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Difficulty: Very hardPlant Science and Agricultural Technology

With reference to plant defense mechanisms and biological crop protection technologies against biotic stresses, consider the following statements:

1. Systemic Acquired Resistance (SAR) is triggered predominantly by biotrophic pathogens and depends on salicylic acid signaling to induce systemic expression of pathogenesis-related (PR) genes.
2. Induced Systemic Resistance (ISR) is typically elicited by non-pathogenic plant growth-promoting rhizobacteria (PGPR) and operates via jasmonic acid and ethylene signaling pathways.
3. Root colonization by beneficial PGPR during ISR activation causes localized hypersensitive tissue necrosis at the site of interaction.

Which of the statements given above are correct?

  1. 1 and 2 onlyAnswer
  2. B
    2 and 3 only
  3. C
    1 only
  4. D
    1, 2 and 3

Answer

Statements 1 and 2 only are correct.
The option stating '1 and 2 only' is correct because Systemic Acquired Resistance (SAR) relies on salicylic acid signaling to induce pathogenesis-related gene expression following biotrophic pathogen attack, whereas Induced Systemic Resistance (ISR) is triggered by beneficial rhizobacteria through jasmonic acid and ethylene pathways. Beneficial rhizobacteria colonize root surfaces without eliciting hypersensitive cell necrosis.

Step-by-Step Solution

1
Analyze Statement 1 regarding Systemic Acquired Resistance (SAR)
SAR is an induced defense mechanism activated primarily by biotrophic or hemi-biotrophic pathogens. It relies on the accumulation of endogenous salicylic acid (SA) and leads to systemic transcriptomic activation of pathogenesis-related (PR) proteins (such as PR-1, PR-2, and PR-5). Thus, Statement 1 is correct.
Salicylic acid serves as the crucial phytohormone signal for broad-spectrum systemic resistance against biotrophic pathogens.
2
Analyze Statement 2 regarding Induced Systemic Resistance (ISR)
ISR is elicited by root-colonizing beneficial microbes, particularly plant growth-promoting rhizobacteria (PGPR) like Pseudomonas fluorescens species. Unlike SAR, ISR does not require salicylic acid hyper-accumulation or PR protein expression; instead, it relies on intact responsiveness to jasmonic acid (JA) and ethylene (ET). Thus, Statement 2 is correct.
ISR functions via a primed state mediated by jasmonate and ethylene pathways without inducing constitutive antimicrobial protein expression prior to actual pathogen challenge.
3
Analyze Statement 3 regarding root interaction and hypersensitive response
Hypersensitive response (HR) involves localized programmed cell death (necrosis) to restrict avirulent biotrophic pathogens. Beneficial PGPR colonize the rhizosphere and root cortex symbiotically without triggering HR or tissue necrosis. Thus, Statement 3 is incorrect.
PGPR root colonization is non-pathogenic and establishes a beneficial interaction without causing localized host cell destruction.

Key Concept

Distinct biochemical pathways of plant immune responses: Systemic Acquired Resistance (SAR) vs Induced Systemic Resistance (ISR)
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Topics

Plant Science and Agricultural Technology

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