Nitric Oxide Signaling System in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 318

both as NO reservoir and NO donor (Lindermayr et al. 2005 ) or independently of
homolytic cleavage to NO (Gaston 1999 ). GSNO acts synergistically with SA in
SAR (Espunya et al. 2012 ). It has been suggested that GSNO would act as a
long- distance phloematic signal in SAR (Durner and Klessig 1999 ).
GSNO may be broken down by GSNO reductase (GSNOR) (Liu et al. 2001 ;
Malik et al. 2011 ). GSNOR reduces GSNO, an essential reservoir for NO activity
(Wünsche et al. 2011 ). It is the main enzyme responsible for the in vivo control of
intracellular levels of GSNO (Espunya et al. 2012 ). GSNOR controls not only the
cellular levels of GSNO but also the levels of S -nitrosylated proteins (Grennan
2007 ). NO bioactivity is controlled by NO synthesis by the different routes and by
NO degradation, which is mainly performed by the GSNOR (Liu et al. 2004 ).
Mutation of the gene AtGSNOR1 in Arabidopsis controls cellular S-nitrosothiols
during plant-pathogen interactions (Feechan et al. 2005 ). GSNOR is encoded by a
single-copy gene in Arabidopsis thaliana (Sakomoto et al. 2002 ).
GSNOR has been shown to play a role in plant defense response (Rustérucci et al.
2007 ). Transgenic Arabidopsis plants with decreased amounts of GSNOR (using
antisense strategy) show enhanced basal resistance against Peronopora parasitica ,
which correlates with higher levels of intracellular SNOs and constitutive activation
of PR-1 gene (Rustérucci et al. 2007 ). SNOs also play important role in systemic
acquired resistance (SAR). SAR is impaired in plants overexpressing GSNOR and
enhanced in the antisense plants, and this correlated with changes in the S-nitrosothiol
content both in local and systemic leaves. The loss of AtGSNOR1 function compro-
mises defense responses in A. thaliana (Feechan et al. 2005 ). GSNOR was found to
be localized in the phloem, suggesting that GSNOR would regulate SAR signal
transport through the vascular system (Rustérucci et al. 2007 ). A reduction in NO
accumulation leads to pathogen susceptibility (Delledonne et al. 1998 ; Zeidler et al.
2004 ), a decrease in SNOs promotes protection against microbial infection (Feechan
et al. 2005 ). Collectively these results show that GSNOR controls SNO in vivo levels
and the SNO content positively regulates plant defense responses.
NO mediates the S-nitrosylation of peroxiredoxin II E (PrxII E), a member of the
peroxiredoxin family consisting of peroxidases that reduce H 2 O 2 and alkyl hydro-
peroxides to H 2 O and the corresponding alcohol using equivalents from thioredoxin
or glutaredoxins (Dietz 2005 ; Horling et al. 2003 ). During H 2 O 2 reduction, the cata-
lytic Cys residues of peroxiredoxins undergo oxidation and must be reduced by
electron donors such as thioredoxins, glutaredoxins or cyclophins before the next
catalytic cycle (Horling et al. 2003 ). S-nitrosylation severely inhibits the peroxidase
activity of PrxII E, thus revealing a novel regulation mode for peroxiredoxins
(Romero-Puertas et al. 2007 ). PrxII E possesses peroxynitrite reductase activity and
S-nitrosylation inhibits this activity (Romero-Puertas et al. 2007 ).
Peroxynitrite (ONOO − ) is a toxic reactive nitrogen species generated by the
interaction of ROS and NO during oxidative burst and PrxII E detoxifi es
ONOO − (Romero-Puertas et al. 2007 ). S-Nitrosylation inhibits the peroxynitrite
detoxifi cation activity of PrxII E (Romero-Puertas et al. 2007 ). GSNO was found to
severely inhibit PrxII E peroxidase activity in a concentration-dependent manner.
This effect could be reversed by the thiol-specifi c reductant DTT, indicating that