Regulation of Phytoalexin Synthesis in Jackbean Callus Cultures

Abstract

Jackbean Canavalia ensiformis (L.) callus tissues synthesized the phytoalexin, medicarpin (3-hydroxy-9-methoxypterocarpan), when treated with spore suspensions of Pithomyces chartarum M.B. Ellis, a nonpathogen of jackbean. Medicarpin was isolated from treated callus tissue and identified by its UV and mass spectra. The minimum spore concentration found to elicit medicarpin synthesis after 26 h was 1 .times. 105 spores/ml; levels of medicarpin in callus tissue increased linearly up to 1 .times. 107 spores/ml, indicating that the recognition sites for presumed elicitors were not saturated. Medicarpin was first detected in callus treated with 1 .times. 107 spores/ml, 6-12 h after application, and the concentration reached a maximum at 48 h, slowly declining thereafter to 72 h. In callus treated with 3.15 mM HgCl2, medicarpin concentrations were also maximum by 48 h. Phenylalanine ammonia-lyase (EC 4.3.1.5) activity increased 2-fold in spore-treated callus after 36 h. Isoliquiritigenin, daidzein and genisten o-methyltransferase (EC 2.1.1.6) activities were increased 3- to 4-fold in treated callus. Caffeic acid and naringenin were more efficient substrates for o-methyltransferase activity than the other flavonoids or apigenin, but there was no increase in these o-methyltransferase activities in spore-treated callus. The phytoalexin response in this callus tissue culture system compares well with natural plant systems and should be an excellent system for investigating regulation of phytoalexin synthesis.