model plant for plant pathogen interaction study in molecular plant pathology

1. CONTENTS
 INTRODUCTION
 WHY ARABIDOPSIS THALIANA?
 HISTORY OF ARABIDOPSIS – PATHOGEN STUDIES
 GENE-FOR-GENE INTERACTIONS
 IDENTIFICATION OF R-GENE LOCI
 ISOLATION OF R-GENE
 PHYSIOLOGICAL RESPONSES OF ARABIDOPSIS
 ACTIVATION OF DEFENSE RESPONSE GENES
 PHYTOALEXIN BIOSYNTHESIS
 SYSTEMIC ACQUIRED RESISTANCE
 CONCLUSION
 REFERENCES

2.  Plant-pathogen interaction is a versatile process, mediated by the
pathogen and plant derived molecules.
 The contact between plant and pathogen lead to a chain of events called
defense responses.
 First and foremost is the recognition. It is mediated by the products of
R-genes (Receptors).
 Initial recognition: Receptors are believed to bind specifically with the
host-specific elicitor molecules produced by the pathogens.
 Recognition is presumed by Signal Transduction Cascade.

3.  Signal transduction:-
 Production of defense response
genes
○ Genes responsible for the production
of :
 Cell wall components
 Hydrolytic enzymes
 Phytoalexins
 Oxidative burst
 Hypersensitive Resistance reaction
Fig 1 : Signal transduction Cascade

5.  Arabidopsis thaliana, Brassicaceae.
 It is not a crop, but a lowly weed.
 Small and grows up to 6 inches high; Grown in the lab in a small space.
Grows rapidly and takes only 6-9 weeks to complete its life cycle.
Produces seeds prolifically (up to 10,000 seeds per plant)
 Seeds can be easily mutagenised (irradiating or by chemical mutagens).
Self pollinated: Recessive mutations quickly become homozygous.
 It has became to plant biology as Drosophila melanogaster to animal
biology.
 It is well suited for position-cloning approaches.
 Smallest genome in the plant kingdom
 135×106 base pairs of DNA distributed in 5 chromosomes
 Almost all of which encodes its 27,407 genes.
 Transgenic plants can be made easily by using Agrobacterium
tumefaciens as vector to introduce foreign genes.
Fig 2 : A Plantlet

6.  As a weed (Thought to possess no agronomic significance) and there was a
little interest until 1980’s.
 However there are two reports of Arabidopsis serving as an alternative host.
 1971 report : Primary host for Sclerotinia of Alfalfa in Maryland.
 1981 report :- P. syringae pv. tomato could be recovered from leaves and
roots of Arabidopsis.
 Arabidopsis is now known to be a host of four different types of virus, three
bacterial species and five nematodes.
 In recent years several reviews of this field have been published and an
entire book emphasizes methods for Arabidopsis-pathogen studies.

7.  Gene for gene interactions requires a specific R-gene in the plant and a
matching avr gene in the pathogen.
 R-genes presumably mediate the specific recognition of the plant pathogen.
 Pathogen avr genes are thought to control the production of host-specific
elicitor molecules.
 A single R-gene in Arabidopsis interacts with two different avr genes of
bacterial pathogen P. syringae.
 Arabidopsis mutants have lesions in the RPS3 disease resistance locus.
RPS3 confers resistance to P. syringae strains that carry the avrB avirulence
gene, and mutants are now susceptible to such strains

8.  Genetic complementation analysis confirmed that the mutations are at RPS3.
 Interestingly, the mutants have also lost resistance to the P. syringae strains
containing an avirulence gene avrRpm1.
 The resistance genes that match avrRpm1 and avrB (RPM1 and RPS3), map to
the same location on chromosome 3.
 This apparently shows that RPM1 and RPS3 appears like a same gene. If true,
this gene is interacting with two dissimilar avirulence genes.
 The resistance response to P. syringae strains expressing avrB is slower and
weaker than the response to strains expressing avrRpm1.
 The available genetic data are consistent with “gene-for-gene” interactions.
Continued…

9.  Several groups have now been demonstrated the existence of classical dominant
(or semi-dominant) R-genes that confer resistance to specific fungal and bacterial
pathogens.
 Nine R-genes have been placed on the Arabidopsis genetic map, including five
that are specific to various races of Peranospora parasitica (downy mildew
pathogen).
 At least one R-gene has been mapped to each of five Arabidopsis
chromosomes.
 R-genes appear in clusters on Chromosome-1 and Chromosome-4.
 The resistance genes RPM1, RPS2, and RPS3 confer resistance to specific strains
of Pseudomonas syringae.
 Identification and mapping of R-gene loci in Arabidopsis represents the first step
towards the isolation via. Position-cloning approach.

