Systemic acquired resistance (SAR) is a whole-plant immune response that is activated upon localized infection by a pathogen. It provides long-lasting, broad-spectrum resistance against secondary infections. SAR involves the production of mobile signaling molecules like methyl salicylate, azelaic acid, and glycerol-3-phosphate in infected tissues that activate defenses in distant, uninfected tissues. This results in increased expression of pathogenesis-related proteins and other defenses. The NPR1 protein is a master regulator of the SAR response.

1. SYSTEMIC ACQUIRED RESISTANCE
Sujata N. Dandale
Ph.D (Pl.Pathology)
Deptt. Of Plant Pathology,
Dr.P.D.K.V.,Akola

4. Presently disease control is largely based on the use of fungicides, bactericides and
insecticides.
Most plant protection methods currently applied use toxic chemicals noxious to the
environment for pathogen and pest control. The hazardous effect of these chemicals or their
degradation products on the environment and human health strongly necessitates the search
for new, harmless means of disease control.
 Induced resistance like SAR may diminish the use of toxic chemicals for disease control,
and thus could be proposed as an alternative, non-conventional, non-biocidal and
ecologically-friendly approach for plant protection and hence for sustainable agriculture.
The induced resistance can be defined as an increased expression of natural defense
mechanisms of plants against different pathogens provoked by external factors of various
types and manifested upon subsequent inoculation.
There are the three different forms of the induced resistance,
(1) Localized Acquired Resistance [LAR],
(2) Systemic Acquired Resistance [SAR],
(3) Induced Systemic Resistance [ISR].

5. The Systemic acquired resistance is a “whole - plant’’ resistance response that occurs
following an earlier localized exposure to a pathogen.
SAR is analogous to the innate immune system found in animals.
The resistance that developed in the distal, untreated portion of pathogen inoculated
plants is termed as systemic acquire.
Systemic acquired resistance (SAR) refers to a distinct signal transduction pathway
that plays an important role in the ability of plants to defend themselves against
pathogens. the formation of a necrotic lesion, either as a part of the hypersensitive
response (HR) or as a symptom of disease, the SAR pathway is activated.
SAR activation results in the development of a broad-spectrum, systemic resistance.
SAR can be distinguished from other disease resistance responses by both the spectrum
of pathogen protection and the associated changes in gene expression. Systemic means
throughout in plant tissues. It is non-specific secondary response.
Systemic Acquired Resistance [SAR]

6. SAR activation results in the development of a broad-spectrum, systemic resistance.
After the formation of a necrotic lesion, either as a part of the hypersensitive response
(HR)a symptom of disease, the SAR pathway is activated. It provides a broad-range
resistance against fungal, bacterial and viral pathogens and leads to pathogenesis-
related (PR) gene expression.
It can be induced by plant pathogens, chemicals and bio-agents and not induced
after the onset of flowering & fruiting in host plant.
Systemic acquired resistance (SAR) is an important component of plant defense
against pathogen infection.
Systemic acquired resistance (SAR) is a mechanism of induced defense that confers
long-lasting protection against a broad spectrum of microorganisms.
 SAR requires the signal molecule salicylic acid (SA) and is associated with
accumulation of pathogenesis-related proteins, which are thought to contribute to
resistance.
SA is apparently not the translocated signal but is involved in transducing the signal
in target tissues.

7. Annual Reviews
MECHANISM OF SAR

8. THE MECHANISM INVOLVED IN SYSTEMIC ACQUIRED RESISTANCE
1. Lignifications and other structural barriers
2. Pathogenesis Related proteins
3. Conditioning
1. Lignification and other structural Barriers
 Deposition of lignin in cell wall is called as lignification
 It observed in many plants
 It is an important mechanism for disease resistance Lignin incorporation
strengthen plant mechanically.
 Lignified cell starved the pathogen
 Lignin pre cursors may might exert toxic effect
 Mycelia of colletotrichum lagenarium become lignified in vitro
 Glycine rich proteins accumulate systemically in cell wall of tobacco plants
infected with TMV and virus infected rice plants

