Like animals, plants have a number of ways of protecting themselves against disease. Their first line of defense is their outer layer of tissue—the epidermis or cork—which is generally covered with a barrier of waxes, cutin, or suberin. However, if a pathogen, such as a virus, bacterium, or fungus, penetrates this barrier, the plant responds by producing other protective molecules. Plants and pathogens have evolved together such that pathogens have mechanisms to attack and penetrate plants, and plants have evolved mechanisms to kill the pathogens and limit the infection.
If a pathogen invades a plant, one of the plant's first responses is to produce polysaccharides. These polysaccharides plug the plasmodesmata between cells and prevent viral pathogens from moving freely from one cell to the next. The polysaccharides also enter plant cell walls and provide a foundation for the laying down of lignin. Lignin enhances the mechanical barrier, and the toxicity of lignin precursor chemicals makes the cell inhospitable to some pathogens. These lignin building blocks are only one example of the toxic substances that plants use as chemical defenses.
In the accompanying animation, we studied responses that a plant performs to limit an invading pathogen from infecting the entire plant. In addition to producing polysaccharides, a plant also produces phytoalexins and PR proteins, both of which destroy a variety of pathogens. The plant may also respond with a hypersensitive response, in which the infected region commits suicide and walls off the pathogen from infecting other parts of the plant. During this hypersensitive response, the infected region produces salicylic acid, a molecule that may then be transported throughout the plant. Salicylic acid triggers the widespread production of additional PR proteins, which provide the plant with systemic acquired immunity.
Textbook Reference: Concept 28.1 Plants Have Constitutive and Induced Responses to Pathogens