Genomics Applications to Salinity Tolerance Breeding in Rice - Translational Genomics for Crop Breeding: Abiotic Stress, Yield and Quality - page 32

cultivation intensifies and spreads to ever more
marginal areas, the impact of salt stress contin-
ues to grow. Rice ( Oryza sativa L.) is considered
a relatively salt-sensitive species. However, con-
siderable genetic variation for salinity tolerance
exists in rice (Akbar et al. 1972; Moradi et al.
2003), and the interest in rice breeding for salt
tolerance is gradually being renewed as a result
of the latest developments in modern breeding
tools and confidence in exploiting this variation.
The effects of salinity on plant growth are
varied, but can be broken into a few broad
categories: toxicity of the Na + ion, osmotic
effects of high salt concentrations, toxicity of
the Cl − ion, secondary effects on mineral nutri-
tion, and secondary effects on plant growth
(Ismail et al. 2007). First and foremost is tox-
icity due to the Na + ion. Excess Na + has a
general inhibitory effect on metabolism, and
many tolerance mechanisms aim at either reduc-
ing the uptake of Na + under conditions when
it is in excess or sequestering the Na + in areas
where its inhibitory effects are less pronounced.
The most well-known tolerance mechanisms fall
into this category. For example, the SOS1 pro-
tein acts as an Na + /H + antiporter, using the
H + gradient established across the cell plasma
membrane to actively pump Na + that enters the
cytoplasm out of the cell (Zhu et al. 1998; Shi
et al. 2000, 2002). Likewise, a related Na + /H +
antiporter, NHX1, actively pumps cytoplasmic
Na + into the vacuole, sequestering it where its
effects on enzyme activity are less pronounced
(e.g., Fukuda et al. 2004, 2011). Both of these
pumps are tied to transmembrane H + concen-
tration (pH) gradients. These gradients are in
turn established and maintained in large part
by the H + -translocating ATPase and pyrophos-
phatase pumps. These energy pumps use chem-
ical energy from phosphate-phosphate bonds to
actively transport H + against its concentration
gradient across the membrane, maintaining this
gradient and thus energizing the aforementioned
Na + /H + antiporter pumps (e.g., Zhu 2003).
Another class of Na + transporters that has
received considerable attention is the HKT gene

family, as two members of this family ( HKT1;5
and HKT1;4 , Platten et al. 2006) are to date the
only transporters yet positively identified as the
cause of naturally occurring variation in salin-
ity tolerance in agriculturally important species
(Ren et al. 2005; Byrt et al. 2007; Huang et al.
2006). This family actually facilitates the trans-
port of Na + into the cell. This may appear to
be a counter-adaptive feature, and indeed when
members of the family are overexpressed they
do result in increased Na + uptake and usually
result in reduced salinity tolerance (Møller et al.
2009; Platten et al. unpubl.). However, the wild-
type genes typically show extremely tissue- and
treatment-specific expression profiles (Ren et al.
2005), and the action of HKT1;4 and HKT1;5
in particular seems to involve sequestering Na +
in xylem parenchyma cells, where it presumably
has fewer toxic effects than in photosynthetic leaf
tissue. Both the HKT and NHX mechanisms are
related to the number and volume of cells; con-
sequently, varieties with increased biomass have
greater reservoirs for sequestration of Na + , and
are known to have comparatively greater salt tol-
erance (Yeo et al. 1990; Flores et al. 2005).
The second broad effect of salinity is the sim-
ple osmotic stress imposed by high salt concen-
tration (Munns et al. 1995; Bahaji et al. 2002;
Castillo et al. 2007). Even if salinity concen-
trations change gradually (avoiding an osmotic
shock), it still becomes increasingly difficult for
plants to extract water and therefore nutrients as
the salt concentration progressively increases in
the soil solution. Nutrient imbalances can also
occur due to displacement of some elements,
such as potassium, under salt stress (Peng and
Ismail 2004; Netondo et al. 2004; Nemati et al.
2011). Osmotic stress is not a consequence of the
NaCl per se, but simply the high concentration of
solutes; NaCl just happens to be the most com-
mon solute that occurs in excess. Tolerance of the
osmotic component of the stress is conferred by
increasing the osmotic potential of the internal
fluids in the plant (Ismail et al. 2007; Nemati et al.
2011). This is achieved by increasing the con-
centration of various “compatible solutes” that