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

Metabolic Engineering for Stress Tolerance
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3.1.9. Protein Kinase
There is a rapid increase in cytoplasmic Ca 2+ levels in plant cells under various abiotic
stress stimuli. Then, Ca 2+ -dependent protein kinases (CPDK) bring about phosphorylation/
dephosphorylation of various proteins, which ultimately mediate Ca 2+ influx
signals (Sanders et. al., 1999). Rice plants engineered with altered levels of cold- and
salt-inducible CDPK gene (OsCDPK7) showed high levels of salt and drought tolerance
on rice plants (Saijo et. al., 2000).
3.1.10. Dehydrins
Plants also synthesize osmoprotectant proteins termed as dehydrins under drought
and salinity stress. Overexpression of dehydrin HVA1 gene from barley (Xu et. al., 1996)
and wheat (Cheng et. al., 2001, 2002) in transgenic rice resulted in resistance towards
salt and water deficit stress.
3.2. Other Stresses
3.2.1. Anoxia/Hypoxia
Both limited supply of water or excessive flooding could cause oxygen deficiency in the
roots. This deficiency triggers ethylene synthesis in the aerial parts of the plant and
produces symptoms, such as chlorosis, senescence and ultimately death of plants.
Certain plants however, are adapted to low oxygen by maintaining adequate supplies of
energy and sugar to avoid self-poisoning and cytoplasmic acidosis (Stearns and Glick,
2003).
In general, plants produce ATP and NAD by means of glycolysis and fermentation
rather than using Krebs cycle under oxygen limited conditions. A set of enzymes,
such as alcohol dehyrogenase (ADH) and pyruvate decarboxylase (PDC) required for
ethanolic and lactic acid fermentation is produced due to this anerobic switch (Andrews
et al., 1994). Genes encoding ADH and PDC have been cloned and described (Bucher
and Kuhlemeier, 1993; Umeda and Uchimiya, 1994; Hossain et. al., 1996, Quimio et. al.,
2000).
Transgenic tobacco plants overexpressing a bacterial pyruvate decarboxylase
gene showed 20-fold higher pyruvate decarboxylase activity in comparison to the
wild-type (Bucher et. al., 1994, Bucher et. al., 1995, Tadege and Kuhlemeier, 1997, Tedege
et. al., 1998). Arabidopsis genes encoding alcohol dehydrogenase, pyruvate decarboxylase
and lactate dehydrogenase and alanine amino transferase were cloned and
expressed in Arabidopsis plants (Dennis et. al., 2000). Transgenic cotton plants
overexpressing cotton alcohol dehydrogenase gene displayed 10-30 fold increase in
alcohol dehydrogenase activity and ethanol fermentation, whereas the cotton plants