1. Plant-fungus interactions can be characterized by gene-for-gene systems where a plant resistance gene corresponds to a fungal avirulence gene. Vertical or race-specific resistance follows this pattern and is not durable due to high selection pressure. 2. R proteins in plants recognize specific pathogen effectors or avirulence proteins through direct or indirect models. Direct models involve recognition of effectors by R protein receptors. Indirect models involve the effector targeting or modifying a host protein that is then recognized by the R protein. 3. Understanding gene-for-gene systems and how plants recognize pathogens at the molecular level can enable new strategies for disease control through deployment of resistance genes and exploitation of avirulence

1. MOLECULAR CHARACTERIZATION OF GENE FOR GENE
SYSTEMS IN PLANT- FUNGUS INTERACTION AND THE
APPLICATIONS OF AVIRULENCE GENES IN CONTROL OF
PLANT PATHOGENS
1

2. 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
2

3. VERTICAL RESISTANCE :
• Race specific , qualitative resistance, differential resistance
• Complete resistance, not durable – high selection presssure
• Follow gene for gene system
3
PATHOGEN
RACE
PATHOGEN RACE
PLANT VARIETY 1 2 PLANT VARIETY 1 2 3 4
A _ + A _ + + +
B + _ B + _ _ +
C _ + _ +
D + _ + _

4. Reaction of plants to attacks by various pathogens in relation to resistance of
the plant
4Agrios (2005)

5. 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 (+)

6. 6
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 + + + +

7. 7Agrios (2005)

8. 8Jones & Dangl (2006)

9. 9

10. 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)
10

11. 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 .
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
11

12. MAJOR CLASSES OF R PROTEINS
S. NO MAJOR R-GENE CLASSES EXAMPLE
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.
12Gururani et.al.,2012

13. 13Agrios (2005)
TYPES OF R-CODED RECEPTOR PROTEINS

14. 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 2002
Pi-ta Rice Magnaporthe grisea(avrPita) 2000
Cf-9 Tomato Cladosporium fulvum(Avr9) 1994
Ve1
Ve2
Tomato Verticillium albo-atrum 2001
Xa-21 Rice Xanthomonas
oryzae pv.oryzae (all races)
1995
Pi-d2 Rice Magnaporthe grisea 2006
14Gururani et.al.,2012

15. Plant proteins belonging to
the nucleotide-binding site–
leucine-rich repeat (NBS-
LRR) family are used for
pathogen detection.
(R-PROTEIN)
Harmful organism
Recognition by
resistance protein
Signal to cell
nucleus
Genetic
material
Defense
Response Defense protein
(R-PROTEIN)
Outsideplantcell
Insideplantcell
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)
15

16. • 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
16

17.  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

18. 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

19. 19Jones & Dangl (2006)

20. 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.
20

21. 21
Jones & Dangl (2006)

22. 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 binding
22

23. 23Zhang et.al., 2013

24.  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 mimics
the 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
24

25. 25Zhang et.al., 2013

26. 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
26

27. 27Collier and Moffett (2009)

28. 28

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30. 30

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39. 39

40. DIRECT UTILIZATION THROUGH TWO COMPONENT SENSOR
SYSTEM (De Witt , 1992)
40Lauge and De Wit (1998)

41. INDIRECT UTILIZATION OF AVIRULENCE GENES
GENE DEPLOYMENT
GENE PYRAMIDING
41
Virulence/ avirulence pattern Gene Deployment
V1 / avr2, avr3 R2, R3, not R1
V2 , V3 / avr1 Only R1
V1, V2, V3/ avr4, avr5 R4, R5
R1
R1 + R2
R1 + R2+ R3
eg. Oat against crown rust

42. 42
MULTILINES
R1
V1
R2
V2
R3
V3
R4
V4

43. 43

44. FLAX 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.
44
Jones & Dangl (2006)

45. 45

46. 46

47. 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.
47

48. 48