5 Tolerance to Combined Stress of Drought and Salinity in Barley 111
abiotic stresses (Chaves et al. 2003 ). Microarray profiling under drought stress has
been carried out in different plant species such as Arabidopsis (Oono et al. 2003 ),
rice (Rabbani et al. 2003 ), barley (Ozturk et al. 2002 ; Talame’ et al. 2007 ), and
wheat (Mohammadi et al. 2007 ). These studies identified differentially expressed
transcripts of genes involved in photosynthesis, ABA synthesis and signaling, bio-
synthesis of osmoprotectants, protein stability and protection, reactive oxygen de-
toxification, water uptake, and a myriad of transcription factors including several
members of the zinc finger, WRKY (c-terminal wrky domain), and bZIP (basic
leucine zipper) families. Du et al. ( 2011 ) showed that two dehydrin genes might
contribute to improved drought and salt tolerance of Tibetan and wild barley. Hv-
WRKY38 is a barley gene coding for a WRKY protein, whose expression is in-
volved in cold and drought stress response which was mapped close to the QTL
region (Mare et al. 2004 ). Hv-WRKY38 was early and transiently expressed during
exposure to low nonfreezing temperature, in ABA-independent manner. Further-
more, it showed a continuous induction during dehydration and freezing treatments.
The aquaporin, dehydrin, C-repeat binding factor (CBF) genes, and Hv-WRKY38
may be putative candidate genes that underlie the QTL effect on salt tolerance.
Differentially regulated proteins predominantly had functions not only in photo-
synthesis but also in detoxification, energy metabolism, and protein biosynthesis.
The analysis indicated that de novo protein biosynthesis, protein quality control
mediated by chaperones and proteases, and the use of alternative energy resources,
i.e., glycolysis, play important roles in adaptation to drought and heat stress (Rollins
et al. 2013 ).
Transcriptional factors (TFs) play important roles in the regulation of gene ex-
pression in response to abiotic stresses such as drought and salinity. TFs are power-
ful targets for genetic engineering of stress tolerance, because overexpression of a
single TF can lead to the up-regulation or down-regulation of a wide array of stress
response genes. Until now, transcription factors have been the most appealing tar-
gets for transgenic barley improvement, due to their role in multiple stress-related
pathways. Dehydration-responsive element-binding protein 1 (DREB1)/CBF and
DREB2 gene function in ABA-independent gene expression while ABA-responsive
element (ABRE)-binding protein (AREB)/ABRE binding factor (ABF) functions in
ABA-dependent gene expression. NAC (nascent polypeptide-associated complex
protein) and MYB (myeloblastosis oncogenes)/MYC (v-myc avian myelocytomato-
sis viral oncogene homolog) are involved in abiotic stress-responsive gene expression
(Uauy et al. 2006 ). In another study, a barley LEA protein, HVA1 (ABA-inducible
protein PHV A1), was also overexpressed in wheat, and the overexpressors were
observed to have better drought tolerance (Bahieldin et al. 2005 ). Transgenic wheat
obtained with Arabidopsis DREB and HVA1 protein overexpression was also shown
to produce higher yield in the field under drought conditions, but further studies are
required to confirm their performance under different environments (Bahieldin et al.
2005 ). The transformation of oat and rice with the barley HVA1 gene also improved
drought and salt tolerance (Xu et al. 1996 ; Oraby et al. 2005 ). It is not unreasonable
to predict in the following decades: genetically modified (GM) wheat will be trans-
ferred to the fields as a common commercial crop. However, to pace this process,