Nitric Oxide Involvement in Incompatible Plant–Pathogen Interactions - Communication in Plants: Neuronal Aspects of Plant Life - page 118

although its activity is fully dependent on the functions of salicylic acid.
In tobacco, NO treatment has been shown to result in the accumulation
of salicylic acid and its conjugates (Durner et al. 1998). PR-1 induction by
NO is mediated by salicylic acid as it is blocked in NahG transgenic plants
that are unable to accumulate salicylic acid (Durner et al. 1998). Moreover,
treatment of tobacco plants with NOS inhibitors or NO scavengers partially
inhibits salicylic acid induced SAR (Song and Goodman 2001).
Although salicylic acid moves through the plant, it is not an essen-
tial signal that activates SAR (Mauch-Mani and Métraux 1998). Several
molecules, such as short peptides and selected lipids and lipid derivatives,
have been suggested to be putative short- or long-distance signals medi-
ating the development of a variety of defense mechanisms. Yet another
candidate for mobile signals is GSNO (Durner and Klessig 1999). GSNO is
a major metabolite in phloem that is distributed throughout the plant and
is believed to act as both an intracellular and an intercellular NO carrier. In
mammals NO has been shown to react with glutathione GSH to form GSNO,
which can serve as a systemic source of NO. In addition, GSNO has been
shown to induce systemic resistance against tobacco mosaic virus infection
(Song and Goodman 2001), and PAL expression in tobacco (Durner et al.
1998). A GSNO-catabolizing enzyme (glutathione dependent formaldehyde
dehydrogenase, GS-FDH) and its encoding gene have recently been charac-
terized. Mutant yeasts that lack this gene showed enhanced susceptibility
to nitrosative challenge, indicating an important biological role for this en-
zyme. The identification of the GS-FDH gene both in pea and Arabidopsis
suggests the ability of the plant to modulate GSNO bioactivity and signaling
functions. (Neill et al. 2003).
8.8
Conclusions and Future Prospects
NO is a gas with a broad chemistry that involves an array of interrelated re-
dox forms with different chemical reactivities. NO was named “Molecule of
the Year” in 1992 by the journal Science, a Nitric Oxide Society was founded,
and a scientific journal devoted entirely to NO was created. A number of re-
cent publications have evidenced the broad spectrum of cellular functions
modulated by NO in plants. NO takes part in the regulation of several phys-
iological processes, but the molecular mechanisms by which NO operates
are still largely unknown. A great effort is now needed for the identification
and characterization of the direct targets of NO. The understanding of NO
signaling functions at the biochemical, cellular and molecular levels will
soon make it possible to discern several important physiological and patho-
logical processes of plants, as has already been demonstrated in mammals.