Plant Defense Pathways

8/12/2019 Plant Defense Pathways

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PLANT DEFENSE

PATHWAYS



Plants respond to attacks by

herbivores and pathogens•Plants use defense systems to deter herbivory, prevent

infection, and combat pathogens

•Plants counter excessive herbivory with physical

defenses such as thorns and defenses such as distastefulor toxic compounds

•Plants damaged by insects can release volatile

chemicals to warn other plants of the same species

•These volatile molecules can also function as an “early

warning system” for nearby plants of the same species.



Plant defense traits

Plant use variety of defenses:-

Mechanical (toughness, spines)

Chemical (alkaloids,phenolics,terpenoids etc.)

Developmental

Phenological defense



Defenses may also be classified with

reference to their production

• Constitutive defence produced by and present

in the plant irrespective of attack

• Induced defence produced by and present in

the plant in response to attack

• both can be structural and chemical



Resistance traits

Direct Indirect



PLANT HORMONES

Auxins

Gibberellins

JasmonateSalicylic

acid

Ethylene



Signaling hormones regulate the

plant immune system activity

• Salicylic acid

•

Jasmonic acid• Ethylene

There can be substantial cross-talk between these

pathwaysThey play an important role in basal and gene-to-

gene resistance to pathogens and herbivores



Salicylic acid

• Phenolic phytohormone

• Role in development,photosynthesis,ion uptake

• Involved in endogenous signaling• role in resistance by inducing production of PGPR

and induce defence against biotrophic pathogen

•

Involved in SAR• can be converted into volatile ester methyl salicylate

COOH

OH



Jasmonic acid

• Member of class jasmonate

• Regulating plant responses and development

• Role in response to wounding of plants and SAR

•induces the transcription of many genes involved in

plant defense

• protein products of genes have anti microbial and anti

fungal activity

• Induce defense against necrotrophic pathogen

• Jasmonic acid turns on genes for proteinase inhibitor • Converted to variety of derivatives like methyl

jasmonate (esters)



Ethylene 2H

4

• Gaseous and ripening hormone

• Present in all higher plants

•

Regulatory function in growth anddevelopment

• Stimulators or inhibitors



Resistance (R) Genes

• These are in plant genomes that convey plant

disease resistance against pathogens by

producing R proteins

• The R protein interacts directly with an Avr

gene (Avirulence gene) product of a pathogen

• confers resistance against specific pathogens

• Can be transfer from one plant to another



Hypersensitive Response (HR)

• A mechanism, used by plants, to prevent the

spread of infection by microbial pathogens

• Burst of oxygen reactive species around

infection site

• Accumulation of Salicylic Acid (SA)

• Restrict pathogen from spreading

• Rapid and local



Pathogen-Related (PR) Genes

• Antimicrobial properties

• Many identified

•

Categorized according to activity• Examples

– PR-2 : beta-1,3-glucanase

–

PR-3 : chitinase – PR-12: defensin



Systemic resistance in plantSAR

• Broad range resistance

• Secondary response

• Initial infection results information of necrotic lesion

• Leads to PR gene expression

• Its activation requiresaccumulation of SA

• Molecular signal

transduction pathway thatis identified by a genecalled NIM1, NPR1 or SAI1 in Arabidopsis thaliana

ISR

• Plant responses to plant

growth promoting

rhizobacteria

•

It does not depend on SAand PR

• It requires both JA and ET

signalling

•

SAR regulatory protein NPR1is required



Case study of: Arabidopsis thal iana

against SLWF nymphal development

Silverleaf whitefly

• Generalist

•

Infests wide variety of crop plantse.g.Brassicaceae members

• Phloem feeders

•

Have different salivary components thanaphids



Mutant and transgenic lines

To test the role of these defense pathways in

basal resistance following five Arabidopsis SA

and JA mutant/transgenic lines were utilized:-

• npr1

• cim10

•

coi1• cev1

• NahG

SLWF h l d l t



SLWF nymphal development on

mutant, transgenic and wild type

plants

0

10

20

30

40

50

60

70

80

90

WT cim10 cev1 coi1 npr1 NahG

Series 1

Series 2

Series 3

series 4



SA,JA and ET defense transcript in

response to SLWF instar feeding

• Transcriptome analysis shows that SA-dependent pathway is induced while JA shows no change

• Arabidopsis respond to SLWF more like a pathogen

• SA responsive gene transcripts accumulated locally

PR1,BGL2,PR5,SID2,EDS5,PAD4

• And systemically

PR1,BGL2,PR5



• JA/ET dependent RNAs

PDF1.2,VSP1,HEL,THI2.1,FAD3,ERS1,ERF1

• Were repressed in infested leaves

• Enhanced JA responses and not SA defenses were responsible

for delaying the nymphal development

• When npr1 plants treated with MejA,dramatic delay in

nymphal development was observed

• SLWFs may provide larger quantities of signals to their host

plant

•

SLWFs introduced inhibitors to prevent JA activation



SLWF development on MejA treated

plants

0

10

20

30

40

50

60

70

80

1st 2nd 3rd 4th

Series 1

Series 2

Series 3



Results

• Showed that SLWF repressed,JA regulated

defenses were associated with basal defense

to the SLWF

• Suppressed JA regulated defenses were

important in slowing nymphal development

• Delay in insect development is an important

resistance mechanism

Plant Defense Pathways

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