Wound-Associated Competency Factors Are Required for the Proximal Cell Responses of Soybean to the Phytophthora Sojae Wall Glucan Elicitor

Intact soybean (Glycine max L. [Merr.]) tissues show distinct proximal and distal cell responses to the Phytophthora sojae (Kauf. and Gerde.) wall glucan elicitor. Proximal cells respond with accumulations of glyceollin and phenolic polymers, whereas distal cells respond with an increase of isoflavone conjugates. Comparison of the activities of the P. sojae glucan in the classical cut cotyledon and a cotyledon infiltration assay suggests that the proximal, but not the distal, responses to elicitor require tissue wounding. Washing the surface of cut cotyledons prior to elicitor treatment also greatly diminishes the proximal responses, which can be restored in a dose-dependent manner by prior treatment of the washed cells with wound exudate from cut "donor" cotyledons. Thus, discrete wound-associated factors, which we term elicitation competency factors, are required for the proximal cell response to the glucan elicitor. The wound factors induce a competent state that is transient in nature. Maximal elicitor response is seen 2 to 3 h after wounding, and cells become elicitor nonresponsive after 4 h. Competency is markedly affected by the age of tissues; cotyledons become more inherently competent as they approach senescence. The time course of attainment of the competent state and its duration are strongly affected by light and temperature. Since the wound-associated competency factors can also be obtained from washings of hypersensitive lesions, we hypothesize that similar competency factors may be released from hypersensitively dying cells in incompatible infections. This event may program the immediately surrounding cells to make them competent for the proximal defense responses.