Abiotic Stress Responses in Plants: Unraveling the Complexity of Genes and Networks to Survive - Abiotic Stress: Plant Responses and Applications in Agriculture - page 56

The use of microarray approaches, and more recently, of the Next Generation Sequencing
(NGS) methodologies have unveiled new regulatory mechanisms that complicate the un‐
derstanding and most of all the possibilities to modulate and control these processes in
view of improving plant responses and productivity.
The regulation of plant genes can be observed at three levels: transcriptional; post-transcrip‐
tional and post-translational. In each level, actions depend on specific molecular elements as
well as molecular networks and cascades.
The transcriptional regulation involves the interplay of three major elements: chromatin and
its modification and remodeling; cis-regulatory elements which are often binding sites, such
as enhancers and promoters, located upstream and downstream the coding region; and trans-
regulatory elements, usually transcription factors. Chromatin modification and remodeling
involved in plant abiotic stress response have been observed in numerous situations [87]. The
sensitization of stress responsiveness is called priming [88, 89]. Priming boosts the plant's
defensive capacity and brings it into an alarmed state of defense. Recently, priming was
correlated with chromatin modification of promoter region of WRKY transcription factors [90].
The involvement of epigenetic mechanisms in the response to environmental cues and to
different types of abiotic stresses has been documented [91,92]. Recent reports have shown
that different environmental stresses lead to altered methylation status of DNA as well as
modifications of nucleosomal histones.
Promoters are regulatory regions of DNA located upstream of genes that bind transcrip‐
tion factor IID (TFIID) and allow the subsequent coordination of components of the tran‐
scription initiation complex, facilitating recruitment of RNA polymerase II and initiation
of transcription [93].
Members of dehydration-responsive element-binding (DREB) or C-repeat binding factor
(CBF), MYB, basic-leucine zipper (bZIP), and zinc-finger families have been well characterized
with roles in the regulation of plant defense and stress responses. Most of these transcription
factors (TFs) regulate their target gene expression through binding to the cognate cis-elements
in the promoters of the stress-related genes [94]. More recently the WRKY transcription factors
are becoming one of the best-characterized classes of plant transcription factors [95]. Several
WRKY proteins were shown to be involved in plant drought and salinity stress responses [96].
For example, overexpression of the Oryza sativa WRKY11 under the control of Heat Shock
Protein 101 (HSP101) promoter led to enhanced drought tolerance [97]. Similarly, the altered
salt and drought tolerance of 35S:OsWRK45 and 35S:OsWRK72 Arabidopsis plants may be
attributed to induction of ABA/stress-related genes [98,99].
NAC (N-acetylcysteine) proteins are plant-specific TFs which have been shown to function in
relation to plant development and also for abiotic and/or biotic stress responses. The cDNA
encoding a NAC protein was first reported as the RESPONSIVE TO DEHYDRATION 26
( RD 26) gene in Arabidopsis [100]. For example OsNAC6 expression is induced by cold,
drought, high salinity, and ABA [101]. OsNAC6 showed high sequence similarity to the
Arabidopsis stress-responsive NAC proteins ANAC019, ANAC055, and ANAC072 (RD26). It
seems that abiotic stress-responsive NAC-type transcription factors, especially the SNAC