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

Photooxidative Stress
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The use of solar energy in photosynthesis primarily depends on the ability to safely
dissipate excess light energy to avoid photoinhibition. The dissipation process employed
by plants in their natural environment is mediated by different groups of plant
pigments which are known as photoprotective pigments. Carotenoids play an important
role in the photoprotection of plant cell against over excitation in excess light and
thus dissipate the excess of absorbed energy (Frank, 1999; Strzalka et al., 2003; Edreva,
2005a). Even under low light, carotenoids act as energetic antenna, harvesting light at
the wavelength not absorbed by chloroplast and transferring electron excitation states
towards photochemical reaction centers. Carotenoids are now known as intrinsic components
of the chloroplast, involved in quenching the 1 O 2
under excess light (Mittler,
2002). This quenching ability of the carotenoids was attributed to chain of isoprenic
residues with numerous conjugated double bonds with delocalized Ð-electrons which
allows easy energy uptake from excited molecules and dissipation of excess energy as
heat (Edge et al., 1997; Edreva, 2005b). Also, â-carotein, lutein and neoxanthine are
known to protect the photosynthetic apparatus against photoexcitation damage by
quenching the triplet states of chlorophyll molecules (Frank, 1999). Carotenoids are
thus potent scavengers of ROS, protecting pigments and lipids from oxidative damage
(Edge and Truscott, 1999). Carotenoids also protect plants from photooxidative stress
by modulating physical properties of photsynthetic membranes with an involvement of
xanthophyll cycle (Demings-Adams and Adams, 1996).The quenching by exchange
electron transfer to produce the carotenoid triplet state ( 3 Car) is the principle mechanism
of carotenoid photoprotection against 1 O 2.
Carotenoids fluidize the membrane in
its gel state and make it more rigid in its liquid crystalline state. Changes in the membrane
fluidity play an important regulatory role in the de-epoxidation of violaxanthine to
antheraxanthine which influences the rate of xanthophyll cycle under high light stress
(Havaux and Niyogi, 1999; Strzalka et al., 2003) (Figure 3).
Under excess light, a rapid change in the carotenoid composition of LHCs is a
common phenomenon. The diepoxide xanthophyll violaxanthin is rapidly and reversibly
converted to epoxide- free zeaxanthin via the intermediate antheraxanthin by the
activity of violaxanthin deepoxidase and the reverse reaction is mediated by zeaxanthin
epoxidase under low light regimes (Havauex and Niyogi, 1999). Zeaxanthin is known to
quench the singlet excited states of chlorophylls or could favour protein – induced
aggregation of the LHCs of PSII leading to energy dissipation, thus protecting the
reaction centers from overexcitation and photoinhibition. Chloroplast membranes are
sensitive targets for photodestruction by different ROS. The xanthophylls cycle is thus
significant in scavenging the free radicals that otherwise would interact with the lipids
surrounding the photosystems.The higher number of conjugated double bonds in
antheraxanthin and zeaxanthin can be presumed to be better protectors than violaxanthin
with a higher efficiency for deexciting 1 O 2
. The xanthophylls cycle is thus a ubiquitous
light-controlled antioxidant system in which a simple chemical substitution in xanthophylls
molecule elicits profound changes in the photostability of the chloroplast membrane
system.