Dc. Fulton et al., ENZYMOLOGICAL ASPECTS OF THE REDIRECTION OF TERPENOID BIOSYNTHESIS INELICITOR-TREATED CULTURES OF TABERNAEMONTANA-DIVARICATA, Phytochemistry, 35(5), 1994, pp. 1183-1186
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.