Recent Advances and Future Prospects on Practical Use of Salicylic Acid - Salicylic Acid: Plant Growth and Development - page 360

suggested that NO is involved in both SA biosynthesis and action. It has been also
shown that SA induced the production of ROS, such as H 2 O 2 and NO (van Camp
et al. 1998 ). In addition, SA may mediate and/or potentiate NO' effects by altering
the activity of various NO-regulated enzymes.
The global picture of ROS-NO-SA interactions is far from being complete, but
it already has been revealed as a fascinating cross talk of mechanisms able to fine-
tune resistance responses and other plant reactions to environmental stimuli, as
well as important developmental aspects in the life of the plant. This dual role of
NO can be accomplished by a signal transduction pathway through a signaling
cascade. Involvement of another signaling molecules, such as salicylic acid, jas-
monic acid, abscisic acid, ethylene and Ca 2+ activate a very complex network

(Beligni and Lamatina 2001 ).
It should also be considered that the SA-induced NO production is probably
controlled by multiple mechanisms, as suggested by the experiments on calcium
signalling. There is increasing evidence of the existence of cross talk between NO
and calcium signalling systems in plants (Lamotte et al. 2004 ). It has been proved
that both ROS and calcium signals are intimately interconnected. How this cross
talk can finally modulate the translocation and/or the activity of nuclear proteins
leading to the control of specific genes is still unsolved problem (Mazars et al.
2010 ).
Wang and Wu ( 2005 ) reported that exogenously supplied MeJA at 100 lM
induced rapid production of NO in Taxus cell cultures, reaching a maximum
within 6 h of MeJA supply. Several other responses were documented, like the
production of H 2 O 2 , and the increases in intracellular malondialdehyde (MDA)
content, lipoxygenase (LOX) and phenylalanine ammonium-lyase (PAL) activi-
ties. The MeJA-induced H 2 O 2 production was suppressed by an NO donor, sodium
nitro-prusside (SNP), but enhanced by NO inhibitors, N'.OMEGA.'-nitro- L -argi-
nine (L-NNA) and 2-phenyl-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide
(PTIO). In contrast, the MeJA-induced MDA, LOX and PAL were all enhanced by
the NO donor but suppressed by the NO inhibitors. According to the authors, the
results are suggestive of a role for NO as a signal element for activating the MeJA-
induced defense responses and secondary metabolism activities of plant cells.
Supplying young excised tomato plants with the NO generators SNP and SNAP
before wounding caused a nearly complete inhibition of the induction of synthesis
of proteinase Inh I, one of several wound-inducible proteinase inhibitor proteins in
tomato leaves. NO also blocked the H 2 O 2 production and proteinase inhibitor
synthesis that was induced by systemin, oligouronides, and JA. Although the
expression of proteinase inhibitor genes in response to JA was inhibited by NO but
not that of wound signaling-associated genes was not. The inhibition of wound-
inducible H 2 O 2 generation and proteinase inhibitor gene expression by NO was not
due to an increase in SA, which is known to inhibit the octadecanoid pathway.
Instead, NO appears to be interacting directly with the signaling pathway down-
stream from JA synthesis, upstream of H 2 O 2 synthesis. The authors suggested that
NO may have a role in down-regulating the expression of wound-inducible
defense genes during pathogenesis (Orozco-Gardenas and Rayan 2002 ).