The Role of Transcription Factors in Wheat Under Different Abiotic Stresses - Abiotic Stress: Plant Responses and Applications in Agriculture - page 371

members have a role in hormone signaling such as OsWRKY71 and OsWRKY51 which were
ABA-inducible and could repress GA signaling transduction in aleurone cells.
Wu et al. [73] obtained sequences for 15 wheat cDNAs encoding putative WRKY proteins.
Phylogenetic analysis showed that the 15 WRKY genes classified to three major WRKY groups
and expression analysis revealed that most genes were highly expressed in leaves. A few of
them such as TaWRKY10 are expressed in the crown intensively and several genes are strongly
up-regulated during the senescence of leaves. Eight isolated genes were responsive to high or
low temperature, NaCl or PEG (polyethylene glycol) treatment. In addition, differential
expression was also measured between wheat hybrids and its parents, and some genes were
more responsive to PEG treatment in the hybrid. The authors concluded that the differential
expression of these WRKY genes in the hybrid might contribute to heterosis by improving the
stress tolerance in hybrids [73].
Orthologous genes are subjected to similar transcriptional regulation by orthologous TFs,
suggesting that the terminal stages of signal transduction pathways leading to defense are
conserved, implying a fundamental role of pathogenesis-related genes, such as PR4 genes in
plant defense. This suggests that diversi fi cation between monocot and dicot plants has most
likely occurred after the differentiation of WRKY functions. Proietti et al. [56] reported the
ability of TaWRKY78 to bind to a W-box-containing region of the wPR4e promoter. Transient
expression assays of TaWRKY78 and AtWRKY20 showed that both TFs are able to recognize
the cognate cis-acting elements present in the wPR4e and AtHEL promoters [56].
Expression analysis by reverse northern blot hybridizations of a group of putative wheat
WRKYs showed that WRKY1 (CN009320] and WRKY2 (CJ873146] were up-regulated in a
stress-tolerant genotype. AtWRKY75 (E value=3e -4 2] is a homologue from Arabidopsis for
WRKY1 which is up-regulated in response to phosphorous deficit stress [15, 17, 59]. This gene
also acts as positive regulator in defense responses to pathogens. Functional characterization
of the WRKY2 homologue in Arabidopsis , AtWRKY33 (E value=4e -1 8], showed that its expres‐
sion in response to salt, mannitol (simulated drought) treatment and cold stress in shoots and
roots increased but this gene was down-regulated during heat stress. It also appears that its
expression is independent of SOS signaling and only partly dependent on ABA signaling, but
forms part of plant responses to microbial infections [27, 40, 59].
5. MYB transcription factors
MYB TFs form one of the largest transcription factor families in plants. More than 200 MYB
proteins are encoded in genomes of Arabidopsis and rice. MYB TFs contain one to four imperfect
repeats [50-53 amino acids) in their DNA-binding domain (MYB domain) near to the N-
terminus and are classified into four subfamilies [58, 83].
According to the number of repeat(s) in the MYB domain: 4R-MYB has four repeats, 3R-MYB
(R1R2R3-MYB) has three consecutive repeats, R2R3-MYB has two repeats, and the MYB-
related type usually, but not always, has a single repeat [16, 28, 61]. Typically, the MYB repeat