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former leads to slow biochemical reactions related to enzymes and membrane transport
activities, while the latter forming ice crystal can cause membrane system disruption
(Chinnusamy et al. 2007).
Numerous TFs in cold stress circumstances have been identified in Arabidopsis, homologs
of which have been reported in other plants also. Significant progress has been made in the
past decade in elucidating the transcriptional networks regulating cold acclimation. Firstly,
AP2/ERF family TFs, CBFs play essential role in controlling genes in phosphoinositide
metabolism, osmolyte biosynthesis, ROS detoxification, hormone metabolism and signalling
(Lee et al. 2005). They can bind to
cis
-elements in the promoters of COR genes to activate
gene expression. Earlier it was proved that
DREB1A/CBF3
,
DREB1B/CBF1
and
DREB1C/CBF2
genes, lying tandemly in the Arabidopsis genome, are induced by cold but
not by dehydration or high salinity (Shinwari et al. 1998).
CBF
genes showed high
expression under low temperature treatment and the transcript was detectable after 30 min
exposed to 4 °C and reached maximum expression at 1 h (Medina et al. 1999). However this
time-linking phenomenon differs from various plants. In rice, detecting
OsDREB1A
and
OsDREB1B
transcript might need to wait 40 min after cold exposure. By the way, CBF
pathway might also have a crucial role in constitutive freezing tolerance (Hannah et al.
2006).
Moreover for many studies, emphasize has been laid on the ICE-CBF-COR transcriptional
cascade. In Arabidopsis, ICE1 (Inducer of CBF Expression1), a MYC-type bHLH TF, can
bind to MYC recognition elements in the
CBF3
promoter affecting its expression during cold
acclimation (Chinnusamy et al. 2003). Besides being the inducer of
CBFs
transcription, ICE1
is also a transcriptional inducer of ZAT12, NAC072 and HOS9 in Arabidopsis (Benedict et
al. 2006). Furthermore, by studying on ice1 mutation under cold stress, the genes in calcium
signaling, lipid signaling or encoding receptor-like protein kinases are found to be affected
(Lee et al., 2005). In conclusion, there do exists network between these components in cold
signaling. Constitutive expressed ICE1 is actived through sumoylation and phosphorylation
induced by cold stress, which then induce the transcription of
CBFs
and reprime
MYB15
. For
CBFs
expression, CBF2 acts as a negative regulator of
CBF1
and
CBF3
expression.
Meanwhile, the expression of
CBFs
is negatively regulated by upstream TF MYB15 and
ZAT12(Chinnusamy et al. 2007).
3. Conclusion
Thanks to so much efforts made by researchers in various fields, we have a pretty clear idea
about how to develop our researching methods in abiotic stress field. That is exactly what
we should concentrate on in the future. For plants' reactions to different kinds of stress, we
must make more efforts in taking measures in focusing on systematic studies which so far
can be taken as the best way to figure out what plants will do under certain circumstances.
As we all know, it is of incredibly great importance to understand more about abiotic stress
which impacts a lot on plants, which will not only change our understanding of current
environment we live, but also bring a plenty of benefits for improving human beings living
