Alarm systems in higher plants

The defenses of higher plants against a variety of biotic and abiotic stress agents can be grouped into two categories: Preformed and Induced. Induced defensive responses by the plant, or “alarms”—the subject of the present review—can be localized or systemic. Certain alarms, especially those which are induced by necrotizing pathogens, are protective against a wide variety of biotic stress agents. Other responses appear protective but the degree of host plant involvement is unclear. Finally, there are a few induced plant responses which, although protective, do not easily fit our criteria of an “alarm.” Oligosaccharins may be involved as signals in both anti-herbivore and anti-pathogen alarm systems. Other specific components of plant alarms appear to be induced by only one type of stress agent. The specificity of protection, mechanisms of various alarms and comparisons between them are presented.