Reactive Oxygen Species and Cognate Redox Signaling System in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 295

NPR1 (for Nonexpressor of Pathogenesis Related genes 1), the transcriptional
regulatory cofactor, is activated by redox signaling in plants (Fedoroff 2006 ). In
unstressed cells, NPR1 is maintained in the cytoplasm in a large complex compris-
ing disulfi de-bonded intermolecular oligomers (Mou et al. 2003 ). NPR1 is activated
when SA accumulates in cells in response to stress signals. Upon activation, the
intermolecular disulfi de bonds are reduced, releasing monomeric NPR1. The
released NPR1 then moves into the nucleus to interact with TGA transcription fac-
tors and activate defense gene expression (Fedoroff 2006 ). Mutations of either
Cys82 or Cys216 render the protein both constitutively monomeric and nuclear and
constitutively activate expression of defense genes (Mou et al. 2003 ). It has also
been shown that an intramolecular disulfi de bond between Cys260 and Cys266 in
TGA1 prevents interaction with NPR1 (Després et al. 2003 ). Reduction of the disul-
fi de bond permits TGA1 to interact with NPR1, which in turn stimulates its DNA-
binding activity (Fedoroff 2006 ). SA promotes the reduction of NPR1 and TGA1
(Fobert and Després 2005 ), probably by producing H 2 O 2 (Torres et al. 2006 ). Along
with increase in H 2 O 2 , the transcript levels of genes encoding antioxidant proteins,
such as peroxidase and glutathione-S-transferases have increased (Joo et al. 2005 ).
The build-up of antioxidants that enhance the general cellular reducing capacity
would have reduced the NPR1 complex (Cumming et al. 2004 ). Thus redox signaling
may be involved in activation of NPR1.
The activities of NPR1 and of the TGA factors TGA1 and TGA4 have been
shown to be modulated by SA-induced oxidoreduction modifi cations of key cyste-
ine residues (Fobert and Després 2005 ). Reduction of two conserved cysteines in
NPR1 leads to its monomerization and nuclear localization, which is required for
the activation of pathogenesis-related (PR) genes. Reduction of conserved cyste-
ines in TGA1 and TGA4 enables their interaction with NPR1, which acts as a
redox-sensitive cofactor in stimulating TGA1 DNA-binding activity (Fobert and
Després 2005 ).
Metallothioneins are small cysteine-rich proteins involved in ROS scavenging
and metallothionein is down-regulated by OsRac1, a G-protein in rice (Wong et al.
2004 ). Down regulation of ROS scavengers may play an important role in redox-
mediated defense signaling (Wong et al. 2004 ). The induced redox regulators
detected in rice cells transformed with OsRac1 included glyceraldehyde-3-P dehy-
drogenase, NADPH-thioredoxin reductase, ferredoxin-NADPH reductase, NADPH
dependent oxidoreductase, quinine oxidoreductase, and glutathione-S-transferase
(GST1) (Fujiwara et al. 2006 ). These results suggest that G-proteins may be involved
in redox signaling system.
Thioredoxins are ubiquitous disulfi de reductases that regulate the redox status
of target proteins. They may act as regulators of scavenging mechanisms and as
components of signaling pathways in the plant antioxidant network (Dos Santos and
Rey 2006 ). A thioredoxin has been shown to interact with the disease resistance
protein Cf-9 and modulate Cf-9 dependent signaling (Fobert and Després 2005 ).
Redox enzymes have been shown to be involved in lignin biosynthesis (Önnerud
et al. 2002 ) and lignifi cation is one of the key defense responses in plants
(Vidhyasekaran 2007 ). Lignin may be formed by a radical polymerization initiated