Elicitation of Defensive Reactions in Conifers

Abstract

There is a general understanding about the different kinds of induced defensive reactions in herbaceous plants, and some hypotheses have been proposed for the possible mechanisms of their induction (Sequeira 1983, Darvill and Albersheim 1984). Two reactions have been distinguished: a localized one and a systemic one. The localized or hypersensitive reaction is seen only in close proximity to the site of wounding and is characterized by a local accumulation of phytoalexins. This reaction is generally caused by the presence of microorganisms (bacteria or fungi) in the wound because sterile wounds do not exhibit this kind of response. Phytoalexins, which play an important role in restricting the extent of penetration by the microorganisms, are released by host plant cells in response to various elicitors, usually compounds from pathogen or plant cell walls (Hadwigeret al. 1981, Sequeira 1983, Darvill and Albersheim 1984). These elicitors are nonspecific and are generally polysaccharides or glycoproteins. The systemic reaction is observed at sites in the host-plant distant from the point of damage or infection and some time after its induction. In these circumstances, the plant is usually more resistant to a challenge inoculation after the original inducing inoculation than to the original one itself. The elicitors responsible for this reaction originate from broken cell walls of the plant and diffuse throughout the plant. They can be released by a simple wound that is caused, for example, by the feeding of chewing phytophagous insects (Ryanet al. 1985).