RESISTANCE GENES
Most R proteins contain amino acid leucine rich domain (LRR- leucine rich repeats),
Depending on R protein LRR reside : cytoplasmic LRRs or extracytoplasmic LRRs.
Leucine-rich repeats (LRR) region of R-genes is involved in recognizing pathogens
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MAJOR CLASSES OF R PROTEINSS. NO MAJOR R-GENE
CLASSESEXAMPLE
1 NBS-LRR-TIR N, L6, RPP5
2 NBS-LRR-CC I2, RPS2, RPM1
3 LRR-TrD Cf-9, Cf-4, Cf-2
4 LRR-TrD-Kinase Xa21
5 TrD-CC RPW8
6 TIR-NBS-LRR-NLS- WRKY RRS1R
7 LRR-TrD-PEST-ECS Ve1, Ve2
8 Enzymatic R-genes Pto, Rpg1 LRR - Leucine rich repeats; NBS - Nucleotide-binding site; TIR -Toll/Interleukin-1- receptors; CC - Coiled coil; TrD –Transmembrane domain; PEST -Amino acid domain; ECS - Endocytosis cell signaling domain; NLS - Nuclear localization signal; WRKY -Amino acid domain; HC toxin reductase - Helminthosporium carbonum toxin reductase enzyme.
12 Gururani et.al.,2012
13Agrios (2005)
TYPES OF R-CODED RECEPTOR PROTEINS
GENE PLANT PATHOGEN YEAR ISOLATED
Pto Tomato Pseudomonas syringae pv. tomato (avrPto)
1993
PBS1 Arabidopsis Pseudomonas syringae pv. phaseolicola(avrPphB)
2001
RPS2 Arabidopsis Pseudomonas syringae pv. maculicola (avrRpt2)
1994
N Tobacco Tobacco Mosaic virus 1994
Bs2 Pepper Xanthomonas campestris pv. vesicatoria (avrBs2)
1999
RRS-1 Arabidopsis Ralstonia solanacearum 2002Pi-ta Rice Magnaporthe grisea(avrPita) 2000Cf-9 Tomato Cladosporium fulvum(Avr9) 1994Ve1Ve2
Tomato Verticillium albo-atrum 2001
Xa-21 Rice Xanthomonas oryzae pv.oryzae (all races)
1995
Pi-d2 Rice Magnaporthe grisea 2006
14 Gururani et.al.,2012
Plant proteins belonging to the nucleotide-binding site–leucine-rich repeat (NBS-LRR) family are used for pathogen detection.
(R-PROTEIN)
Harmful organismRecognition by resistance protein
Signal to cell nucleus
Genetic material Defense
Response Defense protein (R-PROTEIN)
Out
side
pla
nt
cell
Insi
de p
lant
cel
l
Diagram of a plant disease resistance protein in action. A portion of the protein (MAROON) lies outside the cell and specifically recognises the harmful organism. The remaining portion of the protein (RED) resides inside the cell and communicates a signal to the plant’s genetic material, which in turn stimulates a defense response against the invading organism.
R-GENE IN ACTION
NBS-LRRPROTEIN
(R-GENE)
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MODELS FOR R- AVR GENE INTERCTION
• INDIRECT PERCEPTION OF AVR PROTEINS:
Protease dependent defense elicitation model
The co-receptor model
The guard hypothesis
The decoy hypothesis
Bait and Switch model
•DIRECT PERCEPTION OF AVR PROTEINS :Elicitor- receptor model
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Albersheim and Anderson Prouty, 1975 proposed this model.
Avirulence (Avr) gene of a pathogen encodes an elicitor (Avr) protein
that is recognized by a receptor protein encoded by the matching
resistance (R) gene of the host plant.
eg. Pi-ta R gene from rice and AvrPi-ta from Magnaporthe grisea
ELICITOR– RECEPTOR MODEL
17Staskawicz et.al.,1995
PROTEASE DEPENDENT DEFENSE ELICITATION MODEL
Kruger et al., 2002 proposed this model.
eg.Tomato leaf mold –Cladosporium fulvum interaction.
