Structural immunity

Structural defences are natural barriers of two kinds:

(a) Pre-existing structural defences

• – already present in a healthy plant.
• (b) Induced structural defences – formed only as a response to an attack or injury.

(a) Pre-existing structural defences

1. Epidermal cells. The epidermis of green stems and leaves is coated with a waxy cuticle made of cutin. Wax repels water, so water does not stay as a film on the surface; without a film of moisture, fungal spores and bacteria find it hard to germinate or multiply. Many epidermises also bear hairs or thorns, which discourage grazing animals from eating the tissue – and a bite that never happens is a wound (and an entry point) that never happens.
2. The cell wall. Every plant cell is enclosed in a wall made mainly of cellulose. This gives support and protection. The wall can be reinforced further by extra cellulose or by impregnation with lignin, suberin and cutin; a thick, chemically tough wall is very hard for a pathogen to digest or pierce.
3. Cork. The stems and trunks of woody trees are covered in an outer layer of cork: several layers of dead cells whose walls are thickened with suberin. Cork is waterproof, protects against shocks and water loss, and makes the plant more resistant to bacterial and fungal infection. Note that cork is both pre-existing and re-formed: if the outer layer of a stem is cut or torn, the plant produces new cork, like any other tissue repairing itself, to seal the opening.

(b) Induced structural defences

These appear after injury or infection.

1. Cork formation (wound cork). After damage, new cork is laid down beneath the wound, closing the gap through which microbes could enter.
2. Tyloses. When the vascular system is cut or invaded, the protoplast of living parenchyma cells next to the xylem vessels and tracheids bulges outward through the pits in the wall and into the cavity of the vessel. These balloon-like overgrowths are tyloses. They plug the xylem and physically block the movement of the pathogen to other parts of the plant.
3. Deposition of gums. When woody stems are cut, damaged or infected below the outer cork, many plants – for example leguminous trees such as Acacia nilotica – deposit gum at the site. The sticky gum traps microbes and stops them entering deeper tissue.
4. Cellular defence structures. Cells respond morphologically (in shape and structure) to invasion. For instance, the walls of epidermal cells and the cells just beneath them may swell (thicken) at the spot where a pathogen attempts direct penetration, which halts the penetration. Also, the plant may wrap the invading fungal mycelium in an insulating sheath so that the fungus cannot pass from one cell to the next.
5. Sacrificing the infected tissue – the hypersensitive response. The plant deliberately kills a small zone of its own tissue around the infection. Deprived of living cells to feed on, the pathogen cannot spread to neighbouring tissue, and the plant rids itself of the pathogen by letting the injured region die. Think of it as a controlled sacrifice, similar to cutting a firebreak.