Induced resistance: a novel plant defense strategy against pathogens

During the course of their coevolution, plants and pathogens have evolved an intricate relationship resulting from a continuous exchange of molecular information. Pathogens have developed an array of offensive strategies to parasitize plants and, in turn, plants have deployed a wide range of defense mechanisms similar in some respects to the immune defenses produced in animals. The recent advances in molecular biology and plant transformation have provided evidence that sensitizing a plant to respond more rapidly to infection could confer increased protection against virulent pathogens. One important facet in ascertaining the significance of defense molecules in plant disease resistance is the exact knowledge of their spatiotemporal distribution in stressed plant tissues. In an effort to understand the process associated with the induction of plant disease resistance, the effect of biological, microbial and chemical elicitors on the plant cell response during attack by fungal pathogens was investigated and the mechanisms underlying the expression of resistance studied. Evidence was provided that, in all cases, disease-resistance reactions correlated with changes in cell biochemistry and physiology that were accompanied by structural modifications including the formation of callose-enriched wall appositions and the infiltration of phenolic compounds at sites of potential pathogen penetration. Activation of the phenylpropanoid pathway appeared to be a crucial phenomenon involved in pathogen growth restriction and host cell survival under stress conditions. Although examples of practical use of induced resistance as a method of plant disease control are few, a number of field and greenhouse experiments are encouraging and indicate that this approach has the potential to become a powerful strategy against an array of pathogens in a persistent manner.