10.  It is an alternative approach to position cloning for isolating plant R-genes.
 Here an insertional mutagen such as transposable element or T-DNA from
Agrobacterium tumefaciens is utilized to “tag” R-genes.
 In Arabidopsis T-DNA tagging has been especially fruitful.
 Feldman developed a large collection of Arabidopsis lines containing random T-
DNA insertions.
 The difficulty in using gene-tagging technique is obtaining an insertion in the
targeted locus.
 An insertional mutagen can produce one or two mutations per plant, which is at
least ten times fewer than that of what a chemical mutagen produce.

11.  One concern towards the use of Arabidopsis in plant-pathogen interactions will be
fundamentally similar to crop plants (especially non-crucifer crops) in mechanisms of resisting
disease.
 Many secondary products of metabolism extremely different between plant species.
 The analyses confirmed so far that the response of Arabidopsis to pathogens is very similar to
the responses observed in crop species.
 R-gene mediated resistance in plants is always associated with an HR.
 The HR is usually associated with other responses such as increased lignification of surrounding
plant cell walls, peroxidation of membrane lipids, induction of various defense-genes and
production of phytoalexins.
 All the above responses are observed in Arabidopsis when challenged with avirulent pathogens.

12.  Defense response genes are.
 Genes-involved in phytoalexin and lignin biosynthesis.
 Genes encoding hydrolytic enzymes and cell wall constituents.
 Genes involved in oxidation process.
 In Arabidopsis several defense response genes are cloned and characterized. These
include
 Genes encoding PAL, three different BG’s, two classes of chitinase, a LOX, GST and SOD.
 In addition, genes involved in two different steps of aromatic amino acid biosynthesis.
 The predominant phytoalexin made by the Arabidopsis is likely derived from an
aromatic amino acid precursor.

13.  Roles of various defence response genes may be their relative time of expression during
infection by avirulent pathogens is immediate as compared to virulent pathogens.
 Genes involved in arresting the initial infection should be induced very rapidly during
infection by avirulent pathogens.
 While expression of genes involved in subsequent infection (wound dressing) may be
delayed.
 These subsequent response genes may not display a significant difference between
avirulent and virulent pathogen.
 In Arabidopsis at least one of the PAL gene is induced rapidly within 6h after infection
with an avirulent P. syringae strain when compared to the induction of LOX1 gene is not
apparent until 12h after infection by the same avirulent P. syringae strain.
 Avirulent pathogens are known to rapidly induce the production of activated oxygen species
(the “oxidative burst”) in resistant plants.
 The rapid induction of GST1 in Arabidopsis could be a response to lipid oxidation, rather
than a response to an R-gene-linked signal transduction pathway

14.  The predominant phytoalexin produced by the Arabidopsis is 3-thiazol-2̍-
yl-indole (Camelaxin).
 Camelaxin is absent from healthy Arabidopsis tissue. It is more likely
induced by the infiltration of leaves with an avirulent P. syringae pv.
syringae that causes an HR. but not by a virulent X. campestris strain or
by a non pathogenic mutant of P. syringae pv. syringae.
 However Ausubel et.al. have recently reported that camelaxin is induced
in equivalent levels by both virulent and avirulent strains of P. syringae
pv. maculicola (Psm).

15.  Uknes et al. have recently established that SAR can be induced in
Arabidopsis using 2,6-dichloronicotinic acid (INA) and that such
immunised plants become resistant to infection by P. syringae pv. tomato
and Peranospora parasitica.
 The protection against P. parasitica appears to be mediated by an HR-
like mechanism, suggesting that SAR and R-gene-type resistance may
have common components.

16.  Dangl J. L. (1993) The Emergence of Arabidopsis thaliana as a model plant for plant-pathogen
interactions. Ed. Andrews J. H. and Tommerup I. C. Advances in Plant Pathology 10. Academic
Press Ltd, London. PP
127.
 Davis K. R. et al. (1989) A. thaliana as a model system for studying plant-pathogen interactions.
Ed. Lugtenberg B. J. J. NATO ASI Series 36, Springer.
 Dickinson M. (2003) Molecular Plant Pathology. BIOS Scientific Publishers, New York. PP
273.
 Innes R. (1995) Arabidopsis as a model host in molecular plant pathology. Ed. Singh R.P. and
Singh U. S. Molecular Methods in Plant Pathology. Lewis Publishers, CRC Press, Canada. PP
203-
214.
 Caldwell K. S. and Michelmore R. W. (2008) Arabidopsis thaliana genes encoding defense
signaling and recognition proteins exhibit contrasting evolutionary dynamics. Genetics 181: 671-
684.
 Zimmerli L. et al. (2000) Potentiation of pathogen-specific defense mechanisms in Arabidopsis by
β-aminobutyric acid. Proceedings of National Academic Sciences 97: 12920-12925.
 http://www.biology-pages.info/A/Arabidopsis.html
 http://www.books.google.co.in
 https://en.wikipedia.org/wiki/Arabidopsis

17. Email:- kanthrajkanthu46@gmail.com