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

160 A.R. Reddy and A.S. Raghavendra
thesis, are also the potential sources of singlet oxygen ( 1 O 2
) production. These reactive
singlet oxygen molecules are generated by an input of energy by removing the spin
restriction and therefore increasing the oxidizing ability of oxygen (Knox and Dodge,
1985; Niyogi, 1999). The half life time of 1 O 2
is about 200ns in plant cells (Gorman and
Rodgers, 1992). 1 O 2
is known to react with DI protein, thus damaging PSII (Trebst et al.,
2003). Keren et al. (2000) measured the degree of photoinactivation and loss of DI
protein by using series of single turnover flashes. The highly reactive 1 O 2
is also reported
to have a strongly deleterious effect on chloroplast pigment-protein complexes,
as it is generated in the pigment bed (Slooten et al., 1998; Niyogi, 1999). However, the DI
damage is also regarded as physiological defense mechanism as the damaged DI protein
is efficiently replaced by newly synthesized DI (Prasil et al., 1992; Aro et al., 1993).
Suh et al. (2000) showed the production of 1 O 2
in illuminated cytochrome b 6
f complex by
using spin trapping techniques. However, the role of cytochrome b 6
f complex-generated
1 O 2
is still not completely understood. However, it is now known that chlorophyll
sensitizers act as main source of reactive oxygen species and in case the chlorophyll is
activated by energy transfer under high light conditions, 1 O 2
production is increased
(Hippeli et al., 1999).
4.2. Photooxidation-Induced Free Radical Production in Plant Cells
High irradiance produces fluxes of dioxygen and excess electrons leading to overreduction
of electron transport chain (ETC), which might result increased formation of
several free radicals, commonly referred as reactive oxygen species. Thus, high lightdriven
photosynthetic processes are main contributors to chloroplastic-ROS production
in plants. Highly active ETC in chloroplasts under excess growth light operate in an
O 2
-rich environment and leakage of the excess electrons leads to the formation of ROS
(Edreva, 2005a). Unlike the formation of 1 O 2
, chemical activation is the other mechanism
to circumvent spin restriction through univalent reduction of dioxygen which results at
least three intermediates namely superoxide (O 2?¯), hydrogen peroxide (H 2
O 2
) and the
hydroxyl radical (OH . ) (Figure 1). It is also known that these ROS colliding with an
organic molecule may get an electron, rendering it a radical capable of propagating a
chain reaction by forming peroxyl (ROO . ) and alkoxyl (RO . ) radicals (Perl-Treves and
Peri, 2002). Excess electrons from ETC will be derived from ferridoxin to O 2
. In addition,
leakage of electrons to O 2
may also occur from 2Fe-2s and 4Fe-4s clusters of PSI. It is
now well established that Q A
and Q B
sites of PSII are also potential sources of O 2?¯
generation (Dat et al., 2000; Zhang et al., 2003). The addition of an electron to molecular
oxygen by photosynthetic ETC produces O 2?¯ and this reaction is termed as Mehler
reaction (Mehler, 1951). The electron transfer to oxygen will be more at the chloroplast
under high light stress because of high O 2
levels occurring at that site, favouring
markedly high levels of O 2?¯ and 1 O 2
. We will now concentrate on the fate of this