Temperature-induced susceptibility of soybeans to Phytophthora megasperma f. sp. glycinea: production and activity of elicitors of glyceollin

Hypocotyls of soybean (cultivar Altona) are resistant to race 4 of Phytophthora megasperma f.sp. glycinea at 25 °C but susceptible to the same race at 32°C. Lower concentrations of the phytoalexin, glyceollin, accumulate in the temperature-induced susceptible response at 32°C than in the resistant response at 25°C. The possibility that this is due to decreased production or activity of elicitors of glyceollin was examined. Following published procedures, elicitors were obtained from culture filtrates and cell walls of P. megasperma f.sp. glycinea race 4, and for comparison from similar preparations from the compatible P. megasperma f.sp. glycinea race 6. Cotyledon bioassays indicated that preparations obtained from cultures of P. megasperma f.sp. glycinea race 4 grown either at 25 or 32 °C were both active. Preparations obtained from P. megasperma f.sp. glycinea race 6 were active also. The cell-wall elicitor preparations obtained from cultures grown at 25 or 32 °C were active in bioassays performed both at 32 and 25 °C. The abiotic elicitor AgNO₃ had similar activity at both temperatures. It is concluded that temperature-induced susceptibility to P. megasperma f.sp. glycinea race 4 is not due to inability to produce elicitors in culture, to inactivity at 32 °C of the elicitors examined or to inability of the host to produce glyceollin at the higher temperature.