Induced resistance: a novel plant defense strategy against pathogens

During the course of their coevolution, plants and pathogens have evolved a n intricate relationship resulting from a continuous exchange of molecular information. Pathogens have developed an array of offensive strategies to p arasitize plants and, in turn, plants have deployed a wide range of defense mechanisms similar in some respects to the immune defenses produced in ani mals. The recent advances in molecular biology and plant transformation hav e provided evidence that sensitizing a plant to respond more rapidly to inf ection could confer increased protection against virulent pathogens. One im portant facet in ascertaining the significance of defense molecules in plan t disease resistance is the exact knowledge of their spatiotemporal distrib ution in stressed plant tissues. In an effort to understand the process ass ociated with the induction of plant disease resistance, the effect of biolo gical, microbial and chemical elicitors on the plant cell response during a ttack by fungal pathogens was investigated and the mechanisms underlying th e expression of resistance studied. Evidence was provided that, in all case s, disease-resistance reactions correlated with changes in cell biochemistr y and physiology that were accompanied by structural modifications includin g the formation of callose-enriched wall appositions and the infiltration o f phenolic compounds at sites of potential pathogen penetration. Activation of the phenylpropanoid pathway appeared to be a crucial phenomenon involve d in pathogen growth restriction and host cell survival under stress condit ions. Although examples of practical use of induced resistance as a method of plant disease control are few, a number of field and greenhouse experime nts are encouraging and indicate that this approach has the potential to be come a powerful strategy against an array of pathogens in a persistent mann er.