9. 2. Pathogenesis related proteins
 Pathogenesis related protein (PRs) were first described in 1970 in tobacco leaves
infected with TMV (Van and Van, 1970)
 PRs have been defined as plant proteins that accumulate after pathogen attack or
related situation, (Van et al. 1994)
 Several PRs including PR-1, β -1, 3- glucanases (PR-2) Chitinases (PR-3), PR-4 and
Osmotin (PR-5).
 PRs referred as SAR Proteins
 The nature and level of expression of SAR protein vary among Plant species
 In tobacco over expression of PR-1 significantly increase resistance against infection
by Perenospora tabacina and Phytophthora parasitica var. nicotianae (Alexander et
al. 1993)
3. Conditioning
 When plants are pretreated with necrotizing pathogen or a synthetic inducer of SAR,
the systemically protected leaves react more rapidly and more efficiently to
challenge infection with a virulent pathogen. This phenomenon is known as
conditioning or sensitizing.
 Skipp and Daverall (1973) induced resistance in bean hypocotyls though localized HR
caused by Colletotrichum lindemathianum.

10.  A primary local infection can trigger not only effector-triggered immunity
(ETI), which is often associated with programmed cell death (PCD) of the
infected cells, but also production of the immune signal salicylic acid (SA) in
chloroplasts through the activity of isochorismate synthase ICS1.
Mobile immune signals are also produced, including azelaic acid (AzA),
glycerol-3-phosphate (G3P), methyl salicylic acid (MeSA), and
dehydroabietinal amines(DA). AzA regulates the expression of AZI1, which
encodes a predicted secreted protease-inhibitor/seed-storage/lipid-transfer
family protein, and AzA, G3P, and DA all require DIR1, a putative lipid-
transfer protein, for their functions.
Accumulation of SA affects the cellular redox and the NPR1 nuclear
translocation through S-nitrosoglutathione (GSNO) and thioredoxins (TRXs).
The nuclear NPR1 concentration is controlled by SA levels through the SA
receptor proteins NPR3 and NPR4.
SIGNAL TRANSDUCTION IN SAR

11. A high concentration of SA in the local infection site promotes NPR1-NPR3
interaction and NPR1 degradation to allow PCD and ETI to occur, whereas in the
neighboring cells, the intermediate level of SA disrupts NPR1-NPR4 interaction,
resulting in the accumulation of NPR1.
NPR1 can interact with transcription factors (TFs) to activate the expression
of ER genes to facilitate protein secretion; the expression of antimicrobial PR
proteins such as PR1, PR2, and PR5; and resistance to secondary infection.
The SAR primed state is associated with acetylation (Ac) of H3K9 and
methylation (Me) of H3K4 at SAR-associated gene promoters

12. DNA methylation and proteins affecting chromatin architecture (e.g., SNI1) and
DNA repair (e.g., RAD51 and BRCA2) may function in priming plant defense
genes and protecting genome stability not only in the current generation but also in
the progeny.
An avirulent pathogen not only triggers defense responses locally but also
induces the production of signals such as salicylic acid (SA), methyl salicylic acid
(MeSA), azelaic acid (AzA), glycerol-3-phosphate (G3P), and abietane diterpenoid
dehydroabietinal (DA).
 These signals then lead to systemic expression of the
antimicrobial PR(pathogenesis-related) genes in the un inoculated distal tissue to
protect the rest of the plant from secondary infection . This phenomenon is
called systemic acquired resistance (SAR).
SAR can also be induced by exogenous application of the defense hormone SA
or its synthetic analogs 2,6-dichloroisonicotinic acid (INA) and benzothiadiazole S-
methyl ester (BTH) . SAR provides broad-spectrum resistance against pathogenic
fungi, oomycetes, viruses, and bacteria.

13. SAR-conferred immune “memory” in plants can last for weeks to months,
and possibly even the whole growing season .
In contrast to ETI, SAR is not associated with PCD, and instead promotes
cell survival.
The onset of SAR is associated with massive transcriptional reprogramming,
which is dependent on the transcription cofactor NPR1 (nonexpresser
of PR genes 1) and its associated transcription factors (TFs) such as TGAs .
SAR is believed to be conferred by a battery of coordinately induced
antimicrobial PR proteins whose secretion requires significant enhancement of
endoplasmic reticulum (ER) function

14. Systemic signals in SAR for plant defense
 These mobile
signals help in
activation of
SAR.
 Mostly metabolites function as systemic signal,
contributing to long distance signaling in plant
defense.

