References

14 N-Acylethanolamines in Plants 213
and Moore 1993). The NAPE synthase enzyme has been purified to homogeneity
from cottonseed microsomal membranes, but no cDNA sequences
have been identified yet that encode this enzyme. Because this enzyme utilizes
FFAs and PE, two membrane bilayer-destabilizing lipids, as substrates,
and synthesizes NAPE, a bilayer-stabilizing lipid (Schmid et al. 1990), this
might be a mechanism for scavenging FFAs and protecting membrane integrity
during plant cell stress. In fact work in potato cells suggests that
this mechanism may operate in vivo wherein FFAs accumulate in response
to hypoxic stress, followed by a rapid elevation of NAPE (Rawyler and
Braendle 2001).
Plants and animals ultimately synthesize NAPE using the same metabolites
(FFA and PE), but have developed different enzymatic machinery to
accomplish this synthesis. These metabolic differences might relate to different
mechanisms of regulation of overall NAE metabolism in plants and
animals. Reconciliation of these proposed pathways and a better understanding
of the role of NAPE biosynthesis in NAE signaling will be greatly
enhanced by the molecular identification of DNA sequences encoding the
enzymesinvolvedinthispathway.
14.4
Prospective Functions of NAE in Plants
14.4.1
NAEs in Plant Defense Responses
As noted already, the accumulation of NAEs under neurodegenerative conditions
led to the proposal that NAEs might have neuroprotective roles
in animals (Fowler 2003). Evidence supporting the notion that plants and
animals might share common NAE signaling pathways comes from the
observationthatplantcellsalsoaccumulateNAEsinresponsetostress
(Chapman et al. 1998). For instance, the fungal elicitor xylanase stimulated
tobacco suspension cells to release NAEs into the culture medium. However,
in contrast to mammalian cells, which released mostly long-chain,
saturated and unsaturated acyl chain NAEs (NAE16:0 and NAE20:4; Berger
et al. 2004), the NAEs that accumulated following fungal elicitation were
of the shorter acyl chain types (NAE12:0 and N-myristoyethanolamine,
NAE14:0; Chapman et al. 1998). This could be reflective of differences in
the physiological effects and/or targets of these NAE types in plants and
animals. In mammalian systems, the action of one NAE type modulates
the activity of another NAE species by prolonging its signaling activity
(Fowler 2003). This “entourage” effect is best illustrated with NAE16:0 and