This document summarizes a study on the molecular mechanisms of plant defense responses to the tomato powdery mildew fungus Oidium neolycopersici. The study investigated three monogenic genes (Ol-1, ol-2, and Ol-4) that confer resistance to the fungus via different mechanisms. It found that reactive oxygen species and callose accumulation were associated with resistances from both dominant and recessive Ol genes. cDNA-AFLP profiling identified different expression classes of genes, with Class III genes specifically upregulated only during incompatible interactions. The study provides insights into the molecular interactions and defense signaling pathways involved in the plant-pathogen system.
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Molecular basis of plant resistance and defense responses to pathogens
1. The molecular basis of plant resistance and
defense responses to pathogens: Current status
Sruthi.N
2. Introduction
Many plant-associated microbes are pathogens that impair plant
growth and reproduction
Pathogens may proliferate in intercellular spaces (the apoplast)
after entering through stomata or hydathodes (bacteria), enter
plant epidermal cells, or extend hyphae onto the plant cells
(fungi)
Innate immune receptors in plants detect the presence of
microbial pathogens and trigger defense responses to terminate
or restrict pathogen growth
3. Elicitors of defence responses
Any substance that has the capability of activating defense
responses in plants
Include components of the cell surface as well as excreted
metabolites
Elicitors
General Race specific
a) Oligosaccharide elicitors a)avr gene products
b) Protein/peptide elicitors
(Ebel et al.,1998)
4. Perception of elicitor signals
Binding proteins:
Oligosaccharide-binding sites:
-Specific glucan-binding sites on soybean root plasma
membranes
-high-affinity binding sites for chitin fragments in tomato, rice
Glycopeptide- and peptide-binding sites:
-Binding sites for peptidoglycans have been identified in wheat
plasma membranes
5. Plant defense to pathogens
• Plants respond to infection using a two-branched innate
immune system
-Recognition and response to molecules common to many
classes of microbes (basal disease resistance)
-Response to pathogen virulence factors
(Liu et al.,2008)
6. Basal defense
Triggered by trans-membrane receptors that recognize
conserved molecules released by a variety of (unrelated)
microbes
Include cell wall fragments, chitin or peptide motifs in bacterial
flagella - PAMPs or MAMPs
PAMP- triggered immunity (PTI)
(Liu et al.,2008)
7. Secondary defense response
Against virulence effector proteins produced by pathogens
Effector –triggered immunity (ETI)
Mediated by resistance (R) proteins
(Liu et al., 2008)
9. Concepts regarding pathogen recognition and defense
Gene-for-gene resistance (Flor , 1947)
Guard hypothesis (Beizen et al.,1998)
10. Gene-for-gene resistance
For resistance to occur, complementary pairs of dominant genes,
one in the host and the other in the pathogen, are required
(incompatibility)
A loss or alteration to either the plant resistance (R) gene or the
pathogen avirulence (Avr) gene leads to disease (compatibility)
(Hammond-Kosack et al., 1997)
11. Types of genetic interactions between plants and
pathogenic microbes
(Hammond-Kosack et al., 1997)
12. Plant disease resistance genes
Encode proteins that recognize Avr-gene-dependent ligands
Activate signaling cascade(s) that coordinate the initial plant
defense responses to impair pathogen ingress
Capacity for rapid evolution of specificity
Common feature of resistance proteins is a leucine-rich repeat
(Hammond-Kosack et al., 1997)
14. Extracellular LRR class of R genes
Have classic receptor-kinase formats - an extracelluar LRR, a
membrane spanning region and an intracellular protein kinase
domain
Against pathogens that have an extracellular lifestyle
Examples: rice Xa21 against Xanthomonas, cf genes of tomato
against Cladosporium fulvum.
15. NB-LRR R proteins
Consists of four domains connected by linkers
A leucine-rich repeat domain (LRR) fused to a nucleotide binding
(NB) domain
NB-LRR core equipped with variable amino- and carboxy-terminal
domains
(Tameling et al., 2008)
16. NB-LRR subfamilies
TIR NB-LRRs (Toll/interleukin-1 receptor like NB-LRRs)
CC-NB-LRRs (Coiled Coil NB-LRRs)
Solanaceous domain (SD)
BED zinc finger DNA-binding domain
(Tameling et al.,2008)
18. Deviations from gene-for-gene concept
One R gene may confer specificity to more than one ligand
- RPM1 in Arabidopsis confers resistance against P.syringae
expressing either avrRpm1 and avrB
More than one R gene may exist for a given Avr gene
- Pto and Prf genes encode biochemically distinct components
of the same pathway
- Two genes at the Cf-2 locus furnish identical functions
(Bent, 1996)
19. Guard hypothesis
Key points
d) An effector acting as a virulence factor has a target(s) in the host
f) By manipulating or altering this target(s) the effector contributes to
pathogen success in susceptible host genotypes
h) Effector perturbation of a host target generates a “pathogen induced
modified self” molecular pattern, which activates the corresponding
NB-LRR protein, leading to ETI
(Jones et al.,2006)
21. Plant defense responses
Hypersensitive response
Production of reactive oxygen species
Cell wall fortification
Production of antimicrobial metabolites (phytoalexins)
Defense signal transduction
Synthesis of enzymes harmful to pathogen (eg. chitinases,
glucanases) (Nurnberger et al.,2006)
22. Types of activated defense responses
(A) Hypersensitive response in single lettuce mesophyll cells penetrated by haustoria of an incompatible isolate of
the biotrophic fungus Bremia lactucae B) H2O2 generation in lettuce cell walls, in the vicinity of the incompatible
bacterium P. syringae pv phaseolicola(C) Papillae formation. Papillae develop beneath the penetration peg (PP)
and germinating spore (S) of an avirulent isolate of the biotrophic fungusErysiphe graminis f sp hordei on barley
leaves expressing the Mlg gene.
