Reactive Oxygen Species and Cognate Redox Signaling System in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 294

specifi cally modifi ed by a reactive oxygen species through a reaction that is
chemically reversible under physiological conditions and/or enzymatically cata-
lyzed (Forman et al. 2004 ). Signaling through the redox active molecule H 2 O 2 is
important in inducing plant defense responses (Desikan et al. 2005 ).
For proteins to perceive the presence of ROS and to act as signal intermediaries
there should be a direct chemical interaction which leads to signal propagation
(Hancock et al. 2006 ). It is suggested that the ROS sensors must have some specifi c
characteristic that enables them to propagate this signal (Hancock et al. 2006 ).
Modifi cation of thiol residues in the proteins may be the key component in the ROS
signaling system (Cooper et al. 2002 ; Vranová et al. 2002 ; Foyer and Noctor 2005 ).
If there are two cysteine residues in the target protein involved in the reaction, a
disulphide bridge (S-S) may be formed. However, if there is only one cysteine, the
-SH group may be oxidized to varying degrees. The thiol group may be oxidized to
sulphenic acid (−SOH), and this may be further oxidized to sulphinic acid (−SO 2 H)
or sulphonic acid (−SO 3 H). Since, any oxidation of the thiol is dependent on its
mid- point redox potential and its availability to the oxidant, only a low proportion
of the -SH groups within any protein will be able to modifi ed in these ways. Thiols
in different proteins have different mid-point potentials, and hence the proteins will
be differentially controlled by fl uctuations in the intracellular redox state. Some
proteins may be regulated earlier, or later than others as the redox state becomes
more oxidized (Hancock et al. 2006 ).
These oxidation states of the -SH group within cysteine may be restored by
re-reduction. Thioredoxins (Schürmann and Jacquot 2000 ) and glutaredoxins
(Lemaire 2004 ) may act as protein disulphide reductases as well as re-oxidizing
-SOH groups (Collin et al. 2004 ). Sulphinic acid groups may be reduced back to
the sulphenic group by sulphiredoxins. The sulphenic acid group created may
then be reduced further by thioredoxins or glutaredoxins to regenerate the thiol,
−SH (Hancock et al. 2006 ). These observations suggest that there are redox groups
within proteins that can potentially toggle between oxidation and reduction states
in a rapid and ROS dose-dependent manner, and in doing so the structures of the
proteins will be altered and such proteins may partake in H 2 O 2 -mediated signaling
(Hancock et al. 2006 ).
It has been suggested that It has been suggested that H 2 O 2 signaling can activate
responses such as gene expression and reversible protein phosphorylation through
oxidative modifi cation of reactive Cys residues within proteins (Danon 2002 ). It has
been suggested that H 2 O 2 signaling can activate responses such as gene expression
and reversible protein phosphorylation through oxidative modifi cation of reactive
Cys residues within proteins (Danon 2002 ). It has been suggested that H 2 O 2 signal-
ing can activate responses such as gene expression and reversible protein phos-
phorylation through oxidative modifi cation of reactive Cys residues within proteins
(Danon 2002 ). Phosphatases contain readily oxidizable active site cysteine residues
(Stone 2004 ). Since phosphatases are involved in regulation of protein kinases,
redox regulation of phosphatase activity can, in turn, regulate the activity of its tar-
get protein kinases (Tonks 2005 ). Some protein kinases are directly redox regulated
by thioredoxins and peroxiredoxins (Veal et al. 2004 ; Fedoroff 2006 ).