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

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has caused an increase of the activity of superoxide dismutase, catalase and glutathione
reductase, which was more obvious in the salt-tolerant varieties (Sairam et al., 2002).
Rapid increase in the activity of ascorbate peroxidase, catalase, glutathione reductase,
peroxidase and, especially superoxide dismutase observed in salt-treated cotton callus,
suggests the importance of the up-regulation of antioxidant capacity in an early response
to salt stress (Manchandia et al., 1999). The findings concerning the increased
activity of antioxidant enzymes of salt-tolerant varieties in response to salt stress imply
that ROS scavenging might be a part of the general adaptive strategy of plants exposed
to salinity. Expression of many genes is regulated by ROS, especially of those involved
in encoding the ROS scavengers, either enzymes or antioxidants (Jamieson, 1998).
Several genes which encode the antioxidant enzymes have been isolated and
characterized. Examples are csa gene of citrus cells corresponding to phospholipid
hydroperoxide glutathione peroxidase (Avsiankretchmer et al., 1999), and the CAT2 and
Call genes found in Nicotiana plumbaginifolia responsible for NaCl-induced catalase
activity (Savoure et al., 1999). Overexpression of the tomato TPX2 gene and swpa1
gene from sweet potato in transgenic plants contributed to salt tolerance or oxidativestress
tolerance, respectively (Yoshida et al., 2003). In an experiment with salt-adapted
and unadapted tomato cells, adapted cell suspension has exhibited the higher threshold
concentration for down-regulated TPX1 (peroxidase encoding gene) transcripts
(Medina et al., 1999). Differential gene expression of superoxide dismutase isoforms in
rice exposed to drought, chilling and salinity showed that phytohormone (ABA) and
active oxygen species are associated with the regulation of SOD genes under environmental
stresses (Kaminaka et al., 1999).
Complex network of adaptive reactions and interactions at physiological and
molecular level, involving the coordinated interaction of many salt-inducible and regulatory
genes, gene products and signaling athways, permits survival and growth of
plants exposed to salinity. This is through the two main strategies referring to more or
less restricted salt uptake and accumulation evolved in halophytes and non-halophytes,
respectively (Figure 5).
9. GENETIC ASPECTS OF SALT TOLERANCE AND ADVANCES IN
BIOTECHNOLOGY
The salt-adaptive capacity of species may be related to constitutive expression of
genes that encode salt-tolerance determinants (Casas et al., 1992) or the greater capacity
to regulate the expression of these genes in response to salt (Cushman et al., 1990b).
Salt-inducible genes fall into different categories, as per their function, predicted from
sequence homology with known proteins, such as osmoprotectants and molecular
chaperones, proteins involved in ion transport, signaling and oxidative stress defense
systems, proteinases, transcriptional factors, as well as proteins controlling photosynthesis
and other physiological processes.