Suppressors: Determinants of specificity produced by plant pathogens

Plant pathogens secrete suppressors that delay or prevent the host defense responses, with resultant conditioning of host cells such that they become susceptible even to avirulent or non-pathogenic microorganisms. Suppressors have been characterized as glycoproteins, glycopeptides, peptides and anionic and nonanionic glucans. A suppressor itself is non-toxic to plant cells and, thus, it can be distinguished from host-specific toxins produced by certain pathogens. Suppressors disturb fundamental functions of host plasma membranes. For example, the suppressor from a pea pathogen, Mycosphaerella pinodes, inhibits both the ATPase activity and polyphosphoinositide metabolism in pea plasma membranes, causing the temporary suppression of the signal-transduction pathway that leads to the expression of defense genes, which encode key enzymes in the biosynthetic pathway to phytoalexin. In this review, evidence for the role of suppressors in the determination of plant host-parasite specificity is summarized.