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

176 A.R. Reddy and A.S. Raghavendra
β-carotene hydroxylase gene was up-regulated by high light which is involved in xanthophylls-cycle
carotenoid biosynthesis with increased leaf contents of xanthophylls
and higher tolerance to high light and heat stress (Davison et al., 2002). Rossel et al.
(2002) also concluded that photooxidative stress-induced genes also trigger the production
of cold and heat shock proteins providing a comprehensive tolerance mechanism
to the plant against unfavourable environmental variables.
Very interestingly, transgenic rice plants expressing C 4
photosynthetic enzymes
have shown significantly high tolerance to photooxidation compared to wildtype
rice (Jiao et al., 2002). Earlier Jiao and Ji (1996) showed that transgenic rice with PEP
carboxylase may play an important role in adaptation to photooxidative stress. PSII
electron transport efficiency as determined by Chl a fluorescence analysis (Fv/Fm) of
transgenic rice with C 4
enzymes (PEPC, PEPCK and ADPME) was high compared to
wild-type rice after photooxidative stress treatment (Jiao et al., 2002). Consistently, the
ability to dissipate the excess light energy by photochemical and non-photochemical
quenching increased more after photooxidative stress treatment. The transgenic rice
plants also showed higher light intensity for saturation of photosynthesis, high photosynthetic
CO 2
uptake rates and significantly produced 24% more grains than wild-type
plants. These findings suggest that expression of C 4
photosynthetic enzymes in C 3
plants can certainly improve tolerance to photooxidative capacity. However, the exact
mechanism responsible for this improved tolerance to photooxidative stress in transgenic
C 3
plants remains to be elucidated. The induction of genes involved in protection
against photooxidative stress suggests the presence of increased amounts of ROS,
which are also known to be involved in cell signaling processes for cross-tolerance.
8. ROS AS SECONDARY MESSENGERS
Plants are continually in danger of absorbing more light energy than they can use for
their metabolism. The excess excitation energy has to be dissipated to avoid photooxidative
damage to the photosynthetic apparatus, which is often manifested as leaf bleaching,
chlorosis or bronzing of leaves (Niyogi, 2000). Immediate responses to the conditions
that promote excess excitation energy would initiate signaling pathways for plant
acclimation. In response to excess light energy absorption, there would be increases in
the rates of electron transport and consequent redox changes in photosynthetic electron
transport which in turn regulates the expression of both nuclear and chloroplast
genes to encode components of photosynthesis and antioxidant metabolism. Examples
of such genes are Cab (encodes chlorophyll a/b binding protein), LHC (gene for LHCl),
APX1 and APX2 (Kripinski et al., 1999; Oswald et al., 2000). Redox changes in the
vicinity of Q A
and Q B
or plastoquinone have been suggested to be key starting points
for signaling (Mullineaux and Kripinski 2002).
The ROS have been ascribed signal functions both in biotic and abiotic stresses.
As described above, high concentrations of ROS are extremely harmful to plants; while