(Hammond-Kosack et al., 1997)
23. Defense signaling pathways
SA-dependant signaling
Effective against biotrophic pathogens
Activated by the initiation of HR in plants
Rise in SA levels dissociation of NPR1 oligomers to
monomers interaction with TGA-type transcription factors
activation of PR gene expression
TGAs 2,5, and 6 and WRKY70 required for full expression of PR-1
(Glazebrook, 2005)
24. Defense signaling pathways
JA- and ET- dependent signaling
Effective against necrotrophic pathogens
Increase in JA levels and induction of effector genes (PDF1.2, VSP1)
Induction of transcription factors ERF1, RAP2.6 and JIN1 which
activates many defense related genes
Some JA regulated genes also regulated by ET (PDF1.2 , ERF1)
(Glazebrook, 2005)
25. Defense signaling pathways
JA- and ET- dependent signaling
JA levels regulated by cellulose synthases
JAR1 involved in conversion of JA to active form by conjugation with
amino acids like isoleucine
In Arabidopsis, all activities of JA requires the function of CO11.
Some responses to JA require the function of an MAP kinase
encoded by MPK4
(Glazebrook, 2005)
26. Defense signaling pathways
Cross-talk between SA and JA/ET signaling
Helps the plant to minimize energy costs and create a flexible
signaling network that allows the plant to finely tune its defense
response to the invaders encountered
Most reports indicate a mutually antagonistic interaction between
SA- and JA dependent signaling.
(Koornneef et al., 2008)
27. Molecular players in SA/JA crosstalk
NPR1, required for transduction of SA signaling is a suppressor of
JA response
Expression of GRX480 is SA inducible and dependent on NPR1
Overexpression of GRX480 completely abolished MeJA-induced
PDF1.2 expression, but does not affect the induction of the JA-
responsive genes LOX2 and VSP2
(Koornneef et al., 2008)
28. Molecular players in SA/JA crosstalk
Overexpression of WRKY70 caused enhanced expression of SA-
responsive PR genes and suppressed methyl jasmonate (MeJA)-
induced expression of PDF1.2
MPK4 is a negative regulator of SA signaling and a positive
regulator of JA signaling in Arabidopsis
(Koornneef et al., 2008)
29. PR proteins
Coded by host plants as a response to pathological or related
situations
Accumulate not only locally in the place of infection, formed
systemically following infection by pathogens.
Wide array of functions: hydrolases, transcription factors,
protease inhibitors etc.
(Scherer et
al.,2005)
31. Non-Host Resistance
Two mechanisms
In case of a potentially new host, pathogen’s effectors could be
ineffective, resulting in little or no supression of PTI, and failure
of pathogen growth
One or more of the effector complement of the would-be
pathogen could be recognized by the NB-LRR proteins of plants
other than it’s coadapted host , resulting in ETI
Arabidopsis resistance to non-adapted powdery mildew Blumeria
graminis f. sp. hordei
(Jones et al., 2006)
33. How R genes initiate defense signaling pathways
R proteins recognize pathogen effectors in the cytoplasm
Effector perception may result in altered intra and inter-molecular R
protein interactions, including oligomerization.
Activated R proteins cycle into the nucleus and directly bind
transcriptional repressors of innate immunity, resulting in
transcriptional reprogramming of the plant cell
Studies conducted for N, MLA,RPS4, and RX
(Liu et al., 2008)
36. Objective of the study
Study of the interaction of tomato plants with tomato powdery
mildew fungus, Oidium neolycopersici
The monogenic genes Ol-1, ol-2 and Ol-4 confer resistance to
tomato powdery mildew Oidium neolycopersici via different
mechanisms
Study of the molecular and biochemical mechanisms involved
42. Conclusion of the study
ROS, callose accumulation, and upregulation of DE-TDF
associated with resistances conferred by dominant and recessive
Ol genes
cDNA – AFLP profiling clarified that 81% of upregulated DE-TDF
are common for both compatible and incompatible interactions
Class III DE-TDF were up regulated only in incompatible
interactions and are specific for specific resistance genes
DE-TDF profiles of NIL-OL-1 and NIL-OL-4 deviated much but
similarities were observed between NIL-OL-1 and F3-ol-2