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Parasitism and Disease Development
How host plant defend pathogens
Defense barriers (waxy
Recognition of conserved
structural patterns of
Recognition of pathogens
effectors by R gene
Defense induced by
Host-pathogen co-evolution: Central dogma of Plant
Kinase NBS TIR LRR Bent and Mackey (2007)
Plant immune system: zig zag model
PAMP Triggered Immunity (PTI) Effector Triggered Immunity(ETI)
Jones and Dangl (2006)
Effectors can be viewed as “Product
of parasite genes having phenotypic
expression in host bodies” (Dawkins
Specialized plant pathogens can evade or suppress this MAMP
triggered immunity (MTI) by secretion of virulence factors
A subset of these effectors, referred to as avirulence factors
(AVRs), which can be recognized by the resistance proteins
that trigger a second layer of host defense, referred to as
effector triggered immunity (ETI) or R-gene mediated defense.
The pioneering work on genetics of plant disease
resistance involving HR was done by Harold H. Flor in
linseed – Melampsora lini system (Flor 1956).
Martin et al. (1993) provided first evidence of
direct interaction of tomato Pto gene with avrPto
from Pseudomonas syringae pv. tomato.
Apart from direct interaction, evidences
indicate that R proteins also act as guard of a
specific component of the basic defense pathway.
To date, the most convincing evidence for the guard hypothesis has
been found in Arabidopsis thaliana bacterial R-Avr systems where
RIN4 (RPM1- interacting protein 4) was identified as a cellular
protein that is required for the resistance to Pseudomonas syringae
pv. tomato mediated by RPM1 and RPS2.
The RIN4 (guardee) is modified in various ways, depending on the
Avr that it associates with, and these modifications then serve to
activate the corresponding R protein (guard).
“AVR-R gene interaction provides the
molecular basis for flor hypothesis and
Receptor-Effector interaction provides
the biochemical evidences for flor
Molecular recognition in gene for
• When induced in timely manner, the concerted response efficiently
halt pathogen growth with minimal collateral damage to the plant
• No input is required from the farmer and no adverse environmental
• Efficient reduction of pathogen growth
• Minimal damage to the host plant
• zero input of pesticides from the farmers and
• Most importantly the environment friendly nature of such crops.
Advantages of R gene
Disadvantages of R genes
• R genes are quickly defeated by co-evolving pathogens
• Many R genes recognise only a limited number of
• R genes do not provide broad spectrum resistance
• Introgression of R genes into a elite cultivars by
convention breeding is a lengthy process
Classes of Plant Disease Resistance Genes Based on
Numerous R-genes identified, cloned and
characterized in different plants have been
categorized in eight classes (Table 1) based on
their amino acid organization.
(Gururani et al. 2012).
Classification of R gene
Based on structural feature (presence of certain Domains such
as LRR, NBS, TIR, etc)
Localization in the cell (Cytoplasmic, transmembrane or both)
Class Function Example
I Membrane associated, transcription regulating, Broad
II Cytoplasmic, Signal transducing via Serine-Threonine
III Extra cellular LRRs and Transmembrane anchor Cf2 to Cf9
IV Extracellular LRR, transmembrane receptor, has
Cytoplasmic Serine- Threonine kinase (Signal transducing)
V Cytoplasmic, membrane associated, LRR, NBS and TIR
VI Cytoplasmic, membrane associated, LRR, NBS, and a coiled
Structure and classes of plant disease
First R gene cloned was Hm1 in maize against race 1 of
Cochliobolus carbonum in 1992
But, Hm1 was disproved to be a resistance gene……………….
Because race 1 of C. carbonum produces the host specific HC
toxin and this is a pathogenisity factor and essential for
infection. Hm1 produce reductase enzyme which detoxifies the
HC toxin and thereby confers resistance against race 1……….
Therefore, logically the first R gene cloned and sequenced was
Pto in tomato which recognize the avrpto protein and thereby
activates the defence.
Plant pathogen interaction and development of disease
Gururani et al., 2012
How do R gene/protein functions
Alters the gene expression
Elicitors recognized by R protein
Functions of Resistance Genes
Signaling of plant stress hormones, generation
of reactive oxygen species (ROS),ethylene
biosynthesis, defense gene activation leading
to phytoalexin biosynthesis, cell wall
strengthening by the deposition of callose and
(Dixon et al. 2001)
Fungal pathogens and interacting R-genes
Gururani et al., 2012
Viral pathogens and interacting R-genes
Gururani et al., 2012
Cloned nematode resistance
Gene Resistance gene
Resistance to cyst nematode Beta procumnence
Resistance to Meloidogyne incognita & Meloidogyne javanica
isolated fro tomatoes
Gpa2 Resistance to Globodera pallida identified In potato
Hero Resistance to Globodera isolated from tomato
Gro1-4 Resistance to Globodera rostochiensis