Rcr3 required for Cf-2 mediated resistance towards C.fulvum
strains carrying Avr2, encodes a tomato cysteine endoprotease.
Rcr3 might process Avr2 to generate a mature ligand, or Rcr3
might degrade Avr2 - releasing active elicitor peptides that
interact with the extracellular LRR of Cf2. 18
19Jones & Dangl (2006)
THE CO-RECEPTOR MODEL
Jones and Jones,1996 proposed this model.
RPS5 an Arabidopsis NB-LRR protein localized to a membrane
fraction - activated by the AvrPphB cysteine protease effector from P.
syringae.
AvrPphB is cleaved, acylated and delivered to the host plasma
membrane. Activated AvrPphB cleaves the Arabidopsis PBS1 serine-
threonine protein kinase, leading to RPS5 activation.
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Jones & Dangl (2006)
THE GUARD HYPOTHESIS
Van-der-biezen and Jones, 1998 proposed this model.
Interaction between guardee and Avr is recognized by the R
protein
eg. AvrPto of Pseudomonas syringae and Pto gene of tomato,
Prf gene act as guardee.
Evolutionary unstable situation
R protein is absent -evolution of the guardee to avoid binding
R protein is present - selection will favor binding22
23Zhang et.al., 2013
Van der Hoorn and Kamoun, 2008 proposed this model.
Host protein termed as “decoy” - specializes in perception of the effector by the R protein
Not contributing pathogen fitness in the absence of its cognate R protein.
Effector target monitored by the R protein is a decoy that mimicsthe operative effector target
e.g. AvrPto and AvrBs3 - some host targets of effectors act as decoys to detect pathogen effectors via R proteins
DECOY HYPOTHESIS
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25Zhang et.al., 2013
Peter Moffett , proposed this model in 2002.
The NB-LRR protein - primed (signaling competent) but autoinhibited (restrained from signaling) state.
Functional nucleotide binding pocket and multiple intramolecular interactions - fine-tuned balance between the LRR and ARC2.
Avr protein is brought into the NB-LRR system via the bait protein - direct binding or alteration to bait.
Conformational changes within the nucleotide binding pocket- allow signaling motif - downstream signaling components.
Subsequent to signaling, intramolecular interactions within the NB-LRR protein dissociated.
BAIT AND SWITCH MODEL
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27Collier and Moffett (2009)
MOLECULAR BASIS FOR PLANT- FUNGUS INTERACTIONS
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UTILIZATION OF AVIRULENCE GENE FOR CONTROL OF PLANT PATHOGENS
DIRECT UTILIZATION THROUGH TWO COMPONENT SENSOR SYSTEM (De Witt , 1992)
40Lauge and De Wit (1998)
INDIRECT UTILIZATION OF AVIRULENCE GENES
GENE DEPLOYMENT
GENE PYRAMIDING
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Virulence/ avirulence pattern
Gene Deployment
V1 / avr2, avr3 R2, R3, not R1V2 , V3 / avr1 Only R1
V1, V2, V3/ avr4, avr5 R4, R5
R1
R1 + R2
R1 + R2+ R3
eg. Oat against crown rust
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MULTILINES
R1
V1
R2
V2
R3
V3
R4
V4
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CO-EVOLUTION OF RUST AVR AND HOST R GENEFLAX RUST –
All the virulent rust strains retain intact copies of the Avr genes
(AvrL567) but have altered their sequences
Host R genes imposed selection for new variants to escape recognition.
44Jones & Dangl (2006)
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CONCLUSION
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FUTURE PROSPECTS
Functional genomic tools to disease resistance - interactions between defense signaling and other plant processes.
Structural basis of recognition will enable us - design R proteins that recognize essential virulence factors
New transgenic resistant plants by exploiting both avirulence genes and resistance genes in molecular resistance breeding
Using avirulence gene products / race-specific elicitors - events in signal transduction pathways can be studied.
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