59was significantly decreased, suggesting that the salinity stress affects in a negative
way the plant-growth-related pathways at the transcriptional level.
The levels of transcripts encoding late embryogenesis abundant (LEA) (Sanchez
et al. 2011 ; Legay et al. 2009 ; Chao et al. 2005 ; Wang et al. 2013 ; Cramer et al.
2007 ) and heat shock proteins (HSP) (Beritognolo et al. 2011 ; Legay et al. 2009 )
tend to increase after exposure of plants to high salinity. LEA proteins are small hy-
drophilic, largely unstructured, and thermostable proteins that are synthesized in the
seeds during maturation. It is believed that they play a protective role against desic-
cation through multiple functions, including ion binding, hydration buffering, and
membrane and protein stabilization (Battaglia et al. 2008 ). Most HSPs have been
shown to act as molecular chaperones, which are responsible for protein synthesis,
targeting, maturation, stabilization, refolding under stress conditions, and degrada-
tion in a broad array of normal cellular processes. Moreover, the HSPs participate in
the membrane stabilization under stress conditions (Wang et al. 2003 ).
Consistent with the findings from metabolomics, most transcriptomic studies
of the salinity effect on plants record increased the abundance of gene transcripts
involved in the biosynthesis of osmolytes (Sanchez et al. 2011 ; Legay et al. 2009 ;
Gong et al. 2005 ; Chao et al. 2005 ; Evers et al. 2012 ). The abundance of transcripts
related to the ROS scavenging and detoxification has in some studies been reported
as increasing (Beritognolo et al. 2011 ; Gong et al. 2005 ; Chao et al. 2005 ; Cramer
et al. 2007 ) and in some others as decreasing (Legay et al. 2009 ; Evers et al. 2012 ;
Wang et al. 2013 ), after the plants are exposed to salinity stress. This discrepancy
could be an indication that in some cases ROS act as signaling molecules for the
salinity stress and have thus to attain high concentrations to trigger other reactions,
or it could just be a consequence of different durations of plant exposure to stress.
Transcripts that encode ion and amino acid transporters also accumulate in the
plants after exposure to the salinity stress (Beritognolo et al. 2011 ; Kanani et al.
2010 ; Legay et al. 2009 ; Gong et al. 2005 ).
The activity of the salt overly sensitive (SOS) signaling pathway is of particular
interest regarding the response of the plants to the salt stress. This pathway is re-
sponsible for the extracellular and vacuolar sequestration of the Na+ ions with H+/
Na+ antiporters, a process of high significance for the ion homeostasis of the plants
(Zhu 2002 ). The Na+ ion increase caused by the salt stress could be detrimental to
the plants, causing membrane disorganization, impaired nutrient and water acquisi-
tion, metabolic toxicity, inhibition of photosynthesis, and the production of ROS
(Niu et al. 1995 ). In a transcriptomic analysis of A. thaliana plant liquid cultures
under salt stress, the activity of the SOS pathway was indeed observed as signifi-
cantly increased at the transcriptional level (Kanani et al. 2010 ).
3.4.3 Results of Integrated Metabolomic and Transcriptomic
Analyses
Some of the above-mentioned studies carried out both transcriptomic and
metabolomic analyses on the same set of plants, in an effort to comprehensively
investigate the changes in the physiology of the plants due to the high-salinity stress
3 Investigating the Effect of Elevated CO 2 in the Growth Environment ...