Genetic and molecular responses of legumes in a changing environment - Legumes Under Environmental Stress: Yield, Improvement and Adaptations - page 202

mechanisms result from the interplay of genetic and
molecular events, exhibited as cellular and organ-level
changes in the plant body.
Much like other plants, legumes respond to salt stress
by initiating a signalling cascade that involves the per-
ception of a stress factor as a stimulus followed by the
activation of certain molecular pathways to activate and
respond specifically to the physical constraint (Mantri
et al., 2012; Miransari 2012). Figure 12.1 represents the
series of events that are involved in responding to salt
stress. The response may be exhibited directly against
the stressor or by the activation of certain secondary
pathways that ultimately results in the development of
tolerance against the particular stressor. Production of
metabolites, expression of proteins and regulation of
metabolic pathways are the main secondary events
observed in response to salt stress in legumes (Arbona
et al., 2013). Regulators of the secondary pathways
mainly include the transcription factors that regulate
the direct effectors of the salt stress response elements.
In case of a saline-stressed environment, legumes tend
to produce specific response elements targeted either at
decreasing salt intake or regulating salt-rich water
intake. Another class of response elements, however,
may not be directly involved in responding to a
particular stress factor, but may help in coordination
and regulation of responses to the external stimuli
(Bayuelo-Jiménez et al., 2012). The main changes
observed - ionic regulation, metabolite production,

water channelling and genetic regulation - are of prime
importance in case of salt stress, and are discussed
further here.

12.4.1 Ionochemical imbalance
An increase in the salt content in the soil results in a
corresponding increase in the ionic levels of sodium,
chlorine, calcium, potassium and hydrogen in the legume
body (Li et al., 2012; Lindberg et al., 2012; Tahkokorpi
et al., 2012; Turner et al., 2013). This ionic imbalance leads
to irregularity in the transmembrane exchange of ions,
nutrients and energy. The main regulators of intercellular
transport, ATPases, pyrophosphatases and ABC trans-
porters, are affected by the ionic overload (Shabala &
Munns, 2012; Yao et al., 2012; Banasiak et al., 2013).
Additionally, transport across the organellar membranes
is also altered. The changes in H + ions are coupled to the
alteration in the active transport mechanisms across the
vacuolar tonoplast membrane. The thermodynamic and
electrophoretic flux is re-established only once the salt
stress conditions are relieved. Changes in the chemical
and environmental factors lead to the modified expres-
sion of the transport proteins and their regulators, for
instance ATPases and pyrophosphatases. Among the ge-
netic factors responsible for the control of ion transport
in salt-stressed conditions are the Salt Overly Sensitive
( SOS ) genes, primarily SOS3 . The intracellular and trans-
membrane transport of Na + , K + , H + and Ca 2+ ions are all
regulated by this genetic locus (Yu et al., 2012). The SOS

Cell membrane

Signalling cascade

In ux

Transcription

Eff ux

Nucleus

Ion channels

Ion channels

Symporters

Symporters

Vacuole

Antiporters

Cl -
Ca ++

Antiporters

Na+

Secondary metabolites
Proline
Glycine
Trehalose

Figure 12.1 Series of cellular changes in response to saline stress.