Physiology and Molecular Biology of Stress ... - KHAM PHA MOI
Physiology and Molecular Biology of Stress ... - KHAM PHA MOI
Physiology and Molecular Biology of Stress ... - KHAM PHA MOI
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20 A. Yokota, K. Takahara <strong>and</strong> K. Akashi<br />
ciency <strong>of</strong> PSII decreases greatly, but that <strong>of</strong> PSI is not influenced (Golding <strong>and</strong> Johnson,<br />
2003). This observation supports the suggestion that electrons continue to flow in PSI<br />
cyclic electron transport chains under such conditions. This mechanism could relieve<br />
hyper-excitation <strong>of</strong> the PSI complex <strong>and</strong> the flow <strong>of</strong> electrons to oxygen (Miyake et al.,<br />
2005), although it has also been suggested that the oxidized form <strong>of</strong> P700 (P700 + ) participates<br />
in quenching PSI over-excitation (Owens, 1996; Ort, 2001).<br />
The occurrence <strong>of</strong> a reduction in oxygen in PSI under moderate conditions is<br />
under debate; however, repressed expression <strong>of</strong> ascorbate peroxidase <strong>and</strong> a mutation in<br />
the ascorbate-synthetic pathway cause severe inhibition <strong>of</strong> growth in tobacco <strong>and</strong><br />
Arabidopsis, respectively (Orvar <strong>and</strong> Ellis, 1997; Veljovic-Jovanivic et al., 2001). A<br />
reduction in oxygen might also function in the synthesis <strong>of</strong> surplus ATP during the<br />
photosynthetic induction phase (Makino et al., 2002). The transfer <strong>of</strong> electrons from<br />
water in PSII to oxygen in PSI is also thought to play an important role in excess energy<br />
dissipation, as the excitation <strong>of</strong> chlorophyll is so excessive. Superoxide formed in PSI is<br />
reduced by thylakoid-bound Cu,Zn-superoxide dismutase to H 2<br />
O 2<br />
<strong>and</strong> then to water by<br />
thylakoid-bound ascorbate peroxidase; O 2<br />
- <strong>and</strong> H 2<br />
O 2<br />
that escape attack might be decomposed<br />
by stromal is<strong>of</strong>orms <strong>of</strong> these enzymes (Asada, 1999). Thioredoxin peroxidase,<br />
or 2-Cys peroxiredoxin, functions to decompose lipid peroxides (Dietz, 2003).<br />
The above biochemical studies suggest that oxygen reduction in PSI acts as<br />
an important electron sink in chloroplasts under stress. However, the genes <strong>of</strong> the<br />
enzymes involved in decomposing active oxygen in the chloroplasts are not up-regulated<br />
under drought or salt stresses, unlike the genes for the cytosolic counterparts <strong>of</strong><br />
these enzymes (Yabuta et al., 2002; 2004). Furthermore, chloroplast APX is a prime<br />
target <strong>of</strong> oxidative stress (Mano et al., 2001). Inactivation <strong>of</strong> chloroplast APX is much<br />
more severe than that <strong>of</strong> phosphoribulokinase, a light-regulatory SH-enzyme, in tobacco<br />
leaves stressed by drought <strong>and</strong> strong light (Shikanai et al., 1998b). Chloroplast<br />
APX is also known to quickly lose its activity in vitro in the presence <strong>of</strong> H 2<br />
O 2<br />
(Miyake<br />
<strong>and</strong> Asada, 1996). Local imbalance in the ratio <strong>of</strong> ascorbate to H 2<br />
O 2<br />
in the vicinity <strong>of</strong><br />
thylakoids might be a determinant <strong>of</strong> APX activity.<br />
An active flow <strong>of</strong> electrons to PSI cyclic electron transport chains <strong>and</strong> oxygen<br />
<strong>and</strong> activation <strong>of</strong> the malate valve (Fridly<strong>and</strong> et al., 1998) induce transfer <strong>of</strong> protons<br />
from the stroma to lumenal side <strong>of</strong> the thylakoids. However, the thylakoids are not<br />
acidified to a level at which the lumenal proteins are denatured. Uncoupling <strong>of</strong> thylakoid<br />
membranes through a change in the stoichiometry <strong>of</strong> protons transferred in the Q<br />
cycle <strong>and</strong> reduction <strong>of</strong> the γ-subunit <strong>of</strong> ATP synthase have been proposed for suppression<br />
<strong>of</strong> hyper acidification <strong>of</strong> the luminal side <strong>of</strong> the thylakoids in spinach leaves<br />
(Richter et al., 2004). Another example is release <strong>of</strong> coupling factor 1 from the ATP<br />
synthase complex for uncoupling in sunflower plants suffering drought (Teraza et al.,<br />
1999).