Environmental Engineering Reference
In-Depth Information
balancing of Na
+
and K
+
. Most recently, the AtHKT1 was demonstrated to take charge of the
resorption of Na
+
from xylem to reduce the amount of Na
+
reaching shoot in
Arabidopsis
(Davenport et al. 2007). In durum wheat, TmHKT1 is correlated with Na
+
exclusion and a
high K
+
/Na
+
rate in leaves (Huang et al. 2006). In rice, OsHKT1 appears to have the same
function as AtHKT1 (Horie et al. 2007), yet much more work is needed to clarify the multiple
functions of the gene family.
Recently, maintaining intracellular high cytosolic K
+
/Na
+
ratio is accepted as the key
determinant in salinity tolerance (Munns and Tester 2008). K
+
possesses a significant
important role in activating enzymes, regulating osmotic pressure, regulating stoma
movement and balancing turgor (Chérel 2004). In wheat cells, the content of K
+
and Na
+
are
kept at 150 mM at 30 mM, so the K
+
/Na
+
ratio is approximately 5 (Carden et al. 2003).
Indeed, increasingly reports show that it should be the cytosolic K
+
/Na
+
ratio that determine
the plant salt tolerance. Neid and Biesboer put forward that the low level of KNO
3
could
alleviate NaCl-induced stress (Neid and Biesboer 2005). Zheng et al. further acclaimed that
the NaCl stress symptoms can be alleviated by simultaneously applied KNO
3
and the optimal
K
+
/Na
+
ratio should be 100:16 for both the salt-tolerant and the salt-sensitive cultivar (Zheng
et al. 2008). Similarly, overexpression of LeENH1 (a tomato enhancer of SOS3-1) in tobacco
enhanced salinity tolerance by excluding Na
+
from cytosol and retained high K
+
levels in the
cytosol (Li et al. 2013). Studies on NHXs also attest to this point. Recently, mineral analysis
of the transgenic plants with over expressed NHXs seems to indicate that the salt resistance is
offered by a high K
+
/Na
+
ratio through K
+
retention rather than Na
+
exclusion (Hueras et al.
2013). In Arabidopsis, the phenotype of
sos
mutants may be alleviated by an exogenous
supply of K
+
(Zhu 2003). Compared to Arabidopsis, the
Thellungiella
possess stronger adapt
ability to saline soil. One of the reasons is that the cytosolic K
+
concentration in
Thellungiella
is higher. Leidi et al. testified that the transgenic tomato with high resistance depended on
cytosolic K
+
homeostasis rather than on vacuolar Na
+
accumulation (Leidi et al. 2010).
O
XIDATIVE
S
TRESS AND
A
NTIOXIDANT
D
EFENSE
R
ESPONSES
Besides osmotic stress and ion toxicity, salinity also can induce the generation of ROS
and lead to oxidative stress. ROS derives from the excitation of O
2
including singlet oxygen
(
1
O
2
), superoxide anion (O
2
-
), hydrogen peroxide (H
2
O
2
), and hydroxyl radical (OH·)
respectively (Yan et al. 2013). Unlike atmospheric oxygen, ROS has the ability to oxidize the
various cellular components unrestrictedly leading to the oxidative destruction to plant cells
(Mittler 2002). Actually, ROS are the common byproducts of the metabolic pathways such as
photosynthetic electron transport, respiration and photorespiration processes (Ahmad et al.
2010). For example, the salt stress limits the supply of CO
2
causing over-reduction of
photosynthetic electron transport chain and a vast production of ROS (Türkan and Demiral
2009). The overmuch ROS can oxidize cellular components, hinder metabolic activities and
affect organelle integrity (Suzuki et al. 2012). The destructive effect of ROS is the lipid
peroxidation estimated by the content of MDA (Hernández and Almansa 2002). Generally,
under normal conditions, the production and eliminate of ROS are in an equilibrium state
(Jaleel et al. 2009). However, under salt stress conditions, the generation rate of ROS
