Influence of metalaxyl on abscisic acid levels in soybean hypocotyls infected with Phytophthora sojae

Previous studies have demonstrated that susceptible soybean plants treated with the acylalanine fungicide, metalaxyl, and inoculated with a compatible race of Phytophthora sojae develop lesions phenotypically similar to those in an incompatible interaction. The present study explores this similarity further by following the time-course of changes in concentrations of glyceollin and abscisic acid in inoculated metalaxyl-treated and untreated plants. Concentrations of metalaxyl were determined by an enzyme-linked immunosorbent assay to be 0·1-0·2 μg g -1 fresh weight in hypocotyl tissues of soybean seedlings treated by root uptake for 16 h with 1 μg ml -1 metalaxyl. These tissue concentrations are sufficient to cause 20-50 percent growth inhibition in vitro of P. sojae. Following inoculation with a compatible race of P. sojae, restricted lesions containing high levels of glyceollin developed similar to those in an incompatible interaction. During the development of the lesion metalaxyl concentrations within the lesion increased 10-fold in parallel with but preceding by approx. 3 h the time-course of glyceollin accumulation. Abscisic acid levels in the lesions and the tissues immediately surrounding the lesions were measured using an enzyme-linked immunosorbent assay. Abscisic acid concentrations in metalaxyl-treated plants declined rapidly following inoculation with a compatible race of P. sojae. This was similar to the rapid decline in abscisic acid concentration that occurs in untreated plants inoculated with an incompatible race of P. sojae. No comparable changes occurred in treated or untreated plants inoculated with a metalaxyl-tolerant mutant of a compatible race of P. sojae. Thus, the study provides additional evidence that the resistance phenotype induced by metalaxyl treatment duplicates in detail the resistance response that follows recognition of an incompatible race of P. sojae.