ENZYMOLOGICAL ASPECTS OF THE REDIRECTION OF TERPENOID BIOSYNTHESIS INELICITOR-TREATED CULTURES OF TABERNAEMONTANA-DIVARICATA

The elicitor-mediated induction of pentacyclic triterpenoid phytoalexi n accumulation in cells of five-day-old suspension cultures of Taberna emontana divaricata is accompanied by: a rapid and transient increase in HMG-CoA reductase (EC 1.1.1.34) activity; an increase in IPP isomer ase (EC 5.3.3.2), prenyl transferase (EC 2.5.1.1) and squalene synthet ase (EC 2.5.1.21) activity; a rapid inhibition of squalene 2,3-oxide : cycloartenol cyclase activity (EC 5.4.99.8), and a rapid and relativel y transient appearance of squalene 2,3-oxide:amyrin cyclase (EC 5.4.99 .-) activity. These findings are entirely consistent with an elicitor- induced redirection of the cytosolic-microsomal pathway of terpenoid b iosynthesis away from phytosterol biosynthesis and towards pentacyclic triterpenoid phytoalexin biosynthesis. The switch being mediated as a direct result of the rapid inhibition of squalene 2,3-oxide:cycloarte nol cyclase activity just prior to the de novo synthesis of squalene 2 ,3-oxide:amyrin cyclase and the other enzymes on the post-squalene 2,3 -oxide span of the pentacyclic triterpenoid phytoalexin pathway. The i ncreased activities of the enzymes common to both pathways reflects th e fact that the rate of accumulation of pentacyclic triterpenoid phyto alexins in elicited cultures is more rapid than the rate of phytostero l biosynthesis in control cultures. The very rapid and transient incre ase in HMG-CoA reductase activity points to the microsomal form(s) of this enzyme having a key regulatory role in controlling the flux of ca rbon into the cytosolic-microsomal pathway of terpenoid biosynthesis.