Inhibition of ATPase activity in pea plasma membranes in situ by a suppressor from a pea pathogen, mycosphaerella pinodes

A pathogenic fungus of pea, Mycosphaerella pinodes, secretes a so-called "suppressor" in its pycnospore germination fluid. The suppressor blocks the defense responses and induces local susceptibility (accessibility) in pea plants to agents that are not pathogenic in pea. The suppressor nonspecifically inhibits the ATPase activity in plasma membranes prepared from pea, soybean, kidney bean, cowpea and barley plants. However, cytochemical studies by electron microscopy indicate that the suppressor specifically inhibits the ATPase in pea cell membranes, but not in those of four other plant species tested. That is, the specificity of the suppressor appears at the cell and/or tissue level, but is not evident in vitro. Furthermore, the inhibitory effect of the suppressor is temporary because the ATPase activity recovers 9 h after the treatment. A similar effect was observed after inoculation with M. pinodes but not with a nonpathogen of pea, M. ligulicola. The role of the suppressor in host-parasite specificity is discussed.