Chemistry Reference
In-Depth Information
2.5.3. Salinity
Plants growing in high salt concentrations, as we know, will suffer from osmotic stress and
take actions like closing stomata and reducing cell expansion in young leaves and root tips.
Subsequently, accumulation of ions, especially sodium (Na
+
), in the photosynthetic tissues,
will hit photosynthetic components such as enzymes, chlorophylls, and carotenoids
(Davenport et al. 2005), followed by secondary stresses (oxidative stress and nutritional
disorders) (Hasegawa et al. 2000; Chinnusamy et al. 2006).
One of the main strategies taken to improve plant salt tolerance is to re-establish ion
homeostasis by counteracting the osmotic component of the stress to avoid toxic
concentrations within the cytoplasm (Munns and Tester 2008). Recently, in Arabidopsis it
has been confirmed that the effective establishing and maintaining ion homeostasis is
mediated mainly by a Salt Overly Sensitive (SOS) signal pathway, which we refer to in Ca
2+
part. Recently, SOS4 and SOS5 have also been characterized (Mahajan et al. 2008). Similar to
SOS1
,
Arginine Vasopressin1
(
AVP1
) and A. thaliana
Na
+
/H
+
exchanger1
(
AtNHX1
) genes
contribute to ion homeostasis also (Gaxiola et al. 2001; Zhang et al. 2001). Besides these
genes, overexpression of genes encoding LEA proteins, such as the barley HVA1 (Xu et al.
1996) and wheat dehydrin-5 (DHN-5) (Brini et al. 2007), is confirmed to be able to enhance
plant salt tolerance. And regulating Lea gene expression is mediated by both ABA
dependent and independent signalling pathways, both of which use Ca
2+
signaling to induce
Lea gene expression during salinity.
Meanwhile, what do TFs do in salt stress signaling? Based on researches on Cor/Lea salinity
stress responsive genes, whose expression is mediated Ca
2+
and ABA in salt stress signaling,
it has been indicated that various upstream TFs will activate DRE/CRT, ABREs, MYC
recognition sequence (MYCRS) and MYB recognition sequence (MYBRS)
cis
-elements. Also,
with or without the involvement of ABA makes them differs from each other. On one hand,
ABRE and MYB/MYC element-controlled gene expression is ABA-dependent, which
activates bZIP TFs called AREB binding to ABRE element to induce the stress responsive
gene (
RD29A
). However, even if ABA makes these TFs function in their own regulating
ways, it has been shown that ABA-dependent and -independent TFs may also cross talk to
each other in a synergistic way to amplify the response and improve stress tolerance
(reviewed by Dortje Golldack et al., 2011).
2.5.4. Extremes of temperature
Signals of extreme temperature, from freezing to scorching, is perceived by membrane and
transduced by different transduction components results in transcription of several genes.
Cold stress directly inhibit metabolic reactions and indirectly produce harm through cold-
induced osmotic prevents the expression of full genetic potential of plants owing to its direct
inhibition of metabolic reactions and, indirectly, through cold-induced osmotic, oxidative
and other stresses such as water uptake barriers caused by chilling and cellular dehydration
induced by freeze. Cold stress, based on temperature range, are defined as chilling (<20°C)
and/or freezing (<0°C) temperatures, both of which hurt plants in different ways. The
