Plant Defenses 

Physical Defenses

Abiotic factors threaten plants

• E.g., weather and fire.

But daily, there are greater threats in the form of pathogenic viruses, bacteria, fungi, animals, and other plants

Dermal Tissue System

First-line defense of all plants

• Epidermal cells throughout the plant secrete substances to protect plant surfaces from water loss and attack

Above-ground parts also covered with cutin (a water-proof fatty substance that forms the cuticle around fruits and leaves).

• Suberin (another water-proof fatty substance) that is found in cell walls of subterranean plant parts (endodermis, bark)

Silica inclusions (like in horsetails), trichomes, thick sacrificial bark, and even thorns can also offer protection.

Example Pathogen : Fungal Invasion

Fungi seek out the weak spot in the dermal system, or stomata, to enter the plant

Phases of fungal invasion

1. Windblown spore lands on leaves
2. Spore germinates and forms adhesion pad
3. Hyphae grow through cell walls and press against cell membrane
4. Hyphae differentiate into haustoria (a penetrating structure)

Beneficial Fungi and Bacteria

Fungi and bacteria can also be beneficial to plants

• Mycorrhizal fungi.
• Nitrogen-fixing bacteria like Rhizobium.

Chemical Defenses Against Herbivory

Many plants produce toxins that kill herbivores, or at least make them ill

Some are unique to plants

Defensins are found in plants and animals

• These are small, cysteine-rich (containing sulfur) peptides with antimicrobial properties.
  • Reveals ancient origin of innate (non-specific) immunity.
• In some cases, defensins work by limiting protein synthesis.

Secondary Metabolites

Are compound that alter an herbivore’s (plant eater) metabolic pathways

Alkaloids 

• Include caffeine, nicotine, cocaine, and morphine.
• These are bitter-tasting to many animals 

Tannins

• Bind to and inactivate proteins.
• These are also bitter-tasting to many animals 

Plant oils

• Repel insects with strong odors, particularly those found in the mint family.

Animals, including humans, can avoid many of the cumulative toxic effects of secondary metabolites by eating a varied diet

Animals that Protect Plants

Complex coevolution of plants and animals has resulted in mutualistic associations

• Relationships that benefit both.

Acacia trees and ants

• Small armies of ants protect Acacia trees from harmful herbivores.
• Plant provides ants with food and shelter.

Parasitoid Wasps, Caterpillars, and Leaves

As caterpillar chews away, a wound response in the plant leads to release of a volatile compound

• Female parasitoid wasp is attracted.
• Lays fertilized eggs in caterpillar.
• Eggs hatch and larvae kill caterpillar.

Systematic Response to Invaders

Static plant responses to threats have an energetic downside

• Are maintained in the presence or absence of threat.

Energy resources would be conserved if the plant response was inducible

• Defenses launched only when needed.

Wound Response

A wound response occurs when a leaf is chewed or injured

• One outcome leads to rapid production of proteinase inhibitors throughout the plant.
• Bind to digestive enzymes in the gut of the herbivore, which makes them sick.
• Animals can develop Learned Taste Aversions over time.