Elicitation and Metabolism of Phytoalexins in Plant Cell Cultures

Summary

Application of biotic or abiotic elicitors to plant cells induces substantial metabolic alterations directed at establishing plant defence reactions. The elicitor-induced accumulation of antimicrobial phytoalexins deserves special attention for explaining plant-fungal parasite interaction. The great chemical diversity of phytoalexins is reviewed. In chickpea (Cicer arietinum L.), the 6aR:11aR-pterocarpan phytoalexins, medicarpin and maackiain, are induced by both endogenous and Ascochyta rabiei-derived elicitors. An A. rabiei suppressor inhibits in C. arietinum the accumulation of pre-infectional isoflavones, their glucoside conjugates and the phytoalexins. Chickpea cell cultures established from cultivars with high resistance (ILC 3279) and high susceptibility (ILC 1929) to the pathogen A . rabiei show identical patterns of isoflavone accumulation but differ significantly in phytoalexin production. The high phytoalexin producing culture ILC 3279 has been used to characterize new isoflavone hydroxylases and an isoflavone oxidoreductase which are specifically involved in pterocarpan formation. The elicitor-induced changes in enzyme activities measured in cell culture ILC 3279 can be depicted by a metabolic grid of three sets of closely linked enzymes for the general phenylpropanoid pathway, the isoflavone conjugation reactions and the biosynthesis of pterocarpans. After excretion into the growth medium, the pterocarpans are polymerized by peroxidases.