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Salicylic acid mediates the reduced

growth of lignin down-regulated plants

PPH: 504

Course Leader: Dr. Ajay Arora

Presented By-

Vanita Pandey

Ph.D (1styr)

Biochemistry09959

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Salicylic Acid

SA has important regulatory functions in plants.

In thermogenic plants such as voodoo lilies, SA is the natural trigger of heat

production by activating alternative respiration.

SA is as a signal molecule in plant defense responses, activates expression of

plant pathogenesis-related (PR) genes and induces disease resistance.

In resistant tobacco plants, infection of tobacco mosaic virus (TMV) triggers

increased SA levels not only in lower infected leaves that develop hypersensitive

response (HR) but also in upper uninfected leaves that develop systemic acquired

resistance (SAR).

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Biosynthesis of SA

Lignin

Biosyn.

sid2-2

CatecholSA hydroxylase

NahGPR gene

activation

npr1

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Biosynthesis of GA

GA 2 oxGA 2 ox

GA34GA8

cc

Inactive Inactive

Paclobutrazole

X

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Reducing lignin levels by genetic manipulation improves both forage digestibilityand processing of lignocellulosic biomass for liquid biofuel production.

However, transgenic plants down-regulated in the hydroxycinnamoyl CoA:

shikimate hydroxycinnamoyl transferase (HCT) enzyme, which have a strong

reduction in lignin levels, show severe defects in growth.

HCTdown-regulated alfalfa plants contain increased levels of the stress

hormone salicylic acid (SA) and SA-inducible pathogenesis-related (PR) protein

transcripts.

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PR gene expression can be induced by cell-wall pectic fragments via

processes involving SA, and can impact plant growth and development.

Removal of SA by genetic approaches relieves growth inhibition but

maintains low lignin levels in HCTdown-regulatedArabidopsisthaliana.

So, plants can be engineered to contain highly processable cell walls yet

also produce abundant biomass.

The effects of SA are associated with alterations in gibberellin (GA)

responsiveness and signaling.

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Elevated SA levels and

water-soluble pectic elicitors

of PR protein induction in

HCTdown-regulated alfalfa

are not specific to HCT down-

regulation but reflect the

extent of lignin reductionindependent of the lignin

pathway enzyme targeted.

Correlations Between SA Levels and Growth in a Population of

Lignin- Modified Plants.

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Blocking SA Production Restores Growth to HCTDown-RegulatedA.thalianaPlants.

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HCTdown-regulated Arabidopsis

plants in which SA biosynthesis is

blocked no longer display a dwarfphenotype, despite their strongly

reduced lignin levels and

drastically altered lignin

composition.

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Conversion of SA to Catechol Also Restores Growth to HCTDown-Regulated

A. thalianaPlants

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Reduction of SA-Mediated Defense Signaling Does Not Restore Growth to Low-

Lignin Arabidopsis Plants.

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SA-Mediated Growth Effects in Low-Lignin Plants May Operate via GA Signaling.

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SA-Mediated Growth Effects in Low-Lignin Plants May Operate via GA Signaling.

Response of 8-wk-old plants to 105M GA4

GA responsiveness is lost in HCT-RNAi lines at later stages of vegetative developmentwhen lignification is occurring but can be restored by genetic manipulation to block

accumulation of SA

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GASA1: gibberellic acid stimulated

transcript protein homolog 1

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oHCT-RNAi alfalfa plants show reduced GA levels and perception.

oGenes were repressed in the HCT-RNAi Arabidopsis line compared with WT.

oGA responsiveness of these genes (repression for GA20 and GA3 oxidases and induction

for GASA1 and XTH genes) was restored in HCT-RNAi lines in the sid2-2 and NahG

backgrounds after exogenous GA application.

oHence, reduction in growth of HCT-RNAi lines is associated with SA-mediated

impairment of GA biosynthesis/signaling.

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Two pathways have been postulated for SA biosynthesis in plants: fromcinnamate via benzoate or from shikimate via isochorismate.

Lignin down-regulation induces SA formation primarily through the

isochorismate pathway, which operates in Arabidopsis during responses to

pathogens.

Release of pectic elicitors from underlignified secondary cell walls in HCT-

RNAi lines induces SA, and thereby defense responses.

GA-induced growth is restored upon reduction of the SA pool in Arabidopsis,

suggesting that SA may mediate its growth-reducing effects through GAsignaling.

SA-mediated events are central to the orchestration of the reduced growth

response of HCT-RNAi plants.

Summary

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THANK YOU