58
A. thaliana with extreme tolerance to a variety of abiotic stresses, including low
humidity, freezing, and high salinity. As expected, metabolites that act as osmolytes,
i.e., proline, galatinol, and glycine, increased under salt stress in both species. It has
to be noted that, at the control conditions, the concentration of several compounds
that have protective functions was much higher in T. halophila than in the A. thali-
ana plants. Maybe, this concentration profile could partly justify the T. halophila
surviving mechanisms under extreme salt concentrations. Widodo et al. ( 2009 )
studied the long-term responses of two barley cultivars to salinity (i.e., 100 mM
NaCl) stress using GC–MS. After three weeks of high-salinity treatment, the more
sensitive cultivar ceased growing, while the tolerant resumed similar growth to the
control plants. At the metabolic level, the sensitive cultivar exhibited an increase
in the levels of proline, GABA, and the polyamine putrescine, most in accordance
with the previous salinity studies. They suggested, however, that the observed in-
crease in these metabolites is not an adaptive response to salinity but an indication
of slower growth or tissue necrosis. On the other hand, in the tolerant plants, the
levels of TCA cycle intermediates and hexose phosphates increased in response to
salt. However, the response of each cultivar to salinity stress depended heavily on
the duration of its exposure to high salinity.
3.4.2 Transcriptomic Analyses
Many DNA microarray transcriptomic studies of the plant response to high salinity
have been reported in the literature (Sanchez et al. 2008b, 2011 ; Beritognolo et al.
2011 ; Bazakos et al. 2012 ; Kanani et al. 2010 ; Legay et al. 2009 ; Jankangram et al.
2011 ; Gong et al. 2005 ; Chao et al. 2005 ; Evers et al. 2012 ; Wang et al. 2013 ; Cra-
mer et al. 2007 ). Main common observations of these studies are: (a) the significant
decrease in the transcripts related to photosynthesis, i.e., the photosystem I and
II subunits, Calvin cycle enzymes, RuBisCO subunits and the RuBisCO activase,
protein synthesis and energy metabolism pathways (Beritognolo et al. 2011 ; Kanani
et al. 2010 ; Legay et al. 2009 ; Gong et al. 2005 ; Chao et al. 2005 ; Evers et al. 2012 ;
Wang et al. 2013 ) and (b) the simultaneous significant increase in the abundance of
transcripts related to signaling, membrane transporters, and the synthesis of osmo-
protectants and antioxidants (Deyholos 2010 ). These observations are in agreement
with the known decrease in the photosynthesis rate of the salinized plants based on
physiological studies (Chaves et al. 2009 ) while providing molecular insights about
this decrease. Interestingly, however, Cramer et al. report an increase in the tran-
script levels of the photosystem I and II subunits and the RuBisCO activase after
long exposure of grapevines to progressive salinity stress (Cramer et al. 2007 ). This
could be a secondary response of the specific species after long exposure to salin-
ity stress that ensures the survival of the plant. It also underlines the significance
of considering all parameters of the experimental design, including the treatment
duration and strength, when trying to integrate the results among different studies.
In the salt-stressed plants, the abundance of transcripts encoding proteins related
to cellular growth like histones (Kanani et al. 2010 ; Gong et al. 2005 ) and the as
primary metabolism (Beritognolo et al. 2011 ; Legay et al. 2009 ; Evers et al. 2012 )
M.-E. P. Papadimitropoulos and M. I. Klapa