systemic acquired resistance (SAR)
A generalized state of enhanced immunity to infection demonstrated by plants following an initial localized injury. It is thought that the hypersensitive response of the plant to local infection (see hypersensitivity
) produces certain substances that, over a period of hours to days, evoke resistance throughout the plant, even in parts quite distant from the original injury site. There is now strong evidence that one such substance is salicylic acid
(2-hydroxybenzoic acid), whose acetyl derivative, aspirin
, is widely used as an analgesic. One model proposes that salicyclic acid synthesized at the initial injury site is translocated through the plant via the phloem to distant parts. Here it activates genes encoding so-called pathogenesis-related proteins
, which include enzymes capable of degrading microbial cell walls (e.g. chitinase
) and other proteins that block microbial enzymes. The related compound, methylsalicylic acid, is more volatile and may be given off by injured parts to act as an airborne signal.