Immunity: plant defences and the lymphatic system

Some structural defences appear only after an injury or an attack. Tyloses are overgrowths of living cells next to the xylem vessels and tracheids. They push through the pits into the vessels after a cut or an invasion and block them, so the pathogen cannot travel along the transport tissue. Some woody plants, such as the legume tree Acacia nilotica, deposit gums at an injury; the gum traps microbes and keeps them out. Where a pathogen presses on the plant, the walls of the epidermal cells and the cells just under them swell, which stops the pathogen getting through. A fungus’s threads (the mycelium) can be wrapped in a sheath so it cannot pass from one cell to the next. In the hypersensitive response the plant kills the infected tissue so that the pathogen cannot spread: the plant gets rid of the pathogen by letting that small piece of tissue die. The second system is biochemical immunity. Receptors recognize that a pathogen is present and switch on the defences; they exist in healthy plants and increase after an infection. Antimicrobial chemicals may be there before an infection or formed because of it. Phenols and glycosides are toxic to bacteria, which they kill or stop growing, and some amino acids that are not used to build proteins, such as canavanine, are poisonous to pathogens. Antimicrobial proteins appear after an infection; some react with the toxins of the pathogen and turn them into harmless compounds, and detoxifying enzymes do something similar. Finally, some plants strengthen their defences after an infection so that they are better protected against any new one.