Genetic and molecular responses of legumes in a changing environment - Legumes Under Environmental Stress: Yield, Improvement and Adaptations - page 201

genetic players. These responses have been very well
characterized in case of salt, water and temperature
stresses and are reviewed here. Other physical con-
straints activate similar pathways to generate
morphological and physiological changes in the legume
body in order to survive the stress conditions (Hossain
et al., 2013).

Most plants regulate the salt content of their cells by
channelling these ions into the xylem tissue, resulting in
the decreased provision of salt to other plant parts.
Meristematic cells are not directly connected to the
plant's vascular tissues and hence are not directly
exposed to the hyperionic environment (Asensio et al.,
2012). Moreover, in certain plants the response to high
salt concentrations may take the form of closure of the
stomata to reduce the transpirational and ion fluxes
through the plant body. This mechanism is, however, a
long-term survival strategy as reducing photosynthesis
and respiration can help the plant to thrive over a pro-
longed time period (Sabra et al., 2012). Another, more
commonly observed response is the accumulation of
ions in the older leaves of the plants. These leaves, which
ultimately detach from the plant body, serve as sinks for
salt flushed from the plant body. Other morphological
and physiological mechanisms adapted by legumes to
survive salt stress include alterations of growth rate and
transpiration rate, changes in cell structures and alter-
ation of cell-cell interactions (Shabala et al., 2012).
Legumes are glycophytes and they require less than
125 ppm of salt in the soil (Horie et al., 2012). In
comparison, halophytes ( Mesembryanthemum spp.,
Attalea spp.) can grow only when larger amounts of salt
are available to the plant. The mechanisms by which
these extremophytes survive and propagate can provide
valuable insights to generate resistance among the gly-
cophytes to survive in high-salt growth conditions.
Halophytes tend to exhibit intracellular compartmental-
ization to segregate the salt from other cellular contents
(Y. Yang et al., 2012). This results in the protection of the
metabolic machinery of the cell. Moreover, certain hal-
ophytes have evolved specialized structures, known as
glands, to extrude the salt from the plant (Tan et al.,
2013). The segmentation of the intracellular content
into two parts, an organic portion comprising the cel-
lular structures and a salt-rich inorganic part, prevents
damage to cell organelles and other cellular entities.
Halophytes, additionally, show a high degree of salt tol-
erance in comparison to the glycophytes (Krauss & Ball,
2013). The exact origin of these mechanisms, whether
constitutive or adaptive, is still under investigation. At
the cellular level, halophytes tend to show large vacuol-
ization, greater quantities of ions (sodium, chloride,
potassium, hydrogen) and alteration of cellular turgor
pressure to thrive in saline growth conditions (Dasgupta
et al., 2013; Elkahoui et al., 2013). These adaptive

12.4 Genetic and molecular responses
to salt stress

Among the various physical stress factors, soil salinity is
considered to be the most significant in terms of the
resultant losses. These damages are of numerous types,
varying from growth retardation to plant death.
Legumes have developed a number of physiological and
biochemical mechanisms to respond to this abiotic stress
factor (Tuteja et al., 2012). These responses are now very
well characterized owing to recent developments in
plant biotechnology. Modern molecular techniques
have helped in yielding important information about
the subcellular genetic and molecular mechanisms
involved in responding to salt stress (Grieve et al., 2012).
Saline-stressed environments affect plant health by
causing disruption of osmotic and ionic homeostasis
(Shavrukov, 2013). Legumes, therefore, have to respond
by evolving specialized mechanisms to develop toler-
ance against the salt stress.
An elevated salt content in the plant body is the
consequence of uptake of the salt-rich water to carry out
transpiration, photosynthesis and other metabolic
processes (Aroca & Ruiz-Lozano 2012; Geissler et al.,
2013). These pathways are, therefore key to controlling
the salt and water intake of a plant. If not regulated ade-
quately, the salt and water intake can irreversibly
damage the legume's health. At the cellular level,
increased salinity results in hyperionic and hyperosmotic
pressure in the plants (Q. Liu et al., 2013; Rasool et al.,
2013). High levels of sodium and chloride ions in the soil
cause retardation in the growth of legumes. Dysregulation
of cell cycle, metabolic and biochemical disorganization,
oxidative stress and reduced nutrient uptake are the
main events characterizing the legume under saline
stress (Krasensky & Jonak, 2012; Suzuki et al., 2012).
Legumes might respond to salt stress in various ways.
The main aim of all the strategies is to direct the salt
away from the actively growing meristematic tissue.