The Role of Transcription Factors in Wheat Under Different Abiotic Stresses - Abiotic Stress: Plant Responses and Applications in Agriculture - page 374

expression was induced following ABA treatment and gradually increased expression until
72 h after salt or cold treatment. In contrast, PEG treatment lead to an early expression peak
at 6 h after treatment, and then gradually decreased [6].
Zhang et al. [84] identified TaMYB32 as a salt stress-related gene, during the bulk sequencing
of full length cDNAs in wheat ( T. aestivum ). The sequences of TaMYB32 were cloned from
different varieties of hexaploid wheat and its diploid ancestors. Sequence analysis indicated
that two types of sequences existed in the diploid ancestors and four in the hexaploid wheat.
One of the sequences was identical in both diploid and hexaploid wheat. This implied that
TaMYB32 was conserved during the evolution of wheat. The genomic TaMYB32 sequences
proved to be non-intron genes after comparing with their cDNA sequences. TaMYB32 was
mapped onto the homoeologous group 6 of wheat using the electronic mapping strategy, and
two copies of the gene were found in each genome of hexaploid wheat. Homologous analysis
found that TaMYB32 had a similarity with some R2R3-MYB proteins from rice ( Oryza sativa
L.) and maize ( Zea mays L.) as high as 72.4% and 73.7%, respectively. The expression of
TaMYB32 in roots, stems, leafs, pistils, and anthers in wheat, was induced by salt stress [84].
6. NAC transcription factors
The first sequenced cDNA encoding a NAC protein was the RESPONSIVE TO DEHYDRA‐
TION 26 ( RD2 6] gene in Arabidopsis [80]. The NAC domain was characterized based on
consensus sequences from Petunia NAM and Arabidopsis ATAF1/2 and CUC2 proteins [1].
Many NAC TFs, including Arabidopsis CUC2, play important roles in plant development. Some
NAC genes mediate viral resistance [48], while others are up-regulated during wounding and
bacterial infection [11]
NAC domains mediate transcriptional regulation of various biological processes by forming
a helix-turn-helix structure that specifically binds to the target DNA [1]. NAC TFs are quite
diverse in their C-terminal sequences which possess either activation or repression activity.
More than 100 NAC genes have so far been identi fi ed in Arabidopsis and rice which can be
categorized into six major groups. Phylogenetic analyses suggest that these were already
present in an ancient moss lineage. NAC TFs play a range of important roles during plant
development and abiotic stress responses [48]. Many plant growth and developmental
processes are regulated by NAC TFs, including shoot apical meristem formation, lateral root
development, senescence, cell wall development, and secondary metabolism. A large number
of NAC TFs are also differentially expressed in responses to abiotic and biotic stresses [74] and
transgenic Arabidopsis and rice plants overexpressing stress-responsive NAC genes have
displayed improved drought tolerance. These studies indicate that stress-responsive NAC
transcription factors have important roles for the control of abiotic stress tolerance and that
their overexpression can improve stress tolerance via biotechnological approaches [48].
Interestingly, rice plants overexpressing OsNAC6 possessed enhanced tolerance to abiotic
(dehydration, high salinity) as well as biotic stresses (blast disease) [47]. The Arabidopsis NAC
TF, ATAF2, is induced by salicylic acid (SA) and methyl jasmonate (MeJA) treatments, and is