Preparing for Battle: Plant Defense Mechanisms Against Pathogens

Unlike animals, plants cannot move away from danger, so they have developed sophisticated mechanisms to identify, react to, and minimize the damage caused by pathogens (these are microorganisms that cause disease). So, if plants can’t run away from a nasty fungus or life-threatening virus, what do they do?

1. The First Line of Defense: Physical Barriers

When a pathogen first comes into contact with a plant it does so with its exterior. Therefore, physical barriers are the first method that the pathogen comes into contact with. The cuticle is a key physical barrier. Being waxy, any waterborne pathogens are not able to enter through it. Beneath the cuticle, the cell walls of epidermal cells provide structural support and act as a secondary line of defense. They are composed of cellulose, hemicellulose, and lignin, which are challenging for many pathogens to penetrate. Additionally, specialized structures like trichomes (tiny hair-like projections) and thorns discourage herbivores from eating the plant and moderate pathogen entry. Trichomes can also produce antimicrobial compounds that inhibit pathogen growth.

2. The Biochemical Arsenal: Chemical Responses

Plants also implement a variety of chemical compounds that can deter, repel, or kill pathogens. For example, phytoalexins are antimicrobial compounds produced in response to pathogen attack that are synthesized rapidly and locally at the site of infection. These are toxic to many pathogens. Tannins are polyphenolic complexes that bind to proteins. This reduces the supply of proteins to the pathogen, reducing its ability to perform key metabolic functions. Saponins can alter cell membranes of pathogens, resulting in their death. Plants can emit VOCs (volatile organic compounds) in response to pathogen attack. These compounds lure predators and parasitoids that prey on the pathogens. This is an indirect defense as the plant relies on other organisms to aid in killing the pathogen.

3. Immune Responses: Chemical Defense 2

If a pathogen has still managed to enter the plant, the plant’s immune system is activated to attack. How?

  • Pattern Recognition Receptors (PRRs): Plants have PRRs that acknowledge common patterns that a threatening pathogen may have, such as bacterial flagellin or fungal chitin. This recognition prompts a pattern-triggered immunity (PTI) response, which includes the production of antimicrobial compounds and reinforcement of cell walls.

  • Effector-Triggered Immunity (ETI): Pathogens often produce effector proteins to suppress any host plant retaliation to a pathogen attack. In response, plants have evolved resistance (R) proteins that are specialized to detect these effectors. This encourages an even more extreme response often resulting in localized cell death to contain the infection— this is known as necrosis.

  • Systemic Acquired Resistance (SAR): Plants also form long-lasting protection against a pathogen, through the creation of an immune memory. SAR involves the production of signaling molecules like salicylic acid, which travels throughout the plant to enhance defense mechanisms. Therefore, should the pathogen attack again, the defense system is more efficient as the plant ‘remembers’ what to do.

Co-Evolution

Since plants adapt to take the greatest advantage of their environment, they exploit their surrounding to aid their survival, including altering defense strategies based on the other organisms around them. Some plants form favorable relationships with mycorrhizal fungi or nitrogen-fixing bacteria. This is known as mutualistic symbiosis. These symbionts aid plant health and resilience against pathogens.

The Future

Great inspiration has been taken from plant defenses in order to improve human public health and protection from pathogens. Even today, plant defenses are still evolving as pathogens apply a selection pressure onto a specific species. Therefore, it is likely that additional methods of plant defense may be on the horizon for us to unravel in the near future. How exciting!