Alkylresorcinols: Chemical Properties, Methods of Analysis and Potential Uses in Food, Industry and Plant Protection - Natural Antioxidants and Biocides from Wild Medicinal Plants - page 158

bilayer fluidity, as shown for phospholipase
A (Gordeev et al ., 1991). The ARs also inhib-
ited the activity of various dehydrogenase-
type enzymes through interference with
proton transport between co-enzyme and
substrate/product, as occurs with NADH-
dependent enzymes associated with respira-
tion and photosynthesis (Nenashev et al .,
1989). In the latter case, the phenolic nature
of resorcinolic lipids suggests that they may
replace compounds such as ubiquinone or
plastoquinone in mediating processes of elec-
tron transport (Rejman and Kozubek, 1997).

the absence of a host. A resorcinolic lipid
derivative, xenognosin, isolated from roots
of the host plant was able to promote Striga
germination (Kato et al ., 1985). Further
studies showed that its methylated form
enhances the xenognosin-dependent germi-
nation (Orabi et al. , 1991). Another example
is root exudates from rice. Allelopathic
accessions produce higher levels of 5-n-AR
glycosides than non-allelopathic acces-
sions. Nevertheless, it is not clear if these
compounds exert their action on other
plants or regulate microbial populations in
the rice rhizosphere. In this regard, the pos-
sible effect of ARs exuded from rice roots on
soil bacteria was investigated using a set of
biosensors that allows the monitoring of
bacterial gene induction in real time (Miché
et al ., 2003). This set consists of plasmids
where the promoters of genes belonging to
the predominant stress responsive networks
are fused to the bioluminescence genes
luxCDABE of the marine bacterium Vibrio
fischeri (Meighen and Dunlap, 1993).
Members of this set of plasmids have
been described and proved to respond spe-
cifically to environmental-stress-inducing
agents (Belkin et al ., 1997). It was shown
that rice exudates actually exert a general
stress on Escherichia coli sensor strains,
inducing networks responding to protein
damage and oxidative stresses but not to
DNA damage. Moreover, the effects of
alk(en)ylresorcinols extractable from the
root surface of rice seedlings (Bouillant
et al ., 1994) were comparable with whole
exudates, suggesting that they may partici-
pate in the plant-induced selective pressure
on microbial communities in vivo. It is
important to note that the responses induced
would be deleterious effects of ARs exerted
on living membranes more than interactions
of these molecules with specific membrane
receptors. Other works also suggest that ARs
are chemical signals in microbial interac-
tions and the microbial producer would
release ARs to the environment. In this case
ARs would be autoinducers or autoregula-
tors because the own microbial producer
can respond to them. Accumulation of ARs
up to a threshold concentration would
reflect population density, which leads to a

10.7.3 Effects on cell membranes

The ARs can easily be incorporated into the
lipid bilayer of biological membranes where
they induce a range of structural changes. In
a model system ARs can affect phospholipid
membranes differently depending on the
way that they are added (Siwko et al ., 2009).
The addition of ARs before the formation of
phospholipid vesicles stabilizes phospholi-
pid bilayer. In this situation the alkyl
moiety would increase the order of lipid
chains. Then, the membrane thickens, the
interface dehydrates and the membrane
becomes less permeable to water and solutes
such as ions and glucose. In contrast, ARs
disturb membranes, increasing the release of
soluble markers when added to a suspen-
sion of already formed liposomes (Kozubek,
1987). In such a case ARs would generate
transient water pores with an increase in
leakiness. Longer alkyl chains and more
unsaturated bonds enhance the dual effect
of resorcinols on biological membranes.

10.7.4 Effects as chemical signals on
plants and microorganisms

ARs may be chemical signals involved in
plant-plant interactions. The relationship
between maize, sorghum and the parasitic
plant Striga asiatica (scrophulariaceae) is
an example. The seeds of S. asiatica require
a germination stimulus and, once germinated,
Striga survives for less than 2 weeks in