Temperature-induced susceptibility of soybeans to Phytophthora megasperma f.sp. glycinea: phenylalanine ammonia-lyase and glyceollin in the host; growth and glyceollin I sensitivity of the pathogen

Abstract

In soybean cultivars differing in the effect of temperature on the development of resistance, phenylalanine ammonia-lyase (PAL) activity was determined in hypocotyls at 25 or 33 °C, following infection with Phytophthora megasperma f.sp. glycinea or treatment with the abiotic elicitor, AgNO 3. PAL activity was less at 33 °C than at 25 °C in each of six cultivars examined but was lowest in two cultivars previously shown to develop susceptibility at 33 °C. Glyceollin accumulation was determined in response to AgNO 3-treatment and was higher at 33 °C than at 25 °C in four cultivars tested. The increase was marginal with two temperature-sensitive cultivars but more than 50% in two cultivars that remained resistant at 33 °C. There were significant differences among 18 races of the pathogen for growth rates in vitro, the effect of temperature (25 or 33 °C) on growth, sensitivity to glyceollin I and the interaction of temperature and glyceollin I sensitivity. Growth of some races (e.g. race 2) was strongly inhibited at 33 °C, that of others was similar at both temperatures, and that of one (race 19) was faster at 33 °C than at 25 °C. Minimal restriction of growth at 33 °C and relative tolerance to glyceollin I in race 4 combined with a major suppression of PAL activity and little increase in glyceollin accumulation at 33 °C in cv. Altona was consistent with temperatureinduced susceptibility in this race-cultivar interaction. The possibility that combinations of physiological variables in host and pathogen, rather than, or in addition to, putative recognition systems, may define reaction-types is discussed.