| We have hypothesized that plant cell-wall fragments are a trigger for hypersensitive cell death, a fundamental defense response of plants that have been challenged by potential pathogens. The goal of this research was to demonstrate that plant cell-wall fragments released by plant pathogen-secreted enzymes can kill plant cells. The rice blast fungus, Magnaporthe grisea, was found to secrete, when grown on plant cell walls, a heat-labile activity that killed suspension-cultured maize cells. Further, this heat-labile killing activity was shown to release heat-stable maize cell-wall fragments that killed maize cells.;In an attempt to determine whether selected cell wall-degrading enzymes secreted by the fungus were responsible for the heat-labile killing activity, pectin lyase, pectin methylesterase, and a xylanase were purified to homogeneity from the culture filtrate of M. grisea. These enzymes individually or together did not kill suspension-cultured maize cells. However, the xylanase was able to release, to a limited degree, heat-stable maize cell-wall fragments that killed maize cells.;The maize cell-wall fragments solubilized by the fungal-secreted heat-labile killing activity were found to be composed primarily of arabinoxylan oligosaccharides, some of which were feruloylated. The killing activity of the wall fragments was destroyed by base treatment, suggesting that the feruloyl esters were necessary for the killing activity. Purification of the active component was not possible because of the nearly total loss of killing activity during attempted chromatographic separations. The loss of killing activity was shown to result from precipitation or from binding to the walls of glass vessels of a quantitatively minor component of the fragment mixture.;The results presented in this dissertation support the hypothesis that plant cell-wall fragments are elicitors of the hypersensitive response. However, the fact that the killing activity of the cell-wall fragments is due to a quantitatively minor component means that the structural nature of the cell-wall fragment that possesses killing activity remains to be determined. |
