Allelochemicals Biologica... - Name

164
MARK A. BERNARDS, LINA F. YOUSEF, ROBERT W. NICOL
can undergo subsequent 1,2-hydride and/or 1,2 alkyl shifts and further annulation.
Loss of a proton yields pentacyclic saponin precursors such as α- or β- amyrin or
lupeol.
In ginseng, however, only one relatively low abundant ginsenoside (Ro) has a
pentacyclic parent structure similar to most other non-ginseng triterpenoid saponins.
Instead, most ginsenosides are based on the (20S)-protopanaxadiol and (20S)protopanaxatriol
parent carbon skeletons derived from the dammarenediol structure
that results from the quenching of the dammarenyl cation by water. This reaction
(i.e., chair-chair-chair-boat template folding, concerted annulation and final cation
quenching) is hypothesized to be catalysed by dammarenediol synthase (Dewick, 1997;
Haralampidis et al., 2002), although this has not been proven. Other enzymes involved
in triterpenoid biosynthesis have been identified in Asian ginseng (P. ginseng) including
an oxidosqualene cyclase (â-amyrin synthase, Kushiro et al., 1998) as well as other
candidate oxidosqualene cyclases and enzymes involved in modification of the
sapogenin (Jung et al., 2003). Further hydroxylation reactions to yield the (20S)protopanaxadiol
and (20S)-protopanaxatriol parent carbon skeletons from 3,20dihydroxydammarene
have not been described. However, two routes are theoretically
possible (Figure 2). In the first, 3,20-dihydroxydammarene is hydroxylated at C-12 to
yield the (20S)-protopanaxadiol skeleton directly. Subsequent hydroxylation at C-6
yields the (20S)-protopanaxatriol skeleton. Alternatively, the (20S)-protopanaxatriol
skeleton could arise independent of the (20S)-protopanaxadiol skeleton via
hydroxylation at C-6 first, followed by C-12 hydroxylation. In either case, the
assumption is made that the parent carbon skeletons are assembled first, before
glycosylation. No details are yet available with respect to the glycosylation of (20S)protopanaxadiol
and (20S)-protopanaxatriol parent carbon skeletons. Clearly
glycosylation of the unique C-20 hydroxyl group, which is glycosylated in all
ginsenosides except two (20S)-protopanaxadiols (Rh 2 , Rg 3 ) and Ro, represents a novel
glycosyl transferase activity, as does glycosylation of C-6 of the (20S)-protopanaxatriols.
4. GINSENOSIDE FUNGITOXICITY IN VITRO
4.1. Differential Response of Fungi and Oomycetes to the Addition of Ginsenosides
to Culture Media
In commercial gardens, ginseng is attacked by the foliar pathogens Alternaria panax,
A. alternata, and Botrytis cinerea, and the root pathogens Cylindrocarpon destructans,
Rhizoctonia solani, P. cactorum, Py. irregulare, Py. ultimum and several species of
Fusarium including F. oxysporum and F. solani (Brammall, 1994a, b; Reeleder and
Brammall, 1995; Punja, 1997). Although the pathogens in the Pythiaceae (i.e.,
Phytophthora cactorum, Pythium irregulare, Py. ultimum) are superficially similar,
and share the same nutritional mode as the other pathogens, they are not true fungi,
but rather belong in the kingdom Straminipila (Burnett, 2003). Most other ginseng