Physiology and Molecular Biology of Stress ... - KHAM PHA MOI

Photooxidative Stress
177
in lower concentrations, ROS are involved in cell signaling, acclimation and crosstolerance
(Dat et al., 2000; Neill et al., 2002). Of all the ROS molecules, H 2
O 2
is thought to
freely diffuse across biological membranes (Kripinski et al., 1999). Chloroplast-derived
H 2
O 2
could directly influence the functions of extra-plastidial signaling components
which play a potential role in the systemic responses of plants to excess light. H 2
O 2
plays a major role in triggering the expression of antioxidant enzymes (SOD, CAT, APX,
GR) for photoprotection (Gechev et al., 2002; Neill et al., 2002). H 2
O 2
is relatively stable
ROS, unchanged at physiological pH and move out of the sites of its generation propagating
throughout the plant, specially interacting with components of redox-signaling
pathways in plants (Noctor et al., 2002). When low light grown plants were irradiated
with high light, a burst of H 2
O 2
and photoinhibition occurred. H 2
O 2
is known to act as
intracellular signal coming out of the chloroplast into cytosol, inducing a second line of
defense and counteracting the photoinhibition (Mulineaux and Karpinski, 2002). Hence
H 2
O 2
is considered as an intra and intercellular as well as interorgan systemic signal
which is involved in the acclimation of plants to high growth irradiances. The apoplastic
enzyme ascorbate oxidase also regulates the redox state of the apoplastic ascorbate
pool (Pignocchi and Foyer., 2003). The function of ascorbate oxidase is to modify the
apoplastic redox state in such a way as to modify receptor activity and signal transduction
to regulate photooxidative stress. ROS-mediated signaling mechanisms described
above depend on the passage of ROS molecules out of the chloroplast to propagate a
signal leading to nuclear gene expression. It is thus believed that electron transport
chain starts with NADPH in the stroma, spans the chloroplast envelope and ends with
O 2
as the terminal electron acceptor on the outer surface of the chloroplast. Chloroplast
envelope is known to contain several constituents including iron-sulphur proteins,
semiquinones, flavins and α-tocopherol that are involved in the transfer of electrons
and therefore could provide another exit for a chloroplast-derived signal (Jager-Vottero
et al., 1997).
Recently, mitogen-actiavted protein (MAP) kinases are known to be involved
in the signal transduction pathway of plants that senses ROS. At MAPK 3/6 and NL-p
46 MAPK and NtANP1, NtNPK1 have been implicated in ROS signaling in Arabidopsis
and tobacco (Mittler, 2002). When H 2
O 2
is sensed by sensors, calmodulin and MAP
kinase cascade would be activated, resulting in activation or suppression of transcription
factors, thus regulating the response of plants to oxidative stress. The involvement
of ROS in the regulation of stomatal closure and auxin-related cellular responses
also suggest that more signaling pathways might be involving ROS as inducers of
systemic signals in plants under conditions of oxidative stress. The use of transgenic
plants with a comprehensive analysis of ROS-producing and ROS-scavenging systems
by using genomics and proteomics should unravel the further role of ROS in signal
transduction in plants.