Systemic acquired resistance

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Systemic acquired resistance
Systemic acquired resistance was first recognized
as a significant phenomenon in 1933 by
Chester. He and many others since note that
infection of plants with necrotizing pathogens
(causing HR) often results in enhanced resistance
to subsequent infections by a variety of fungal,
bacterial and viral pathogens. This physiological
immunity was termed systemic acquired resistance
(SAR).
SAR confers a broad spectrum type of resistance
SAR is effective against some but not all
pathogens Tobacco Phytophthora parasitica,
Cercospora nicotianae, Peronospora
tabacina tobacco mosaic virus, tobacco necrosis
virus, Pseudomonas syringae pv. tabaci, Erwinia
carotovora Not effective against Botrytis
cinerea or Alternaria alternata Arabidopsis
Phytophthora parasitica turnip crinkle
virus Pseudomonas syringae pv. tomato DC3000
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Some key events in understanding regulation of SAR
Systemic acquired resistance was associated with
the coordinated induction of a set of SAR
genes encoding proteins known as
Pathogenesis-related (PR) proteins (Van Loon and
Gianinazzi (early 1970s). (1979) White found
that acetyl salicylic acid application sufficient
to induce PR gene expression and enhanced
resistance to tobacco mosaic virus in tobacco
plant. Discovery came out the interest in
developing chemical control methods for viral
infection. After that several groups went on to
show that salicylic acid application on tobacco
leaves mimics pathogen induced expression of PR
genes and pathogen resistance in treated
tissues. (1990) Two groups one led by Klessig
and Raskin and another led by Metraux found that
salicylic acid accumulates in cucumber and
tobacco plants prior to pathogen infection,
but before the onset of resistance. The work by
these and many others led to the hypothesis that
salicylic acid (SA) is the endogenous signal
molecule that is required for the induction of
systemic acquired resistance. (1993/1994) The
group headed by Ryals made tobacco plants that
could not accumulate SA and found that these
plants were defective in their ability to develop
systemic acquired resistance. This work
demonstrated a central role for SA in
establishing systemic acquired resistance. The
group also demonstrated that these tobacco plants
were defective in their ability to accumulate PR
proteins. (1997) Cloning of NPR1, a key
regulator of SAR
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PR (Pathogenesis-Related) proteins
PR proteins first identified as major proteins
induced by necrotizing pathogens (pathogens that
induced the hypersensitive response)
Proteins secreted predominantly into
intercellular spaces in response to wounding or
infection. Soluble at pH 3 Basic homologs also
found (in vacuole). Proteinase resistant (but not
proteinase inhibitors). Some are developmentally
expressed as part of normal plant development
in absence of wound or infection (e.g.
flowering).
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Traditional PR protein gels
Acidic gel
Basic gel
PR proteins
Proteins first isolatedfrom apoplast
ofTMV-infected tobacco. Induced by many other
pathogens. Some PR proteins are also induced
byabiotic stresses.
old nomenclature
All tobaccoPR proteins
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PR genes induced after HR or SA treatment
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What do PR proteins do?
Type
Family member Biochemical function
Resistance PR-1a,b,c Tob 1a
Unknown Oomycetes (1a) PR-2 Tob. N, O 1,3
?-glucanase antifungal (w/ chitinase) PR-3
Tob. P, Q chitinases anti-Rhizoctonia, other
fungi PR-4 Tob. R Unknown antifungal PR-5 Tob.
S Homologous to amylase /proteinase
inhibitor and sweet protein thaumatin.
Anti-oomycete PR-6 Tom. Inhib I proteinase
inhibitor PR-7 Tom. P69 endoproteinase PR-8 Cuc.
Chitinase chitinase PR-9 Tob. Lignin- peroxidase
forming peroxidase PR-10 Parsely
PR-1 ribonuclease-like PR-11 Tob. V
chitinase chitinase PR-12 Defensins antifungal
PR-13 Thionins antifungal PR-14 Lipid-transfe
r proteins antifungal
Modified from Table 21.4, Biochem Mol Biol of
Plants
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Chitin is 1,4 N-acetyl-D-glucosamine (GlcNAc)
polymer Chitosan is partially deacetylated
chitin.
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Constitutive expression of chitinase PR protein
confersresistance to Rhizoctonia solani
Broglie et al. (1991) Science 254, 1194-1196
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Application of salicylic acid mimics SAR
(1979) White found that the application of
aspirin, salicylic acid, and benzoic acid
resulted in enhanced resistance to TMV. Used 3
tobacco cultivars that contained the N
resistance gene that confers HR to TMV. Found gt
90 reduction in lesion number in treated leaves
versus water control.
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SA accumulation is associated with acquisition of
resistance
SA
Lesions obtained after second inoculation
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Science (1993) 261, 754-756.
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Central role of SA in SAR
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Central role for SA in defense continued
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Enhanced susceptibility
Loss of resistance
INA induces resistance in presence of nahG
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Signaling steps between SA and PR protein
expressionand disease resistance.

