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

Photooxidative Stress
173
and this reduction of LHC II in response to high light was partially attributed to
acclimative proteolysis of apoproteins of outer LHC II (Anderson et al., 1995; Boekema
et al., 1999; Jackowski et al., 2003). However, the biochemical identity of products of
LHC-gene family and their possible role as irradiance-responsive protein is yet to be
understood. Jackowski et al. (2003) demonstrated that plant acclimation to high light
irradiance was accompanied by progressive decline in Lhcb 2 and 3 abundance, whereas
decline in Lhcb1 level was identified only at excessive irradiance causing moderate
stress to PSII and there was an acclimation related decline in LHC II apoproteins in
spinach. Ascorbate exhibited the most dramatic acclimatory response to growth photosynthetic
photon flux density among all antioxidants (Grace and Logan, 1996). This can
be attributed to multi-faceted roles of ascorbic acid in plant cell metabolism, particularly
in photoprotection of the chloroplast. In addition, it is also believed that the redox
signals derived from photosynthetic electron transport play an important regulatory
role in acclimation to high light stress. The redox signals are known to modulate the
expression of many plastid and nuclear genes encoding photosystems (Walters, 2004).
Plant acclimation to high light results in an increase in the photosynthetic rate which
has the potential benefit to the plant as increased photosynthetic rates increase the
growth rates. In contrast, antisense plants with reduced levels of cyt bf complex have
marked reductions in net photosynthetic rates indicating that large changes in the
levels of cyt bf complex under low light are intimately linked to the changes in photosynthetic
capacity (Price et al., 1998). Also, high light-grown plants often have substantially
increased capacities for ∆ pH-dependent protective energy dissipation (qE),
which were related to different energy dissipation characteristics of a larger light harvesting
system (Park et al., 1996; Bailey et al., 2004). These studies suggest that redox
regulation and antioxidant systems in plant cells appear to be part of acclimatory responses
of plants to high growth light intensities. Furthermore, mutants defective in
acclimation to photooxidative stress will be critical tools to understand the adaptive
benefits to photoacclimation. It would be possible to consider the ways in which modifying
acclimation behavior of plants might help to improve the agriculture productivity
in crop plants under changing global environmental conditions.
7. MOLECULAR AND GENETIC ASPECTS OF PLANT RESPONSES TO
PHOTOOXIDATIVE STRESS
Light is highly unpredictable resource for plants and the changes in growth irradiance
induce several changes in biochemical and molecular composition of the plant cell.
Murchie et al. (2005) showed that there are 99-light responsive genes which were down
regulated and 130 were up-regulated in rice during light treatment. Majority of these
genes showed reduced levels of expression in response to high light, whereas stress
related genes showed increased level of expression. In order to avoid over-excitation of
chlorophyll protein complexes and photooxidation, a regulated degradation of LHC
was observed in rice leaves along with a decline in CP-24, PSI genes and a 10 kD PSII