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

174 A.R. Reddy and A.S. Raghavendra
gene was also noticed under high light (Murchie et al., 2005). PS I has long been
reported to be less affected than PSII by high light (Kok, 1956). PSI in isolated thylakoid
membranes was inactivated by high light (Sonoike, 1995). Since PSI is the terminal
electron carrier in the chloroplast, it was identified as a major site producing ROS and
shown to be closely associated with ROS-scavenging systems in the chloroplast (Ogawa
et al., 1995). The role of ROS inactivating PSI reaction center and degradation of psaA
and psaB under high light conditions has been studied (Sonoike, 1996; Tjus et al.,
1999). Very recently, Jiao et al (2004) demonstrated that high light stress readily
photoinhibited PSI, following the loss of psaC as well as degradation of PSI reaction
center proteins (psaA and psaB). The findings suggest that PSI photoinhibition can be
a limiting factor in crop productivity under high light.
Photoinhibition and photooxidative stress reflect the photoinactivation of
photosynthetic apparatus especially the PSII and thus decreasing the photosynthetic
function which is often referred to PSII photoinhibition and degradation of D1 proteins
(Long and Humphries, 1994; Kettuhen et al., 1996). Damaged PSII centers do not usually
accumulate in the thylakoid membrane due to a rapid and efficient repair mechanism
.As most of the damage is targeted to D1 protein, the turnover and repair of the protein
subunits is a complex process involving reversible phosphorylation of PSII core subunits,
monomerization and migration of PSII core, highly specific proteolysis of the
damaged proteins and multi-step replacement of the damaged proteins with de novo
synthesized copies (Aro et al., 2004). In addition to D1 protein, it was also reported that
D2 protein also occasionally becomes damaged in light (Sansen et al., 1996). More
recently, one of the small PSII subunits, the psbH protein was also shown to be frequently
replaced (Bergantino et al., 2003). Although phosphorylation of all the major
LHCII antenna proteins are not involved in PSII photodamage or repair, the phosphorylation
of Lhcb4 (CP29) has been assigned a role in the photoprotection of PSII centers
and this protein is a relevant candidate to possess a functional role in the dissipation of
excess excitation energy and the protection of PSII against photodamage (Bassi et al.,
1997; Bergantino et al., 1998; Pursiheimo et al., 2003). On the other hand, another PSII
protein in thylakoids, the 22 kD psbS protein is now known to function in the regulation
of photosynthetic light harvesting and is necessary for photoprotective thermal dissipation
(qE) of excess absorbed light energy in plants (Niyogi et al., 2004; Hieber et al.,
2004). Arabidopsis thaliana mutants lacking qE required psbS in addition to low lumen,
pH and the presence of de-epoxidized xanthophylls for protoprotection (Li et al., 2002).
The expression of LHC genes is tightly regulated by light (Niyogi, 1999). High
light intensities inhibit transcription of LHC genes and activate a set of proteins known
as early light-induced proteins (ELIPs), a class of proteins structurally related to LHCs
(Hutin et al., 2003). ELIPs are predicted to have three trans-membrane helices like LHCs
and are known to bind chlorophyll and carotenoids. Recently, a number of ELIP-type
polypeptides in response to high light have been discovered in higher plants (Jasson et
al., 2000). The induction of ELIPs in plants by high light intensities suggests a role in
the acclimation to light stress rather than a light harvesting function. ELIPs are also