57transcripts of the glyoxysomal and chloroplastic malate dehydrogenases and the de-
creased abundance of transcripts of the cytoplasmic and mitochondrial malate dehy-
drogenases. Moreover, the accumulation of proline was consistent with an increase
in the transcript abundance for delta 1-pyrroline-5-carboxylate synthetase (P5CS),
the enzyme that catalyzes the first two steps in the proline biosynthetic pathway.
Kanani et al. ( 2010 ) observed that after a short-term (i.e., 30 h) continuous ex-
posure to high salinity, A. thaliana plant liquid cultures accumulated fatty acids and
sterols including tocopherol, a known antioxidant. A significant increase was also
observed in the levels of homo-serine, β-alanine, methionine, glycine, N-acetyl-
glutamate, allantoin, and the TCA cycle intermediates from citrate to fumarate
throughout the treatment period. Homoserine and methionine are precursors of the
S-adenosyl-methionine, which is required along with glycine for the biosynthesis of
glycine-betaine, the main osmoprotectant in A. thaliana, and along with β-alanine
for the production of β-alanine-betaine. Polyamines and betaines are known osmo-
lytes in plants. As it was the case with the previously discussed studies, these ob-
servations are in accordance with the need of the plants to produce osmoprotectants
and antioxidants to counteract the stress conditions. At the same time, the increased
production of amino acids/amine group containing metabolites that are precursors
of osmoprotectants and antioxidants was accompanied by a significant decrease
in the concentration of metabolic intermediates that are required for plant growth.
Moreover, based on their time-series analysis, Kanani et al. were able to observe
a change in the metabolic physiology of the plants even from the first hour of the
salinity treatment.
Sanchez et al. (2008a) studied comparatively the metabolic responses of A. thali-
ana, Lotus japonicus, and rice after long exposure and potential acclimation of the
plants to salinity stress (i.e., 75, 150, and 100 mM NaCl for each plant species,
respectively). They reported a salinity dose-dependent increase in the concentra-
tion of sucrose and amino acids like proline, glycine, serine, threonine, leucine,
and valine, in all the three species. The TCA cycle intermediates, citrate, succinate,
malate, and other organic acids, such as oxalic and maleic acids, which are direactly
related with the TCA cycle flux, exhibited conserved reduction in their pool sizes in
response to long-term salinity stress. Reduction was also observed in the concentra-
tions of the glycolysis intermediates glucose, fructose, glucose-6-phosphate, and
fructose-6-phosphate. The authors suggest that a reason for the reduced acid levels
under salt stress may be their involvement in the compensation of the ionic imbal-
ance. At physiological pH levels, organic acids exist as carboxylic anions and coun-
terbalance inorganic anions, so a depletion of organic acids may actually reflect
preferential uptake of anions compared to cations. Moreover, the increased amino
acid biosynthesis may also serve the plants to absorb excess ammonium while pro-
ducing osmolytes. Excess organic acids could be recruited from the TCA cycle and
sequestered into the biosynthesis pathways of amino acids and amines. Thus, the
maintenance of the charge balance, the ammonium detoxification, and the compat-
ible solute accumulation could all be met by a common mechanism.
Gong et al. ( 2005 ) compared the short-term responses to salinity (i.e., 150 mM
NaCl) stress of A. thaliana and Thellungiella halophila, a species related to
3 Investigating the Effect of Elevated CO 2 in the Growth Environment ...