Induction of Medicarpin Biosynthesis in Ladino Clover Callus by p-Chloromercuribenzoic Acid Is Reversed by Dithiothreitol

Treatment of Ladino clover (Trifolium repens L.) callus with 1 millimolar p-chloromercuribenzoic acid (PCMBA) stimulated the biosynthesis of the phytoalexin medicarpin. Increased medicarpin concentration was detected at 6 hours, and maximum level was achieved by 36 hours. Dithiothreitol (DTT), added to callus at 2, 6, 10, and 18 hours following addition of PCMBA, partially counteracted the effect of PCMBA in the cells. The greatest effect on reversal of medicarpin accumulation was found at the earlier times for DTT addition. Compared to a 24-hour PCMBA control, final medicarpin concentrations were 9 to 19%, 37 to 45%, and 92 to 105% of the control value when DTT was added at 0.5, 1 or 2 hours, 3 or 4 hours, and 6, 8, 10, or 12 hours, respectively, after PCMBA addition. The results indicate that stimulation of medicarpin biosynthesis is reversible, but once activated, the pathway is not shut down by removal of elicitor.