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of PC reaction is catalysed by glutamyl-cysteine synthetase (Piechalak et al.
2002
).
PC-metal complex regulates the PC synthase (Cobbett
2000
). In many plants
PC synthase is present in the cytoplasm and also in the roots of
Pisum sativum
(Klapheck et al.
1995
) metal ion is most likely to be activated although de novo
synthesis may occur (Cobbett
2000
).
OxidativeStressandAntioxidativeSysteminPlants
Oxidative stress is an essential regulated process, as the equilibrium between the
oxidative and antioxidative capacities determines the fate of the plant. The antioxi-
dant defence system provides sufficient protection against active oxygen and free
radicals under non-stressful conditions. Natural and anthropogenic both stresses
provoke the high production of toxic oxygen derivatives. In this condition, the re-
sponse of the capacity of the antioxidative defence system is increased, but in many
situations the response is moderate. In addition, important sites like reaction center
and apoplastic space have very little protection against this oxidative damage.
About two billion years ago, ROS have been the unwanted companions of aero-
bic metabolism. Apart from molecular oxygen (O
2
), partially reduced or activated
derivatives of oxygen (O
2
-1
, H
2
O
2
and HO) are highly reactive and toxic and cause
the oxidative destruction of cells. This results that evolution of all aerobic organism
has been dependent on the development of efficient ROS scavenging mechanisms.
Recently new rate for ROS was known like the control and regulation of biological
processes such as cell death, stress responses, hormonal signaling and development.
Therefore the understandings of ROS suggest its dual role in plant biology:
• Toxic products of aerobic metabolism
• Key regulators of metabolic and defense
TheROSCycle
Rate of ROS in different cellular compartments is determined by the relationship be-
tween multiple ROS-producing pathways and ROS scavenging mechanisms. ROS
signal transduction pathways control these mechanisms and form the 'basic ROS
cycle'. This pathway monitors the level of ROS, produced by aerobic metabolism,
and controls the expression and ROS-scavenging pathways during normal growth
and development. The ROS cycle also performs good metabolic timing e.g. Photo-
synthesis control to reduce the production rate of ROI. The ROS source in plants be-
longs to the aerobic metabolic reactions, such as photosynthesis and respiration and
others belong to pathways enhanced during abiotic stresses like photorespiration.
Recently, NADPH oxidase, amino-oxidases and cell wall bound peroxidases were
identified to be the new sources of ROS in plants. They participate and are tightly
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