Plant Immune System

• Plants live in complex environments in which they intimately interact with a broad range of microbial pathogens.

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• The evolutionary arms race between plants and their attackers provided plants with a highly sophisticated defense system.

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• Recent advances in plant immunity research have provided exciting new insights into the underlying defense signaling network.

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Plant immune system

Pre-existing defense:

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• An array of structural barriers
• Preformed antimicrobial metabolites

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Post infection primary immune response:

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Common features of microbial pathogens (eg. Chitin,

flagellin, glycoprotein, lipopolysaccharides) which are

called pathogen associated molecular patterns.

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Recognized by pattern recognition receptors (PRRs)

of host

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Initiate downstream signaling

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Activation of basal resistance called

PAMP-triggered immunity (PTI)

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Pathogen transports effector molecules into host cell

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Effector-triggered

susceptibility (ETS)

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Plant immune system

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Post infection secondary immune response:

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Plants have developed a range of resistance (R) proteins that recognize effector molecules of pathogen

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Effector-triggered immunity (ETI)

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Induced plant defense responses (PTI & ETI):

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• Cell wall fortification by callose, lignin, etc.
• Production of phytoalexin
• Accumulation of PR proteins like chitinase, glucanase, etc.

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Plant immune system

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Systemic acquired resistance (SAR):

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SAR is a mechanism of induced defence that confers long-lasting protection against a broad spectrum of microorganisms. Once plant defence responses are activated at the site of infection, SAR is often triggered in distal plant parts to protect these undamaged tissues against subsequent invasion by the pathogen.

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Steps:

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PTI & ETI-mediated pathogen recognition

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Signal transmitted to distal plant parts by methyl salicylate, jasmonate, etc.

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Accumulation of specific type of PR proteins

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SAR

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Plant immune system

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Induced systemic resistance (ISR):

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Beneficial soil-borne microorganisms, such as mycorrhizal fungi and plant growth–promoting rhizobacteria, can induce a phenotypically similar form of systemic immunity called induced systemic resistance.

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Steps:

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Molecular patterns of beneficial microorganisms

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Recognized by host receptors

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Mild activation of immune response

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ISR

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Small-molecule hormones in plant immunity

Phytohormone:

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• Substances that, at low concentration, influence plant growth and differentiation.

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• Formerly referred to as plant hormones or phytohormones, these terms are now suspect.

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• Better called plant growth regulators.

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• Roles of SA JA & ET as primary signal molecule in plant defense is well established.

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• Recently, roles of ABA, auxin& gibberellin have also been reported.

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• Intensive interplay between hormones & other small molecule mediated signaling pathways help plants to fine tune their immune response against individual attackers.

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• Hormone signals:
• Biotrophic pathogens are regulated by SA signaling.
• Necrotrophic pathogens are regulated by JA/ET signaling.
• Herbivory & insect bite are regulated by JA signaling.

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Small-molecule hormones in plant immunity

SA signaling pathway

EDS1/PAD4

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SA

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Change in cellular redox potential

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NPR1Oligomer NPR1 Monomer

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Transported to nucleus

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Binds to TGA/WRKY/GRX480 transcription factors

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Act as transcriptional co-activator of SA-responsive genes (e.g. PR1)

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Defense against biotrophs

JA signaling pathway

JA accumulation .

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JA-isoleucine binds to F-box protein COI1 in the SCF^COI1 complex

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Jasmonate ZIM-domain (JAZ) protein complex.

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JAZ proteins ubiquitinated & degraded in proteosome 26S

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MYC2 activated ERF1/ORA59 activated

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Defense against necrotrophic pathogens

Transcription of JA responsive genes (e.g. VSP2)

Transcription of JA responsive genes (e.g. PDF1.2)

ET signaling pathway

Gaseous ET

Active ETR1

Inactive ETR1

CTR1 active

CTR1 inactive

EIN2

EIN2

SCF^EBF1/2

SCF^EBF1/2

EIN3

Transcription of ET responsive genes (e.g.PDF1.2 )

Defense against necrotrophs

SA-JA CROSS-TALK

• Antagonistic in nature.

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• Biotic pathogen induced SA pathway suppresses JA mediated defense against caterpillar feeding and necrotrophic pathogen.

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• JA responsive marker genes – PDF1.2 & VSP 2 – are suppressed by exogenous application of SA.

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• Synergistic action of SA-JA: Application of low concentration of SA & JA leads to synergistic activation of PDF1.2 (JA responsive gene) and PR 1 (SA responsive gene).

• Proteins involved in SA-JA cross-talk include:

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• MAPK4
• EDS1(ENHANCED DISEASE SUSCEPTIBILITY1)
• PAD4 (PHYTOALEXIN-DEFICIENT4)
• NPR1
• SSI2 (SUPPRESSOR OF SA INSENSITIVITY2)
• GRX480 (Glutaredoxin)
• WRKY transcription factor proteins, e.g.. WRKY70

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• NPR1 plays a central role in SA signaling.

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• SSI2 plays NPR1-independent role in SA-JA cross-talk.

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SA-JA cross-talk

JA-ET CROSS-TALK

• Synergistic in nature.

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• Regulation of the Arabidopsis plant defensin gene PDF1.2 requires concomitant activation of the JA and ET response pathways.

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• ERF1 and ORA59 are members of the large plant-specific APETALA2/ETHYLENE RESPONSE FACTOR (AP2/ERF) superfamily of transcription factors.

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• The expression of both ERF1 and ORA59 is induced by JA and ET synergistically.

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• Over-expression of the transcription factor genes ERF1 or ORA59 in the JA-insensitive mutant coi1, or ERF1 in the ET-insensitive mutant ein2, constitutively activated the PDF1.2 gene point of convergence of JA and ET signaling.

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SA-ET CROSS-TALK

• Synergistic in nature.

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• ET is essential for the onset of SA-dependent SAR.

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• ET enhances the response of Arabidopsis to SA, resulting in expression of the SA-responsive marker gene PR-1.

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• This synergistic effect was blocked in the ET-insensitive mutant ein2 which indicates that EIN2 of ET responsive pathway modulates the SA pathway.

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SA-JA-ET crosstalk in plant immune response

• Arabidopsis has been demonstrated as an excellent model species in elucidating plant-pathogen interaction.

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• Roles of small molecule hormones in regulating plant defense has been uncovered in this model.

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• However, the actual nature is far more complex and laboratory findings needs to be tested in ecological context.

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• Enormous amount of data generated by different plant biologists calls upon a systems approach where computational biology helps to integrate and comprehend these complex datasets.

Concluding remarks

Thank you

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