Enzymological aspects of the redirection of terpenoid biosynthesis in elicitor-treated cultures of Tabernaemontana divaricata

Abstract The elicitor-mediated induction of pentacyclic triterpenoid phytoalexin accumulation in cells of five-day-old suspension cultures of Tabernaemontana divaricata is accompanied by: a rapid and transient increase in HMG-CoA reductase (EC 1.1.1.34) activity; an increase in IPP isomerase (EC 5.3.3.2), prenyl transferase (EC 2.5.1.1) and squalene synthetase (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 relatively 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 biosynthesis 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:cycloartenol 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 increased activities of the enzymes common to both pathways reflects the fact that the rate of accumulation of pentacyclic triterpenoid phytoalexins in elicited cultures is more rapid than the rate of phytosterol biosynthesis in control cultures. The very rapid and transient increase in HMG-CoA reductase activity points to the microsomal form(s) of this enzyme having a key regulatory role in controlling the flux of carbon into the cytosolic-microsomal pathway of terpenoid biosynthesis.