Salt stress and leguminous crops: Present status and prospects - Legumes Under Environmental Stress: Yield, Improvement and Adaptations - page 44

part delivers the selectivity required for certain groups
of metabolites. Among all analysers that can be used
with the separation techniques mentioned, the most
popular in metabolomics are MS analysers, particularly
those providing accurate mass measures such as hybrid
quadrupole/time-of-flight (TOF) or orbitraps. However,
more targeted techniques are still extensively used for
the quantitation of several plant metabolites and
hormones due to their enhanced sensitivity and speci-
ficity (Arbona et al., 2013).
Comparative targeted metabolic profiling shows
marked differences in the metabolite composition bet-
ween salt-sensitive and salt-tolerant soybean varieties.
Principal components analysis clearly confirmed that it
is possible to use secondary metabolites, for example,
isoflavones and saponins, to discriminate between
closely related soybean genotypes. Genistin and group B
saponins were recognized as the key secondary metabo-
lites connected with salt tolerance. These individual
metabolites may provide additional insight into salt tol-
erance and adaptation of plants (Wu et al., 2008). The
future challenge is the integration of transcriptomics,
proteomics and metabolomics within a single frame-
work that will allow a better understanding of how
plants respond to a changing environment.

Many agriculturally important legumes are salt
sensitive. However, there is variability in the salt toler-
ance of the legumes within and between species, and of
the rhizobia and their functional symbiosis with the
plant. A number of curious physiological and molecular
components in the salt response pathways of legumi-
nous crops have been exposed. They provide crucial
information for testing new hypotheses in ROS scav-
enging, ion/osmotic homeostasis, protein synthesis/
turnover, cell structure modulation and energy metabo-
lism. Despite the significant progress, large gaps remain
in our knowledge of transmemberane ion transport,
cellular compartmentalization, sensors/receptors in
signal transduction and metabolites in energy supply.
Therefore, further physiological and molecular studies
are needed to focus research in this area. Based on the
results presented here, it also seems that the responses
to salinity of nitrogen fixation closely follow biomass
production in the most tolerant legumes. Symbionts like
rhizobia and arbuscular mycorrhizal fungi are generally
found to be more salt tolerant than their host plants.
Functional symbiosis sometimes makes an important
difference in the salt tolerance and productivity of
plants. Therefore this line of research should be
continued to develop agriculturally important legumes
with improved salt tolerance.
More systematic studies using both forward and
reverse genetics tools in leguminous crops are required.
Reverse genetics study of the homologous genes in legu-
minous crops would provide information about the
similarity and differences of the functions of the trans-
porters important for salt tolerance. Forward genetics
screening for mutants that are sensitive or tolerant to salt
stress would classify novel genes and/or salt-tolerance
mechanisms unique to leguminous plants. It can be
envisaged that more and more genome sequences of
leguminous crops will be available in the near future
along with rapid developments in sequencing technol-
ogies and falls in the associated costs. Genome sequence
information will also be very beneficial in identifying
the natural variant genes bestowing salt tolerance in
salt-tolerant lines within a species. Such information on
salt tolerance will prove important for molecular
breeding of salt tolerance in leguminous crops. The link
with functional genomics may also enable identification
of candidate genes as tools to further interpret the
mechanisms involved in the efficiency of rhizobial sym-
biosis and its adaptation to salt stress.

2.6 Conclusions and future prospects

Leguminous crops are often subject to various environ-
mental stresses that can severely affect plant
development and eventually productivity. Production of
stress-tolerant cultivars of leguminous crops is a press-
ing need today because of the growing threat to crop
productivity of global warming and changing climatic
patterns, in addition to the anticipated human
population explosion in the near future. Plant salinity
tolerance is a multifaceted physiological trait involving
adaptations to signalling and metabolic networks.
Studies on salt stress responses and tolerance in many
crop plants have progressed rapidly in recent years.
Morphological, physiological, biochemical, genetic and
genomic analyses have made important discoveries of
salt-responsive genes, proteins and metabolites involved
in different cellular pathways. In this review, we have
attempted to summarize significant contributions
related to salt stress responses and tolerance in legumi-
nous crops, particularly grain legumes.