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

Metabolic Engineering for Stress Tolerance
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is observed in plants naturally resistant to stress (Hayashi et. al., 1997; Nuccio et. al.,
2000; Sulpice et. al., 2003). Since accumulation of small quantities of glycine betaine
appears to protect plants from a variety of stress factors, glycine betaine may have a
role in stress tolerance by mechanisms other than it being an osmoticum.
Escherichia coli choline dehydrogenase has been introduced into tobacco
alone and together with E. coli gene for betaine aldehyde dehydrogenase (Lilius et al.,
1996; Holmstrom et al., 2000). Engineering the codA gene encoding choline oxidase
from the soil bacterium Arthrobacter globiformis (Deshnium et. al. 1995) provided salt
tolerance to transgenic to Arabidopsis (Hayashi et. al. 1997; Alia et. al., 1998), when
grown on media containing 200 mM NaCl. Rice is a non-accumulator of glycine betaine
(Ishitani et. al., 1993), but transgenic rice expressing codA gene exhibited normal growth
at a faster rate than the wild type under salt stress (Sakamoto et. al., 1998). In addition,
transformed Brassica juncea and Japanese persimmon (Diospyros kaki) overexpressing
a gene for choline oxidase have shown tolerance to salt stress (Gao et. al., 2000; Prasad
et. al., 2000a, 2000b). When a chimeric gene for betaine aldehyde dehydrogenase alone
was expressed in the chloroplasts of carrot cells, some betaine was detected with a
reported increase in salinity tolerance (Kumar et al., 2004). The nature of choline availability
and choline oxidation step in this system is unclear because glycine betaine level
has been shown to increase in plants that were engineered with the gene for betaine
aldehyde dehydrogenase, the second step in the two step pathway to glycine betaine.
Glycine betaine synthesis in transgenic plants was limited by the availability
of free choline because choline is essential for the synthesis of phosphatidyl choline of
the membranes (Nuccio et. al., 2000). However, choline synthesis could now be engineered
(McNeil et. al., 2001).
3.1.6. Beta-Alanine and Beta-Alanine Betaine
Beta-alanine is a non-protein amino acid and behaves as an osmoprotectant in microbial
bioassays. In all plants, beta-alanine is one of the precursors for pantothenate, an
essential vitamin (Figure 4). In most members of the stress tolerant plant family
Plumbaginaceae, beta-alanine is the precursor for beta-alanine betaine, an excellent
osmoprotectant (Hanson et al., 1991). We used Limonium latifolium, a member of the
Plumbaginaceae as a model to investigate the synthetic pathway to beta-alanine betaine.
Beta-alanine betaine synthesis is catalyzed by a trifunctional N-methyltransferase
(Rathinasabapathi et. al., 2001). A full length cDNA for beta-alanine N-methyltransferase
from L. latifolium was cloned and characterized recently (Raman and Rathinasabapathi,
2003). Experiments in our laboratory showed that in a variety of plant species, pantothenate
synthesis was limited by d-pantoate and not by beta-alanine (Rathinasabapathi
and Raman, 2005). Therefore, it should be possible to engineer the cDNA for betaalanine
N-methyltransferase in plants that do not naturally accumulate beta-alanine
betaine, without significantly depleting pantothenate.