Elicitor Induction of Cytochrome P-450 Monooxygenases in Cell Suspension Cultures of Chickpea (Cicer arietinum L.) and Their Involvement in Pterocarpan Phytoalexin Biosynthesis

A yeast glucan elicitor causes the accumulation o f the pterocarpan phytoalexins medicarpin and maackiain in chickpea ( Cicer arietinum) cell suspension cultures established from seeds. A cell culture line from a chickpea cultivar resistant against its main fungal pathogen Ascochyta rabiei accumulates large amounts (944 nmol/g fr. wt.) whereas a cell culture line from a susceptible cultivar accumulates only low amounts (38 nmol/g fr. wt.) o f the phytoalexins. This is consistent with differential accumulation o f pterocarpan phytoalexins in intact plants [1], The first reactions in the pterocarpan-specific branch o f biosynthesis are hydroxylation of the isoflavone intermediate formononetin in position 2' or 3', catalyzed by microsomal cyto­ chrome P-450 monooxygenases. Upon elicitation formononetin 2'-hydroxylase undergoes a strong transient induction in the cell suspension culture o f the resistant cultivar, whereas in the cell culture from the susceptible cultivar it is only slightly induced. In both cell suspension cul­ tures the induction o f cinnamic acid 4-hydroxylase and o f formononetin 3'-hydroxylase does not show a clear correlation with phytoalexin accumulation. Experiments with different elici­ tor concentrations confirm that formononetin 2'-hydroxylase is much more induced in cell cul­ tures from the resistant cultivar than from the susceptible one. It is concluded that the massive difference in phytoalexin accumulation between cell suspension cultures from the resistant and susceptible cultivar is determined mainly by the differential induction o f formononetin 2'-hydroxylase activity.