Innate immune responses in plants
Plants possess innate immune responses to defend against a wide range of pathogens, including bacteria, fungi, viruses and insects. These responses are necessary as they cannot move to avoid threats. Plant immunity involves both physical and chemical defence strategies.
Physical Defence Strategies
- Barriers:
- Epidermal Layer: The outermost layer of plant tissues, consisting of tightly packed cells, acts as a primary physical barrier against pathogen entry. This layer helps prevent the invasion of pathogens through the surface.
- Cuticle: The cuticle is a waxy layer covering the epidermal cells of leaves and stems. It provides a hydrophobic barrier that inhibits the entry of pathogens and reduces water loss. The thickness and composition of the cuticle can be modified in response to pathogen attack.
- Cell Walls: Plant cell walls, composed of cellulose, hemicellulose, and pectin, act as physical barriers. They are reinforced with lignin to provide additional strength and resistance against pathogen penetration.
- Leaf Structures:
- Trichomes: These are small hair-like projections on the surface of leaves and stems. Trichomes can physically obstruct pathogen entry and can produce defensive chemicals.
- Stomata: While primarily involved in gas exchange, stomata can also play a role in immune responses. Plants can close their stomata in response to pathogen detection to limit pathogen entry.
Chemical Defence Strategies
- Plant Defensins:
- Definition: Plant defensins are small, cysteine-rich proteins with antimicrobial properties. They inhibit the growth of various pathogens by disrupting their membranes or cellular processes.
- Mechanism: Defensins bind to pathogen cell membranes, leading to membrane disruption and cell death. They can also inhibit enzymes essential for pathogen survival.
- Production of Toxins:
- Phytotoxins: Plants produce a variety of toxic compounds known as phytotoxins that deter herbivores and pathogens. These toxins can interfere with the metabolic processes of the invading organisms.
- Alkaloids: These are nitrogen-containing compounds with potent biological activity. Examples include nicotine in tobacco and morphine in poppy, which act as deterrents to herbivores and have antimicrobial properties.
- Phenolic Compounds: These include flavonoids and tannins, which have antimicrobial and antioxidant properties. They can inhibit pathogen growth and neutralise harmful free radicals produced during infections.
- Saponins: These compounds can create pores in pathogen membranes, leading to cell lysis. They also have antifungal and antibacterial properties.