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

170 A.R. Reddy and A.S. Raghavendra
four mitochondrial MnSODs and a novel type of chloroplastic FeSOD (Zhu and
Scandalios, 1993; Kenodle and Scandalios, 1996). MnSOD, a homotetrameric mitochondrial
enzyme of 85 kD has been isolated from Nicotiana, Pea, Arabidopsis, Rubber tree,
Wheat and Rice (Breusegem et al., 2002).
5.2.2. Ascorbate Peroxidase (APX)
The APX in plant cells destroy harmful H 2
O 2
via ascorbate-glutathione pathway (Figure
3), ascorbate-glutathione cycle provides protection against oxidative stress by a
series of coupled redox reactions in photosynthetic tissues (Foyer and Halliwell, 1976),
in mitochondria and peroxisomes (Jimenez et al., 1997). Based on the available sequence
data, seven different APXs are distinguished in plants; two soluble cytosolic forms,
three types of cytosol membrane bound, including a glyoxysome bound form, one
chloroplastic stromal and one thylakoid membrane-bound (Jespersen et al., 1997). Among
these, chloroplastic isoforms are very specific for ascorbate as electron donor while
cytosolic APX transcript levels are induced mostly by drought and excess light (Vansuyt
et al., 1997). Although APX activity was demonstrated in mitichondria and peroxisomes,
intact mitochondria and peroxisomes had no latent APX activity indicating that
the active site of APX is exposed to the cytosol and scavenges H 2
O 2
leaking from
mitochondria and microbodies (Jimenez et al., 1997). Glyoxysomal membranes of pumpkin,
cotton and spinach were found to contain APX activity (Zhang et al., 1998;
Yamaguchi et al., 1995). Recently Yabuta et al (2002) suggested that thylakoid APX is a
limiting factor of ROS scavenging systems. Under photooxidative stress in chloroplast
and that the enhanced activity of thylakoid APX functions to maintain the AsA content
in the redox status of AsA under photooxidative stress.
5.2.3. Glutathione Reductase (GR)
The GR completes “Asada-Halliwell pathway” by regenerating glutathione pool with
NADPH as electron donor (Foyer et al., 1994). Although most of GR activities were
studied in chloroplasts, mitochondrial in cytosolic isoenzymes were also described in
plant cells (Creissen et al., 1996). It was also reported that GR and glyoxalase II reduce
GSSG to GSH and help in maintaining the plant cell in its homeostatic GSH/GSSG ratio
(Kumar et al., 2003).
5.2.4. Glutathione-S-Transferase (GST)
The GST is another important enzyme which protects the plant cells by detoxifying
harmful compounds. The GST is known to catalyze the conjugation of GSH to a variety
of cytotoxic compounds (Kumar et al., 2003). Glutathione transferases (GSTs) are also
known to play important role in protecting the plants from wide range of biotic and
abiotic stresses including xenobiotic toxins, UV-radiations and photooxidative stress