References

214 E.B. Blancaflor, K.D. Chapman
NAE20:4. Although NAE16:0 does not bind to CB1 receptors, it is capable
of downregulating FAAH expression, leading to the enhancement of anandamide
effects in the cell (De Petrocellis et al. 2002). It will be interesting
to see whether a similar scenario occurs between the different NAE types
in plants.
Tobacco cell cultures exhibit a short-term alkalinization of the culture
medium upon exposure to xylanase (Felix et al. 1993). When added without
elicitors, NAE14:0 did not affect the alkalinization response, but when
added together with xylanase, the elicitor-induced alkalinization response
was inhibited. Most plant NAE types, including the mammalian NAE anandamide,
showed this inhibitory effect toward xylanase-induced alkalinization
(Tripathy et al. 1999). Moreover, NAE14:0 inhibited the alkalinization
response induced by other pathogen elicitors and the effect of added
NAE14:0 was dependent on concentration as well as the timing of addition
(Tripathy et al. 1999).
There is also evidence that NAEs participate in gene expression changes
in plants responding to pathogens. The induction of phenylalanine ammonia
lyase (PAL) gene expression, a critical enzyme in the phenylpropanoid
pathway, typically accompanies pathogen attack (Dixon et al. 2002). These
changes in PAL transcript abundance are likely related to the cell’s ability to
establish a coordinated defense response to ward off the invading pathogen.
Interestingly, PAL2 gene expression was induced by NAE14:0 in a manner
that mirrored the induction by elicitors. More importantly, NAE14:0 accumulated
10–50-fold in elicitor-treated tobacco leaves and these concentrations
were capable of activating PAL2 gene expression. On the basis of these
observations it is possible that NAE formation following elicitor treatment
is part of the signal transduction machinery that leads to plant defense
responses.
As discussed earlier, NAEs in vertebrates are perceived by transmembrane
CB receptors at the cell surface (Wilson and Nicoll 2002). A similar
mechanism for NAE perception might exist in plants to facilitate signaling
during pathogen elicitor perception. Evidence in support of this assumption
comes from the recent identification of a high-affinity NAE binding
protein in cell suspensions and microsomal membranes from a variety
of plant sources (Tripathy et al. 2003). Interestingly, CB receptor antagonists
were able to reduce NAE14:0-specific binding when included in the
assays and these antagonists diminished the NAE14:0-induced PAL2 expression
in leaves of tobacco plants as well as the NAE inhibition of the
short-term elicitor-induced alkalinization response (Tripathy et al. 2003).
Taken together these results support the concept of a CB receptor-like
NAE binding protein participating in defense signal transduction. The
successful isolation of a functional protein and the generation of knockouts/overexpressors
for this binding protein will represent an invaluable