The role of suppressors in determining host-parasite specificities in plant cells

Plant pathogens secrete the compounds that delay or prevent defense responses only of the host plants, with resultant conditioning of host cells such that they become susceptible even to avirulent microorganisms. The priniciples, which are called suppressors, have been characterized as glycoproteins, glycopeptides, peptides, or anionic and nonanionic glucans. Suppressors do not evoke drastic and visible damages of plant cells and, thus, they can be distinguished from host-specific toxins produced by several fungal species almost belonging to genera Helminthosporium and Alternaria. The mode of action of these suppressors has been found to disturb fundamental functions of host cells. The suppressor from a pea pathogen, Mycosphaerella pinodes, inhibits both the ATPase activity and the polyphosphoinositide metabolism in pea plasma membranes, causing the temporary suppression of the signal transduction pathway leading to the expression of defense genes encoding key enzymes in the biosynthetic pathway to phytoalexin. Moreover, it affects the function of cell wall in a strict species-specific manner even in vitro. In this chapter, evidence for the role of suppressors in the determination of plant host-parasite specificity is summarized.