Inhibition of phytosterol biosynthesis in elicitor-treated cultures of Ammi majus

Abstract

The elicitor-mediated induction of coumarin phytoalexin production by cell-suspension cultures of Ammi majus is accompanied by the inhibition of both biomass production and phytosterol biosynthesis [from mevalonic acid (MVA)]. However, cell-free preparations of cells from elicitor-treated cultures are able to support the synthesis of squalene from MVA, isopentenyl diphosphate (IPP) or (2 E, 6 E)-farnesyl diphosphate (FPP), even though radioactive squalene is not found following administration of [2- 14C]MVA to elicitor-treated cultures. The preparations, as expected, also exhibit umbelliferone:O-dimethylallyl transferase activity. These results indicate that in vivo there is a specific inhibition of one of the enzymes on the MVA to dimethylallyl diphosphate (DMAPP) span of the cytosolic microsomal pathway of terpenoid biosynthesis in elicitor-treated cultures, and that in cell-free preparations this deficiency is made good by the corresponding plastid enzyme. The finding that the coumarin phytoalexins are not labelled on feeding either [2- 14C]acetate or [2- 14C]MVA to elicitor-treated cultures precluded us from establishing whether the inhibition of phytosterol biosynthesis is a means of increasing the input of cytosolic IPP into the plastid for coumarin phytoalexin biosynthesis (in which case cytosolic IPP isomerase would be the enzyme inhibited in elicitor-treated cultures) or whether the cytosolic and plastid pathways of terpenoid biosynthesis are not linked and the site of inhibition (between MVA and DMAPP) is unrelated to the provision of plastid IPP for phytoalexin biosynthesis. The feeding experiments with [2- 14C]acetate suggested that, somewhat unexpectedly, polar lipid biosynthesis is not repressed in elicitor-treated cultures.