Omics approaches and abiotic stress tolerance in legumes - Legumes Under Environmental Stress: Yield, Improvement and Adaptations - page 222

phenotype of an organism than mRNA or even the pro-
teins (Wienkoop et al., 2011). Out of all the 'omics'
technologies, metabolomics has proven to be the most
transversal and can also be used for studying a number
of different organisms without requiring any major
modifications (Arbona et al., 2013). The molecular phe-
notypes of plants, after exposure to abiotic stress, have
been studied by the application of metabolomics, with
the aim to discover particular patterns related to stress
tolerance. The crucial role of primary metabolites like
sugars, amino acids and Krebs cycle intermediates in
photosynthetic dysfunction and as osmotic readjust-
ment effectors was highlighted in these studies. It was
also found that secondary metabolites are rather more
specific to species and genera and thus respond to
particular stresses by generating reactive oxygen species
(ROS) scavengers, antioxidants, etc. They may also
serve as regulatory molecules (Arbona et al., 2013). It is
also speculated that secondary metabolites could be
induced by several abiotic stresses, thus effectively
becoming a mechanism of cross-protection against abi-
otic stresses. Metabolomics can also be used to screen
for the presence or absence of certain metabolites and
also for assessing gene expression in order to provide
accurate markers for the selection of stress-tolerant
plants in breeding programmes.
The metabolomics approach has the potential to
bridge the distance between the genotype and pheno-
type of an organism. Metabolic changes play a great role
in plant development and responses induced by stress,
hence metabolic information can accurately be used as a
reflection of biological endpoints. This information can
give a more precise analysis when integrated with the
transcript and protein analysis. Thus the only way to
fully understand a plant's response to stress at the gene
expression and molecular levels would be through the
integration of transcriptomics, proteomics and metabo-
lomics (Dixon, 2001).
The metabolomics approach has also been applied to
the study of legumes (Olivares et al., 2011); one example
is its use to determine the responses of M. truncatula
suspension cells to various stimuli (Bell et al., 2001).
Although a comprehensive metabolomic approach is
challenging for large-scale studies, quite a few targeted
analyses have been carried out to assess the level of
involvement of various subsets of metabolites in a
number of stress-induced responses. Although combining
the metabolomics approach with transgenetics seems to

have the potential to increase the level of intrinsic resis-
tance against various stresses and also to strengthen the
role of biotechnology in the improvement of plants,
emphasis should be placed on the fact that metabolic
pathways never function alone and are usually part of
highly complex networks (Wu & Van Etten, 2004). This
means that interfering with one metabolic pathway
could have a negative impact on others, resulting in con-
comitant detrimental traits in the modified plant. Hence
large-scale analysis is extremely important to determine
the metabolic networks that are responsible for the
growth and development of a plant under various envi-
ronmental conditions.
The first step of metabolomics experiments is acqui-
sition of the metabolic fingerprints or profile by using
a number of analytical instruments and separation
techniques based on the physicochemical characteristics
of each of the metabolites (Jogaiah et al., 2012). Since
no single technology is able to detect all the metabolites
present in an organism, the use of a combination of
techniques is required for studying the metabolome.
These techniques include gas chromatography (GC),
liquid chromatography (LC), nuclear magnetic reso-
nance (NMR), capillary electrophoresis (CE) and mass
spectrometry (MS).

13.5.1 Common bean
The common bean ( Phaseolus vulgaris L.) is one of the
most commercially important legume crops as far as
human consumption is concerned. One of the abiotic
stresses it usually faces is low phosphorus level in the
soil (Broughton et al., 2003; Singh & Schwartz, 2010). A
non-biased metabolomics approach, coupled with tran-
scriptome profiling, was used to assess the dynamics of
gene expression and the overall metabolism that
occurred in common bean plants kept under P-deficient
conditions (Hernández et al., 2007). A sum of 81 metab-
olites were detected, out of which 42 had been
differentially expressed in the P-deficient bean plants.
Stress-related metabolites like polyols were also iden-
tified in P-deficient plants' roots, thus strengthening
speculation of a function for these metabolites in P stress.

13.5.2 Lotus japonicus (model legume)
The model legume Lotus japonicus has been used in
studies focusing on drought stress. Through a non-tar-
geted metabolomic approach, attempts have been made
to study the response to drought conditions of model as