Agriculture Reference
In-Depth Information
16.2 Drought and Salt Stresses Induce ABA-Responsive
Gene Expression
As sessile organisms, plants cannot adjust their location during unfavorable
circumstances such as drought and high salinity. Facing these stressful triggers, a
long-distance signaling transduction event in plants will start with signal percep-
tion, move to signal relay, and end with the stress response, that is, gene expres-
sion in the cell nuclei. The expression of stress-responsive genes will finally
determine whether plants adapt and survive or succumb to the stressor.
Both drought and high salinity induce the expression of osmotic stress-
responsive (OR) genes, which are activated under stressful conditions but silent
under normal conditions (Kawasaki et al. 2001 ; Roychoudhury et al. 2013 ;
Seki et al. 2001 , 2002 ). The ABA-responsive genes increased by drought and
high salinity can be classified into two general groups. The first group includes
genes that produce important metabolic proteins to protect cells and tissues
from stressful conditions. These genes encode enzymes for the biosynthesis of
osmolyte, acid metabolism, and cell detoxification, as well as those for water
channel proteins, membrane transporters, proteins for protection of macromol-
ecules such as late embryogenesis abundant (LEA) proteins and osmotins, etc.
The second group includes genes primarily encoding regulatory proteins for
modulating downstream genes expression in response to stressful conditions,
such as protein kinases, protein phosphatases, transcription factors, and so on.
The transcription factors interact with cis-acting elements to regulate the expres-
sion of stress-inducible genes and finally confer tolerance to stress on the plants
(Agarwal et al. 2006 ; Shinozaki and Yamaguchi-Shinozaki 2007 ). Epigenetic
mechanisms have also been reported to participate in controlling the transcrip-
tion of stress-inducible gene expression, including DNA methylation, histone
modifications, histone variants, and non-coding RNA (Chinnusamy and Zhu
2009 ).
In the first group of genes discussed above, Rab16, Salt, and Osem from rice,
Rab18 from Arabidopsis, and Rab17 and dehydrins from maize all belong to LEA
or LEA -like genes. Overexpressing Rab16A , which originated from a salt-toler-
ant rice variety called pokkali, in tobacco resulted in enhanced tolerance to salt
stress as well as increased gene expression under ABA, drought, and high salinity
(Roychoudhury et al. 2007 ). Similarly, overexpressing the Rab16A gene in indica
rice also conferred enhanced salt tolerance (Ganguly et al. 2012 ). ABA-activated
protein kinase (AAPK) binds to dehydrin mRNA in response to both drought and
ABA. The OPEN STOMATA1 ( OST1) gene is induced by exogenous ABA in pro-
toplasts of guard cells. Both AAPK and OST1 belong to ABA-activated SNF1-
related protein kinases (SnRKs), which function in stomatal closure under drought
stress (Himmelbach et al. 2003 ). Arabidopsis overexpressing the AtHD2C gene, a
member of the histone deacetylases (HDACs) family, show enhanced drought and
salt tolerance in addition to an up-regulation of the LEA class of genes. This indi-
cates that histone deacetylation plays a role in ABA and osmotic stress responses
(Sridha and Wu 2006 ).
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