13.9 - Plant Defences

Physical plant defences

• Plants don't have an immune system of specialised cells and antibodies like animals do.
• Instead, they use physical defences - barriers that stop pathogens getting into the plant in the first place.

The two physical barriers

1. Waxy cuticle - A tough layer covering most leaves and stems. It blocks pathogens from entering, and stops water collecting on the leaf - which lowers the risk of infection by pathogens carried in water.
2. Cellulose cell wall - Every plant cell is surrounded by a cell wall made of cellulose, sitting beneath the cuticle. It forms a further barrier against any pathogen that gets past the cuticle.

Chemical plant defences

As well as physical barriers, many plants make chemical defences - substances that kill pathogens or deter the animals that would eat the plant.

Antimicrobial chemicals

• Also called antibacterials or antiseptics, these chemicals kill or stop the growth of bacteria and fungi.
• Examples: mint and witch hazel.

Poisons to deter herbivores

• Some plants make poisonous chemicals that deter herbivores (plant-eating animals).
• Examples: tobacco plants, foxgloves, and deadly nightshade.
• The poison digitalis comes from the foxglove.