Physical plant defences: cell walls, cuticle, bark

Infection and response • Plant disease (biology only)

Flashcards

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What is the immediate benefit of a continuous cuticle and intact cell walls?

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Continuous cuticle and intact walls reduce the probability of initial infection and slow pathogen establishment .

Key concepts

What you'll likely be quizzed about

Cellulose cell walls

Definition: The cell wall is an external rigid layer around plant cells made mainly from cellulose, a long-chain carbohydrate polymer. Cause → effect: Cellulose fibres increase wall strength, which resists mechanical penetration by pathogens and limits their movement into living tissues. Limiting factors: Damage to the cell wall by wounds, insects or environmental stress permits opportunistic pathogens to bypass this barrier. The cell wall also provides structural support so that turgid cells press tightly together, reducing gaps through which pathogens might enter .

Waxy cuticle on leaves and stems

Definition: The cuticle is a thin, hydrophobic layer of wax produced by epidermal cells on aerial parts of the plant. Cause → effect: The waxy cuticle repels water and creates a smooth surface that reduces the ability of pathogens and spores to attach and penetrate. The cuticle also reduces surface wetness, which lowers the chance for many fungi and bacteria to germinate. Limiting factors: Stomata and physical damage create entry points; some specialized pathogens enter through stomata or by producing enzymes that degrade the cuticle .

Layers of dead cells (bark)

Definition: Bark consists of outer layers of dead, often lignified cells that surround the stems and larger branches of woody plants. Cause → effect: Dead cell layers form a tough external shield that blocks pathogen entry and prevents water loss. Bark sheds naturally or after damage, and shedding removes attached pathogens and spores from the plant surface. Limiting factors: Deep wounds or insect galleries that penetrate bark provide routes for pathogens; saturated or decaying bark can become colonised more easily by fungi .

When physical defences fail

Definition: Failure of physical defences occurs when barriers are breached by wounds, insect feeding, environmental damage, or by pathogens that specialise in penetrating barriers. Cause → effect: Once a physical barrier is breached, pathogens access living tissues and trigger induced chemical defences or structural responses such as increased cellulose deposition or localized cell death. Limiting factors: The speed and effectiveness of subsequent chemical and cellular responses determine whether infection becomes established; weak or slow induced responses allow pathogens to spread.

Key notes

Important points to keep in mind

Cellulose is the main polymer that strengthens plant cell walls.

Waxy cuticle repels water and reduces pathogen attachment and germination.

Bark consists of layers of dead cells that protect stems and shed pathogens.

Stomata and wounds provide major entry points for pathogens.

Surface wetness increases risk of fungal and bacterial infection.

Plants thicken cell walls near infection sites to limit pathogen spread.

Young tissues often lack full physical protection and are more vulnerable.

Insect feeding often precedes successful pathogen invasion.

Physical defences act before chemical or cellular responses are required.

Damage to barriers is a common limiting factor for physical defence effectiveness.