Allelochemicals Biologica... - Name

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Figure 3 middle trace). Therefore, the differential response of these two organisms to
the presence of ginsenosides in their growth medium is coincident with differences in
the profile of ginsenosides that can be recovered from their spent medium.
Interestingly, preliminary observations also show that the recovery of ginsenosides
from the spent broth of Py. irregulare is dependent on the presence of sucrose in the
original culture medium, since significantly smaller amounts of ginsenosides were
recovered from a spent broth lacking sucrose (Yousef and Bernards, unpublished
data). This observation implies that Py. irregulare is using ginsenosides as a source
for carbon. However, the additional observation that mineral broth supplemented with
a concentration of glucose equimolar to that expected to be released into the medium
by ginsenoside de-glycosylation, does not support the same degree of growth increase
in Py. irregulare biomass observed when the same broth is supplemented with
ginsenosides (Yousef and Bernards, unpublished data), argues against a simple carbon
source mechanism. Since Pythium spp. have been reported to incorporate sterols into
their membranes (Olsen, 1973a) and because sterols are known to mediate growth
(Nes, 1987), we now hypothesize that Py. irregulare secretes saponinases to (partially)
deglycosylate ginsenosides, the latter of which are then incorporated as “sterol”
triterpenoids into its membrane. It is further assumed that under the experimental
conditions employed, this incorporation favours growth.
5.1. RETS and Soil Analysis
MARK A. BERNARDS, LINA F. YOUSEF, ROBERT W. NICOL
5. GINSENOSIDES IN THE RHIZOSPHERE
When the soil associated with three-year-old ginseng roots was extracted and analysed
by HPLC, six major ginsenosides were tentatively identified by co-elution with
standards. While much of the soil chemical HPLC profile remains unidentified, HPLC-
MS analysis confirmed the presence of the six major ginsenosides (Rb 1 , Rb 2 , Rc, Rd,
Re and Rg 1 ) plus pseudoginsenoside F 11 and another protopanaxadiol ginsenoside in
the soil extracts (Nicol et al., 2003). The amount of ginsenosides as percent weight of
dry soil was calculated to range from 0.02% to 0.098% (average 0.06%).
In order to confirm that ginsenosides were present in the exudate of intact ginseng
roots, (and not isolated from residual root tissue in our soil preparations) root exudates
were collected from pot-grown ginseng plants using a root exudate trapping system,
or RETS (Tang and Young, 1982). HPLC analysis of the trapped exudate revealed
the presence of peaks that had the same retention times as the ginsenoside standards.
These peaks were not present in the exudate collected from control pots (no ginseng
plants) and were taken as evidence of ginsenosides in the exudate of pots containing
ginseng plants. HPLC-MS analysis of the trapped exudate confirmed the presence of
the same suite of ginsenosides as found in the soil (Nicol et al., 2003). After
quantification of the ginsenoside content of the root exudates, the individual ginseng
plants were determined to be losing approximately 25 µg of ginsenosides per day
(i.e., amount of recovered ginsenoside / number of plants / number of days the
experiment ran).