Hydroxyphaseollin production by soybeans resistant and susceptible to Phytophthora megasperma var. sojae

Abstract

Soybean ( Glycine max (L.) Merr.) varieties carrying either of two allelomorphic resistance genes ( Rps and rps2) accumulated 6a-hydroxyphaseollin from 10 to 100 times faster when challenged with incompatible Phytophthora megasperma Drechs. var. sojae A. A. Hildb. races than lines inoculated with compatible races. In incompatible host-parasite combinations, the hydroxyphaseollin levels attained were 100 to 400 times the ED 50 concentration for inhibition of mycelial growth of Phytophthora megasperma var. sojae; however, 6a-hydroxy-phaseollin accumulated to only 1 to 4 times the ED 50 concentration in compatible host-parasite combinations. Hydroxyphaseollin was not detected in non-wounded, unchallenged soybean hypocotyls and did not begin to accumulate until 10 to 12 h after inoculation. Both resistant and susceptible soybean hypocotyls had the capacity to accumulate more 6a-hydroxy-phaseollin and were more resistant as a function of plant age. As reported previously by others, heat treatment broke resistance conferred by the Rps allele. This treatment con-comitantly decreased the rate of 6a-hydroxyphaseollin accumulation in inculated hypocotyls by 90%. The evidence indicated that 6a-hydroxyphaseollin may be the soybean phytoalexin described previously by other workers and that its rapid production is the mechanism of expression of genetically defined resistance to Phytophthora megasperma var. sojae.