The Role of Transcription Factors in Wheat Under Different Abiotic Stresses - Abiotic Stress: Plant Responses and Applications in Agriculture - page 376

obvious effect on TaNAC4 expression. Similar to TaNAC8 , abiotic stresses such as high salinity,
wounding, and low-temperature also induced TaNAC4 expression, suggesting a role of Ta‐
NAC4 as a transcriptional activator during biotic and abiotic stresses responses in wheat [75].
Rahaie et al. [59] have shown that NAC67 (BU672229], a putative member of the NAC family
was up-regulated during salt stress treatment. The encoded protein has a close homologue in
wheat (TaNAC69, E value=2e -15 1] [59, 78]. Xue et al. [78] demonstrated the role of TaNAC69
in response to abiotic stresses including drought, cold and ABA treatments. Expression
analysis of three highly homologous TaNAC69 genes showed that these were up-regulated
during the above-mentioned stresses, especially drought stress. Besides their up-regulation by
drought, TaNAC69 genes were expressed at high levels in the root under unstressed conditions.
This suggests that TaNAC69 genes are not just involved in drought stress, but may also be
required in normal cellular activities of roots [78]. Over-expression of TaNAC69 in transgenic
wheat leads to enhanced dehydration tolerance and improvement of water use efficiency [79].
AtNAC2 is also a NAC67 homologue in Arabidopsis which is involved in salinity stress, ABA,
ACC and NAA treatment in Arabidopsis , but AtNAC2 induction by salt stress requires the
ethylene and auxin signaling pathways. It has been shown that the expression level of AtNAC2
in roots and flowers has been higher than in other tested tissues [20, 59].
7. Enhanced abiotic stress resistance by genetic manipulation of a
transcription factor linked to crop yield improvement in the field
In the past decade numerous transgenic plant studies have demonstrated that the improve‐
ment of abiotic stress resistance can be achieved by genetic manipulation of transcription
factors. However, many resistant transgenic lines with constitutive over-expression of a
transcription factor exhibit a slower rate of growth under non-stress conditions. Field trials
have also shown that some transgenes tend to have a negative effect on grain yield under
normal growth conditions [76]. This phenomenon can theoretically result from the following
two causes: (i) genes that are induced during stress generally have a negative impact on the
growth and yield, and (ii) the energetic cost of the stress-related metabolite accumulation due
to over-expression of a transcription factor. Therefore, the expression of a transcription factor
needs to be tailored to meet the requirement for plant stress adaptation if the crop yield is
concerned. Any reduction of crop yield under normal growth conditions could potentially
override a marked yield advantage under stress.
The expression of a transcription factor can be tailored to stress adaptation by using a stress-
inducible promoter. For example, transgenic Arabidopsis plants carrying a drought inducible
promoter-driven DREB2A gene exhibit the improved drought resistance with no significant
difference in growth rate under normal growth conditions [64]. Other aspects for consideration
of minimizing the negative impact of transgene expression on growth and yield include the
appropriate expression level of the transgene and cell specificity. Recently, a root-specific
promoter has been used for driving expression of drought-upregulated transcription factors
for engineering drought tolerance [26, 60]. Most interestingly, a number of transcription factors