(c) plant defences against pathogens

The idea

Plants defend against pathogens with physical and chemical barriers. Physical defences include the waxy cuticle, cellulose cell walls, bark, and stomatal closure or guard-cell responses that limit entry. Chemical defences include antimicrobial compounds such as tannins, alkaloids and terpenoids, plus enzymes and the oxidative burst. Some tissues respond to infection by producing callose to thicken walls and block spread, and by localised cell death (hypersensitive response) that isolates the pathogen. Teach learners to separate prevention of entry (barriers) from responses after entry (chemicals, callose, cell death), and to avoid importing animal immune ideas such as antibodies or phagocytes as plant defences. A useful example is the thickened cuticle and antimicrobial compounds of many leaves.

What earns the marks

• Names a physical plant defence (waxy cuticle, cellulose cell wall, bark, stomatal closure) and explains how it limits pathogen entry.
• Names a chemical plant defence (tannins, alkaloids, terpenoids, antimicrobial enzymes) and explains its role against pathogens.
• Explains callose deposition thickening cell walls to block pathogen spread.
• Explains the hypersensitive response as localised cell death that isolates the pathogen.
• Rejects animal immune features such as antibodies or phagocytes presented as plant defences.

Common mistakes and how to put them right

• Describing antibodies as a plant defence.

  Put it right: Plants rely on physical barriers and antimicrobial chemicals, not antibody-mediated immunity.

• Treating the cuticle as a response to infection.

  Put it right: The waxy cuticle is a pre-existing physical barrier.

• Confusing callose with a chemical toxin.

  Put it right: Callose is a polysaccharide deposited to thicken and block cell walls.

• Assuming stomata cannot respond to pathogens.

  Put it right: Stomatal closure is a recognised physical defence limiting entry.

Exam tips

• Sort each option into physical barrier, chemical defence or post-entry response before choosing.
• Eliminate options that name animal immune cells or antibodies, which do not apply to plants.
• Look for the mechanism in the option, not just the name of a substance, since a correct term with a wrong role is a distractor.