15. Methyl Salicylate (MeSA)
• Mobile signal, moves systemically, found in phloem exudates of infected
leaves, and is required for SAR
• Accumulation of salicylic acid induces the secretion of pathogenesis-
related (PR) proteins with antimicrobial activities.
• SAR requires SABP2's MeSA esterase activity in the systemic tissue to
convert biologically inactive MeSA to active SA. (SABP2’S = Receptor in
systemic tissue)
• Nicotiana tabaccum contains N resistance gene that governs gene –for –
gene type resistance to TMV, MeSA functions as enhance the resistance to
subsquent infection by TMV.

16. Jasmonates
 Oxylipns ,synthesized from polyunsaturated fatty acid.
 Methyl jasmonate (MJ) function as volatile signal and also translocated
through the vasculature.
 JA activates gene encoding protease inhibitor which protect plants against
insect attack.
 Treatment of potato with jasmonate increase resistance to Phytophthora
infestans.

17. Azelaic acid
 A nine–carbon dicaboxylic acid.
 Pathogen infection induces the release of free carbon 18 fatty acids (C18
FA) from membrane lipids, these results in azelaic acid (AzA) production.
 AZI1 (AZELAIC ACID INDUCED1) gene, which is expressed at elevated
level in azelaic acid-treated plant, was required for defence priming by
azelaic acid.

18. Salicylic Acid Biosynthesis
Genetic studies in Arabidopsis have shown that SA is synthesized mainly
through the pathway involving ICS1
Biochemically, SA can also be synthesized from cinnamate produced by
phenylalanine ammonia lyase (PAL) but this pathway seems to play a minor
role in SAR-associated SA synthesis.
 The chorismate pathway has similarities to the bacterial SA-biosynthesis
pathway:
1. ICS1 converts chorismate to isochorismate
2. SA is generated from isochorismate catalyzed by isochorismate pyruvate
lyase (IPL) .

19. Proteins whose roles in SA synthesis have been proposed but not specifically
confirmed:
EDS5 (enhanced disease susceptibility 5)- hypothesized to be involved in
transporting SA or a precursor of SA .
PBS3 (avrPphB susceptible 3)- belongs to the GH3 acyl adenylase family of
enzymes. The related JAR1protein is known to adenylate JA, leading to the
conjugation of isoleucine to JA to form the bioactive
 EPS1 (enhanced Pseudomonas susceptibility 1)- provide the enzymatic activity
that is equivalent to IPL found in bacteria.

20. Avr R gene
SA
NPR1
PR-1 PR-2 PR-5
SAR
SAR

21. The Arabidopsis NPR1 (nonexpresser of PR genes 1) protein is a
master regulator of SAR. Recent study has shown that salicylic
acid directly binds to the NPR1 adaptor proteins NPR3 and NPR4,
regulates their interactions with NPR1, and controls NPR1 protein
stability

22. Production of pathogenesis related protein
 PR protein are plant protein that are induced in
pathological and related situation.
 These proteins are accumulated 7-10 days after infection
and indicate the attainment of SAR.
 It is accumulated in the intercellular spaces(first line of
defence) and vacuole (second line of defence by lytic
enzyme).

23. CONCLUSION
Methyl salicylic acid, azelaic acid, glycerol-3-phosphate, and abietane diterpenoid
dehydroabietinal have been identified as mobile signals for SAR.
Azelaic acid, glycerol-3-phosphate, and abietane diterpenoid dehydroabietinal all
require DIR1, a lipid transport protein, for their functions.
NPR1, as a transcription coactivator, is a master regulator of plant defense required
for ER and PR gene induction, local defense, and SAR.
Besides inducing SAR, SA is known to inhibit the hypersensitive response during ETI
and trigger thermogenesis in plants and inhibit plant growth, chloroplast development,
and photosynthesis.