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

Metabolic Engineering for Stress Tolerance
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3.2.11. Amino Acids and Derivatives
Heat shock is also known to influence the level of certain free amino acids. Heat shock
resulted in a significant increase of gamma-aminobutyric acid, beta-alanine, alanine,
proline and tyrosine in cowpea cells (Mayer et. al., 1990). Arabidopsis plants engineered
to overproduce glycine betaine, showed increased thermotolerance (Alia et. al.,
1998), suggesting that nitrogenous osmoprotectants can have a role in cellular protection
against heat stress. We have engineered tobacco plants to express bacterial aspartate
decarboxylase, the enzyme catalyzing the decarboxylation of aspartate to betaalanine.
The transgenic plants expressing the bacterial gene contained significantly
increased levels of beta-alanine and total free amino acids and exhibited a thermotolerant
phenotype, when compared to control plants (Fouad and Rathinasabapathi, unpublished).
Our results are consistent with the hypothesis that beta-alanine could have a
role in thermotolerance. Further research is in progress to explore whether beta-alanine
and other amino acid synthesis pathways could be potential avenues to improve crop
tolerance to heat stress.
3.2.12. Fatty Acid Unsaturation
The cold sensitivity of the plant is closely linked with the degree of unsaturation of
fatty acids in the membranes. Plants with elevated levels of cis-unsaturated fatty acids,
like spinach and Arabidopsis, are resistant to cold, whereas species having only a small
quantity, like squash are not. Many plant species produce high amounts of polyunsaturated
fatty acids, especially 18:3 (linolenic acid) in membrane when exposed to low
temperatures (Smolenska and Kuiper, 1977; Clarkson et. al., 1980; Kodama et. al., 1995).
Following increase in polyunsaturated fatty acids, restriction of membrane permeability
(Kuiper, 1974) and reduction in membrane-associated enzymes activities (Cronan and
Gelmann, 1975) bring about an increase in membrane fluidity (Chapman, 1975). Glycerol-
3-phosphate acyl transferase is an important determining factor for the level of
phosphatidylglycerol fatty acid unsaturation. Tansgenic tobacco plants expressed a
fair degree of cold tolerance following the manipulation of cholotoplastic enzyme glycerol-3-phosphate
acyl transferase from squash and Arabidopsis (Murata et. al., 1992;
Murata and Tasaka 1997; Sakamoto et. al., 2003). The ù-3 fatty acid desaturases are
membrane-bound enzymes found in plastids (Mazliak, 1994) that catalyze the conversion
of 18:2 (linoleic acid) to 18:3. The overexpression of the plastidal ù-3 fatty acid
desaturase genes (NtFAD7 and NtFAD3) in transgenic tobacco provided increased
chilling tolerance (Kodama et al., 1994; Hamada et. al., 1996, 1998). Transformation of
Arabidopsis with a codA gene encoding choline oxidase enhanced cold tolerance
during germination and early growth (Alia et. al., 1998).