Genetic and molecular responses of legumes in a changing environment - Legumes Under Environmental Stress: Yield, Improvement and Adaptations - page 204

serve as molecular chaperones to regulate the transcription
and act as inhibitors of ionic solutes preventing retar-
dation of enzymatic activity. Hence, the response
demonstrated by these players is dependent upon the
type of tissue and the initial stress factor. Moreover,
these metabolites are not only directly involved in
osmotic regulation but also may result in the manipula-
tion of other pathways and players. Some additional
metabolites produced as a result of salt stress conditions
include charged molecules like glycine betaine and pro-
line, which are involved in the adjustment of water
content in the plant body (Nounjan et al., 2012; Waditee-
Sirisattha et al., 2012). Transcriptional activation of
various genes, including pyrroline 5-carboxylate syn-
thase, leads to an increase in the accumulation of
proline. This proline, via a NADP + -mediated pathway,
then helps in protecting the plant against various abiotic
stress conditions (Kim & Nam, 2012). Apart from pro-
tecting plants from salt stress, certain metabolites may
also play a role in tolerance against reactive oxygen
species. In legumes, these mechanisms are shown by
adaptive mediators including superoxide dismutase
(SOD), ascorbate peroxidase (APX) and phospholipid
hyperoxide glutathione peroxidase (PHGPX) (Morais
et al., 2012; Ramana et al., 2012; Zelinová et al., 2013;
Sultana et al., 2014). Enhanced peroxidase activity in
response to salt stress in Ammopiptanthus species has
been recently studied using expressed sequence tags
(ESTs) (M. Liu et al., 2013). Similarly, dimethylsulpho-
niopropionate (DMSP) is also regulated in salt and
accompanying osmotic stress in legumes and other
plants (Arora et al., 2012; Wdowiak-Wróbel et al., 2013).
The pathways involved in the production of these
secondary metabolites also affect the basic metabolic
pathways including amino acid and sugar synthesis and
catabolic processes, as most of the secondary metabo-
lites are either derivatives of amino acids or sugar
molecules. Hence, glycine betaine, proline, inositol and
pinnitol are all involved in the regulation of basic meta-
bolic processes in salt-stressed legumes.

the root. The physiological basis of this phenomenon
is an alteration in the number and degree of water
channels (aquaporins) or water/solute channels (aqua-
glyceroporins) that are available for osmotic exchange
(Betti et al., 2012; Bhardwaj et al., 2013). These water
channels are activated and phosphorylated by calcium-
dependent protein kinases (Osakabe et al., 2013). Other
proteins involved in water channelling include tonoplast
intrinsic protein (TIP) and plasma membrane proteins
(PM proteins) (Marino et al., 2012; Shavrukov, 2013).
The response of a plant to salt stress takes the form of
a coordinated strategy involving different tissues of the
body. Hence, communication between those different
plant parts has a basic role in initiating the genetic and
physiological changes of a response to saline stress. This
coordination mainly involves chemical substances but
in certain cases the generation and manipulation of
action potentials may solely be involved in the process.
Generally, the signalling molecules involved in cellular
communication in legumes include enzymes (kinases,
phosphatases) or plant growth hormones (auxins, cyto-
kinin, abscisic acid, ethylene). Abscisic acid (ABA), for
instance, initiates a complex genetic and signalling cas-
cade in response to salt stress in legumes (Miransari,
2013). The biosynthetic pathways (ABA), ion channels
and related catalytic pathways (abscisic acid insensitive
1, ABI1 ) and genes involved in providing resistance to
plant growth hormones (e.g. auxin resistance gene 2,
AXR2 ) are sequentially involved in coordination and
control of stress response signals. Potassium and calcium
ion channel conduction is mainly regulated by activation
by these signalling pathways. The ion channels involved
in this case may be K + or Ca 2+ conductors. Osmolytes
and a number of enzymes (oxidases, invertases and oxi-
doreductases) are also involved in signalling cascades
(Lopez-Gomez et al., 2012). These and many other
chemical entities cause signal mediation and effector
cell excitation, thereby yielding the responses observed
to a particular stress signal.
The chemicals produced as a result of a stress factor
do not necessarily act on the neighbouring tissues and
cells. In response to an osmotic stress, for instance, the
photosynthetic capacity of leaves was altered accompa-
nied by a decrease in Na + ion uptake from the roots.
The regulation of photosynthesis in response to a salt
stress is mainly attributable to the production of myo -
inositol. Moreover, indole acetic acid (IAA) assists the
generation of phosphatidylinositol metabolites that,

12.4.3 Water channelling and inter-tissue
coordination
As observed in many plant species, another mechanism
employed by most legumes is the ability to channel
water away from meristematic tissue. This is accompa-
nied by a decrease in osmotic penetration into cortical
tissue, which is marked by decreased water uptake from