• Previously Linked a PR protein promoter called
  BGL2 to GUS.
• Screened thousands of mutant transgenic BGL2-GUS
  plants forABSENCE of GUS activity induced by SA
  treatment.
• Using standard Arabidopsis genetic mapping
  methods, identified asingle mutant gene, npr1.
  Phenotype
• Complete absence of GUS activity in response to
  SA
• Absence of PR-1, PR-5, BGL2 expression in
  response to SA
• Is now susceptible to Peronospora parasitica
• and to Pseudomonas syringae pv maculicola (Psm).
  

Cao et al. (1997) Cell 88, 5763,
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wt
npr1-2
genotype
Cloned NPR1 by standard 1990s methods.
Chromosome walking, YAC library
non-compl.
none
NPR1
NPR1
transgene
none
Proof of cloning by transgenic comple-mentation
of mutants w/ wildtype NPR1.
wt
npr1-1
npr1-1 NPR1
symptoms
Psm inoculated
GUS
Cao et al. (1997) Cell 88, 5763,
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NPR1 has ankyrin repeats
Ankyrin repeats arein lots of differentproteins.
Involved in protein-protein interactions. Especi
ally in proteinsthat control trans-cription.In
NF-kB and I-kB inmammals. Inducedby many
pathogens, stresses
Cao et al. (1997) Cell 88, 5763,
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NPR1 is reduced to a monomer during plant defense
Mou et al., (2003) Cell, 113935944
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Expression of PR-1 is associated with NPR1
monomerization
Mou et al., (2003) Cell, 113935944
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Monomeric NPR1 localizes to the nucleus
Mou et al., (2003) Cell, 113935944
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NPR1 enhances TGA1 binding to the as-1 element
under reducing conditions
Despres et al. (2003) Plant Cell. 1521812191,
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Identification and Cloning of a Negative
Regulator of Systemic Acquired Resistance,
SNI1, through a Screen for Suppressors of
npr1-1 Xin Li, Yuelin Zhang, Joseph D. Clarke,
Yan Li, and Xinnian Dong Cell, 98, 329339,
1999.
Screened for EMS mutants of npr1-1 plants
containing BGL2-GUSreporter. Look for plants
that turn blue in response to INA (SA analog)
like NPR1 wild type plants. But which, of
course still harbor the npr1-1 mutation. Found 11
loci that gave increased GUS, out of 7000 plants
screened.
Li et al., Cell 98, 329339.
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Li et al., Cell 98, 329339.
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Li et al., Cell 98, 329339.
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SNI1 is similar to mouse Retinoblastoma (Rb). Rb
is a tumor suppressor that represses function of
E2F transcription factor
Li et al., Cell 98, 329339.
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SNI1 is in the nucleus
nucleus
TGA TFs
Li et al., Cell 98, 329339.
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Everyone interested in regulation of gene
expression in plant defense should know about
WRKY transcription factors

• Recognize the motif (T)(T)TGAC(C/T).
• Have the conserved WRKYGQK at N-terminal end.
• Have a novel zinc-finger-like motif.
• Bind DNA via divalent cation (probably zinc).
• Approx. 100 members of WRKY family in
  Arabidopsis.
• NPR1 has a WRKY motif in its promoter TTGACTTGAC
  TTGGCTCTGCTCGTCAA

The WRKY superfamily of plant transcription
factors Thomas Eulgem, Paul J. Rushton, Silke
Robatzek and Imre E. Somssich (2000) Trends Plant
Sci 5, 199-205.
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Conserved amino acidsin WRKY proteins
ofArabidopsis (red).
Putative Zn-finger ligandsare highlighted in
black.
Eulgem et al. (2000) TIPS 5, 199-205.
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The Plant Cell, Vol. 13, 15271539, July
2001. Evidence for an Important Role of WRKY DNA
Binding Proteins in the Regulation of NPR1 Gene
Expression Diqiu Yu, Chunhong Chen, and Zhixiang
Chen 1
W-box TTGAC
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Binding of Arabidopsis proteinsto W-box sequence
in NPR1s promoter (PN1 34 nt dsDNA).
Npr1 upstream seq.
-99
-132
Mutated Npr1 upstream seq.
Antibodies to WRKYGQK peptideinhibited binding
of both AtWRKY18and SA-induced binding activity
to PN1 (Fig. 3).
SA
-
-


-
-
AtWRKY18
AtWRKY18
Plant extract
Pure protein
Yu et al. (2001) Plant Cell 13, 1527-1539.
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W-box is necessary for basal and SA-inducible
NPR1 promoter activity.
mRNA
Reporter constructs contain 2419 ntfrom upstream
of NPR1 ORF placedin front of GUS reporter. Used
25 indep. transformants/construct.
Yu et al. (2001) Plant Cell 13, 1527-1539.
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SA-inducible WRKY genes.
Yu et al. (2001) Plant Cell 13, 1527-1539.