Genetic and molecular responses of legumes in a changing environment - Legumes Under Environmental Stress: Yield, Improvement and Adaptations - page 203

stress compensation mechanism has been studied in a
number of legumes including chickpea and Medicago
species (Molina et al., 2011; Samineni et al., 2011;
Arraouadi et al., 2012). Other genes implicated in alter-
ation of transmembrane ionic transport (especially Na +
ions) include the sodium-hydrogen exchangers and
superoxidase dismutase gene families (Asensio et al.,
2012; Hedrich et al., 2012). The increased transporter
activity leads to alterations in metabolic and ionic poten-
tials, changes in protein morphology and regulation of
transcription.
The genetic regulation of transport proteins results in
alteration of the rate of ionic exchange across the plasma
membrane. Soon after the induction of salt stress, the
influx of Na + ions is coupled with the rapid exit of Cl −
ions from the cell. The movement is usually attributed
to the use of certain anion transporters. Another hypo-
thesis involves the symport of H + /Cl − ions across the
plasma membrane. Cations like Na + and K + also com-
pete for their entry into the cell, using the same cellular
transporters (Nilius & Honoré, 2012). These transporters
can be divided into two functional categories depending
upon the degree of specificity for the cations. The non-
selective transporters utilize the same proteins for the
influx of more than one kind of ion. The ion-selective
transporter proteins, on the other hand, like the K + ion
transporter proteins, are involved mainly in the trans-
membrane exchange of a single ionic entity. Potassium
ion transport proteins, for instance, include K + channels
and transporters, Na + /K + symporters and voltage-gated
ion channels, among various types (Takahashi & Tateda,
2013). These transport mechanisms, either alone or in
combination, contribute significantly to the development
of tolerance against the salt-stressed environment in
legumes. The increased demand for K + ions in comparison
to Na + ions results in the intracellular accumulation of
various ions even in saline-stressed environments (Thiel
et al., 2013).
Apart from manipulation of transmembrane ion
transport mechanisms, ionic sequestration into the
plant vacuoles results in the ability to tolerate salt stress
more effectively. Osmotic adjustment within the cell is
carried out by the regulation of different ions, chiefly
Na + and Cl − ions (Jiang et al., 2013). Induction of cyto-
plasmic acidification and vacuolar alkalization follows
the ionic modification of the cellular environment.
Overexpression NHX genes in legumes (Na + and H +
exchange) are responsible for these stress-compensation

mechanisms (Bassil et al., 2012; Roy & Tester, 2013).
Studies on soybean have been especially helpful in deci-
phering the molecular mechanisms involved in NHX-
mediated stress compensation (Ha et al., 2013; Roy &
Tester, 2013). Similarly, solute regulation, especially, in
the form of alteration of calcium ion levels, leads ulti-
mately to the generation of signal transduction and
adaptation to the salt stress.

12.4.2 Metabolite production
An essential compensatory mechanism adapted by
many legume species is the production of certain metab-
olites or response elements that can prevent, neutralize
or counteract the physical constraints. The salt and
drought response elements usually encountered are
involved in the regulation of osmotic balance and,
therefore, are referred to as osmolytes. They help in sus-
taining the water levels within the cell. Table 12.1 reviews
the various metabolites produced in plants in response
to a saline stress. Among the different metabolites,
sugars (sucrose, trehalose and fructans) and sugar
alcohols (inositols and glycerols) are the main players
for osmoregulation in legumes (L. Yang et al., 2012;
Joshi et al., 2013; Lyu et al., 2013). Sugars and sugar
alcohols complex with proteins to act as osmoprotec-
tants by lining the cellular surfaces. Moreover, they may

Table 12.1 Metabolites produced by legumes in response to
salt stress.

Molecules Possible role(s) References

Amino acids
Ectoine
Proline

Osmoprotection Saxena et al., 2013

Redox control,
respiration regulation

Hayat et al., 2012

Carbohydrates
Trehalose
Polyols

Osmoprotection Redillas et al., 2012

Osmoregulation,
redox control

Palma et al., 2012

Proteins
Dehydrins
Chaperones

Dessication control,
redox control

Galani et al., 2013

Protein folding, heat-/
cold-/salt-shock
proteins

Nakaminami et al.,
2012

Amines
Glycine betaine
Dimethyl sulphonium

Osmoprotection Chen & Murata, 2011

Protein protection Parvaiz & Satyawati,

2008