Rapid localized changes in abscisic acid concentrations in soybean in interactions with Phytophthora megasperma f. sp. glycinea or after treatment with elicitors

Abstract

Abscisic acid concentrations were measured with an enzyme-linked immunosorbent assay in tissues of etiolated soybean hypocotyls inoculated with Phytophthora megasperma f. sp. glycinea or treated with an abiotic (AgNO 3) or a biotic (from P. megasperma f. sp. glycinea culture fluids) elicitor. Abscisic acid concentrations were determined in tissues excised from infected or treated sites, in the tissue of the hypocotyl section surrounding the sites, and in sections above and below the inoculated sites. There was a rapid decrease in abscisic acid concentration in the infected site between 2 and 4 h after inoculation in the incompatible interaction to about 35% of control values. Significant decreases in concentrations of abscisic acid occurred between 4 and 6 h after inoculation in tissues surrounding or remote from infected sites. Abscisic acid concentrations in silver nitrate-treated hypocotyls declined similarly but more slowly than in the incompatible interaction. No significant decline in abscisic acid concentrations was detected in compatible interactions until 8 h after inoculation. A small decrease was demonstrated also in wounded hypocotyls treated with the biotic elicitor. Abscisic acid was detected in inoculum drops, but the pattern of accumulation differed in compatible and incompatible interactions. It is concluded that a major decrease in localized abscisic acid concentrations is one of the earliest responses to infection in the incompatible, in contrast to the compatible, interaction. A possible role of abscisic acid in the regulation of resistance responses is discussed.