The Role of Transcription Factors in Wheat Under Different Abiotic Stresses - Abiotic Stress: Plant Responses and Applications in Agriculture - page 368

referred to as regulon [49]. Transcription factors (TFs) provide a possibility for plants to
overcome and respond to biotic and abiotic stresses and are also involved in modulating
developmental processes [45, 83].
Until now, several major regulons involved in response to abiotic stress have been identified
in Arabidopsis . Recent studies have demonstrated that DREB1/CBF, DREB2, AREB/ABF, and
NAC regulons have important functions in response to abiotic stresses in rice [49]. Significant
advances have been made in recent years towards identifying regulatory genes involved in
stress responses which confer abiotic stress tolerance in plants [18].
In this review, we provide an overview of the functions of different TF family members with
particular emphasis on the role of bZIP, bHLH, WRKY, MYB, and NAC TFs and their in‐
volvement in abiotic stress responses in wheat.
2. bZIP transcription factors
Basic region/leucine zipper (bZIP) TFs possess a basic region that binds DNA and a leucine
zipper dimerization motif. The bZIP domain comprises two structural features located on a
contiguous α-helix: a basic region of about 16 amino acid residues with a nuclear localization
signal, an invariant N-x7-R/K motif to contact the DNA as well as a heptad repeat of leucines
or other bulky hydrophobic amino acids located exactly nine amino acids towards the C-
terminus, to create an amphipathic helix. When binding to DNA, two subunits adhere through
interactions of the hydrophobic sides of their helices, which create a superimposing coiled-coil
structure (zipper). The capability to form homo- and heterodimers is governed by the electro‐
static attraction and repulsion of polar residues adjacent to the hydrophobic interaction surface
of the helices. Proteins with bZIP domains are present in all eukaryotes analyzed to date and
bZIP proteins typically bind to DNA sequences with an ACGT core. Plant bZIPs bind to the
A-box (TACGTA), C-box (GACGTC) and G-box (CACGTG), but there are also reports of
nonpalindromic binding sites for bZIPs [24]. Based on the sequence similarities of common
domains, 75 bZIP protein members have been divided into ten subgroups in Arabidopsis [24,
35]. In plants, bZIP transcription factors present a divergent family of TFs which regulate
processes including light and stress signaling, seed maturation, pathogen defense, and flower
development [24, 59].
The plant hormone abscisic acid (ABA) plays an essential role in maturation and germination
in seeds, as well as mediating adaptive responses to abiotic environmental stresses. ABA
induces the expression of many genes, including late-embryogenesis-abundant (LEA) genes.
HVA1 is one of the LEA genes whose expression is affected by ABA. Analysis of the interplay
between ABA and TaABF1 as a bZIP factor in the aleurone cells of imbibing wheat grains by
Keyser [32] indicated that the two are not additive in their induction of the HVA1 promoter.
It has been shown that TaABF1 may undergo an ABA-induced posttranslational modification.
However, the lack of synergism between ABA and TaABF1 overexpression in HVA1 induction
does not support this conclusion. These findings indicate that the branch of ABA signaling
leading to HVA1 is more complex [32].