Lepidimoide is a naturally occurring disaccharide reported to be an oligosaccharin,
i.e. to exhibit ‘hormone-like’ biological activity. It was found in cress (Lepidium
sativum) root exudates and exerts apparently allelopathic effects on neighbouring
Amaranthus seedlings. In the present study the effect of cress root exudates on
hypocotyl and root length of Amaranthus caudatus and Lactuca sativa was studied.
The seedlings of both species grown with Lepidium sativum seedlings had longer
hypocotyls and shorter roots as compared to the control. In this study I found an
active principle with biological effects similar to those of lepidimoide to be more
abundant in cress seed-coat mucilage than in root exudates. The active principle
peaked 24 hours after seed soaking, and thereafter plateaued. I also for the first time
confidently proved that the bioactive compound(s) were exuded by cress and were
not microbial digestion products or seed treatment chemicals. Quantitative tests of
cress root exudates and cress seed-coat mucilage showed the presence of hexoses,
pentoses, uronic acids and unsaturated uronic acid. The presence of unsaturated
uronic acid might be of interest because the known structure of lepidimoide includes
an unsaturated uronic acid.
Active principle from mucilage was partitioned into the aqueous phase when
shaken with ethyl acetate at pH 2, 6.5 and 12, showing it to be hydrophilic, unlike
auxins and gibberellins. The mucilage was also heated at 130°C for 48 h and severe
heating did not affect its biological activity, suggesting that if the compound is
lepidimoide then it is heat-resistant.
In an attempt to test whether the compound is of high or low Mr, the mucilage
was partitioned into 75% ethanol-precipitated and non-precipitated fractions. The
biological activity in the non-precipitated fraction was very high, and was further
separated by gel-permeation chromatography (GPC). GPC on Bio-Gel P-10 and P-2
suggested that the active principle had Mr ~500–750, compatible with
oligosaccharide(s), suggesting that a particular oligosaccharide may be the active
principle. TLC separation of bioactive fractions from P-2 showed that the bioactive
compound migrated between GalA and Gal but co-migrated with sucrose; however,
paper chromatography separation proved that the compound is not sucrose and might
be a different disaccharide (lepidimoide).
From the structure of lepidimoide, Fry et al. (1993) proposed that lepidimoide
is formed by the lyase-catalysed cleavage of a pectic polysaccharide,
rhamnogalacturonan-I (RG-I). So I tried to prepare lepidimoide or lepidimoide-like
compounds by the action of RG-I lyase from Pichia pastoris on purified potato RG-I.
The lyase showed its activity but the digest did not demonstrate biological activity,
which might be due to presence of tris-HCl buffer in the solution. An attempt was
also made to prepare lepidimoide by methyl esterification and -elimination of
purified potato RG-I but again the product did not show any biological activity,
which might be due to presence of borate buffer in the solution. This part of research
might be useful for future work on preparation of lepidimoide and lepidimoide-like
compounds
