A generalized state of enhanced immunity to infection demonstrated by plants following an initial localized injury. The hypersensitive response of the plant to local infection (see hypersensitivity) produces certain substances, such as methylsalicylic acid, that over a period of hours to days evoke resistance throughout the plant, even in parts quite distant from the original injury site (see systemic signalling). Signal molecules, such as systemins and jasmonates, are disseminated throughout the plant and cause the increased expression of genes encoding pathogenesis-related (PR) proteins. These include enzymes capable of degrading microbial cell walls (e.g. chitinases and glucanases), defensins, and other proteins that block microbial enzymes (e.g. protease inhibitors). Salicylic acid (the precursor of the analgesic drug aspirin) is also a well-known signal molecule in plant defences. See also immunity (sense 2).