Phosphoinositide Breakdown During the K⁺/H⁺ Exchange Response of Tobacco to Pseudomonas syringae pv. syringae. Merelee Atkinson. Department of Plant Pathology, University of Wisconsin, Madison 53706 U.S.A. James Bina, and Luis Sequeira. Department of Plant Pathology, University of Wisconsin, Madison 53706 U.S.A. MPMI 6:253-260. Accepted 14 December 1992. Copyright 1993 The American Phytopathological Society.

The hypersensitive response (HR) of tobacco to Pseudomonas syringae pv. syringae involves activation of a K⁺/net H⁺ exchange response (XR). Dependence of this response on calcium influx suggests that it is triggered by a signal transduction pathway. To test this hypothesis, we studied in vivo phospholipid metabolism during the XR. Suspension-cultured tobacco cells were incubated with ³H- or ¹⁴C-labeled fatty acids to radiolabel endogenous lipids. Cells were then inoculated with P. s. pv. syringae or with a hrp mutant that does not induce the HR or the XR in tobacco. In XR-induced cells, radioactivity in phosphatidylinositol decreased 20-30% within 4 hr, relative to cells inoculated with the hrp mutant. In contrast, radioactivity in other phospholipids remained constant or increased over this time period. When tobacco cells were pulse-labeled with ³²Pi during the XR, radioactivity in phosphatidylinositol and phosphatidylinositol 4-phosphate was reduced while radioactivity in other phospholipids was markedly increased. Phosphatidylinositol breakdown, calcium influx, and the XR were inhibited by bromophenacylbromide, a phospholipase inhibitor, and by neomycin, which blocks metabolism of polyphosphoinositides. These results suggest that increased activity of phosphoinositide-specific enzymes, possibly phospholipase C and/or phosphoinositide kinases, play a role in XR activation.