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

Metabolic Engineering for Stress Tolerance
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with tools to transform almost any crop species (Li et. al., 2001). However, the efficiency
of gene transfer remains low for many important crops and needs to be improved. There
is an increased availability of expression elements such as stress inducible promoters,
vectors, selectable markers and data on the stability of recombinant proteins expressed
in transgenic plants (Curtis and Grossniklaus, 2003; Fischer et. al., 2004; Herbers and
Sonnewald, 1999; Penna et. al., 2002; Schunmann et. al., 2003). Now, progress has been
made to insert a single gene or multiple genes in a single transformation event (gene
stacking), allowing one to build a multi-step pathway (Chen et. al., 1998, van Bel et. al.,
2001). Expression of transgenes in the plastids via chloroplast transformation (Daniell
and Dhingra, 2002) is used for high level expression of genes. Some of the technical
problems surrounding gene transfer in particular crops will eventually be overcome.
2.2. Wild Species to Understand Specific Stress Adaptations
Many wild plants have evolved interesting, and often unique and complex stress adaptations.
Therefore, it is possible to identify unique pathways, transport processes and
regulatory networks, not present in current model species, like Arabidopsis thaliana
and rice. For this purpose there is a need to develop genomic and bioinformatic resources
on highly stress-tolerant plants and other organisms. One approach is to use
comparative genomics to identify genes implicated in stress tolerance. For example, the
genomes of wild stress-tolerant relatives to Arabidopsis and Oryza can first be exploited
for novel genes.
Based on physiological and molecular characterizations available in the literature,
many plant species could be suggested to be models to study specific stress
adaptations. These include but not limited to Thellungiella halophila (Volkov et. al.,
2003), Limonium latifolium (Rathinasabapathi et. al., 2001; Raman and Rathinasabapathi,
2003), Mesembryanthemum crystallinum (Vera-Esterella et. al., 2004), Atriplex hortensis
(Shen et. al., 2003), Porteresia coarctata (Majee et. al., 2004), Tortula ruralis (Wood et.
al., 1999) and Xerophyta viscosa (Garve et. al., 2003). Depending upon the kind of gene
mining strategy, the whole genome sequence may not be required for all the stresstolerant
models. Genomic research tools such as cDNA libraries, expressed sequence
tag (EST) libraries, profiles of stress modulated genes and gene products using
microarray and proteomics technologies are required for various stress-tolerant wild
plants and other organisms.
2.3. Selection of Targets for Metabolic Engineering
Specific metabolites and their synthetic pathways have been implicated in stress tolerance
in plants. This has been done over the years using comparative physiology
between stress-tolerant and non-tolerant species. Metabolic engineering for stress
tolerance can be achieved by amplifying constitutive concentration of antioxidants and
assembly of compatible solutes in the plant tissues (Bohnert and Shen, 1999; Verpoorte