Agriculture Reference
In-Depth Information
In both animal and plant cells, cyclic ADP ribose (cADPR) is another
molecule that serves as a second messenger for NO signaling. In both
systems, cADPR functions as a second messenger to stimulate Ca
2+
re-
lease through intracellular Ca
2+
-permeable ryanodine receptor channels.
cADPR was shown to induce
PAL
and
PR-1
gene expression in tobacco,
and this induction was blocked by a cADPR-gated Ca
2+
channel inhibitor
(Durner and Klessig 1999). Moreover, a cADPR antagonist such as 8-Br-
cADPR partially suppressed NO-dependent induction of
PR-1
transcripts,
suggesting that NO activation of the defense response may occur through
more than one pathway (Delledonne et al. 2003). Since the expression of
PR-1
and
PAL
genes was increased when cGMP and cADPR were added
simultaneously, these two second messengers appear to act synergistically
to increase gene expression. Recently, Lamotte et al. have shown in tobacco
cells that NO participates in the cryptogein-mediated elevation of cytosolic
free Ca
2+
through the mobilization of Ca
2+
from internal stores. In addition,
cryptogein-mediated cytosolic Ca
2+
elevation was shown to be partially in-
hibited at low concentrations of an antagonist of RYR channels (Lamotte et
al. 2004). These results suggest that NO contributes to the release of Ca
2+
from internal pools.
It is now recognized that NO and its related species can introduce post-
translational modification of proteins. These modifications are due to the
high reactivity of NO with the thiol groups present in reactive amino acids
such as cysteine and tyrosine as well as the transition metal centers of
a wide functional spectrum of proteins (Stamler et al. 2001). Tyrosine
nitration and methionine oxidation can introduce irreversibile modifica-
tion of proteins, leading to loss of function, while cysteine nitrosylation is
a reversibile modification that can modulate protein functions (Sokolovski
and Blatt 2004). These modifications are both reversible and specific, al-
lowingcellstoflexiblyandpreciselyalterproteinfunctioninresponseto
environmental signals (Mannick and Schonhoff 2002).
8.7
Systemic Acquired Resistance and NO
SAR is activated after pathogen infection and leads to the induction of the
plant defense response in uninfected parts of the plant. As a result, the
entire plant is more resistant to a secondary infection. Salicylic acid plays
an important role during incompatible interactions for the amplifications
of early signals deriving from plant-pathogen recognition (Shirasu et al.
1997). Exogenous application of salicylic acid has been found to mimic
SARandinducesthetranscriptionof
PR
genes. Several lines of evidence
highlight the role of NO in the modulation of signaling leading to SAR,
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