Omics approaches and abiotic stress tolerance in legumes - Legumes Under Environmental Stress: Yield, Improvement and Adaptations - page 225

salt tolerance in Arabidopsis thaliana . Another recent
attempt to transform soybean plants involved over-
expression of certain transcription factors (e.g. TF
DREB1A) that regulate gene expression involved in
response and defence against abiotic stresses. As com-
pared to the non-transformed plant, the transformed
DREB (dehydration response element-binding protein)
plants responded with better growth in terms of number
of seeds and pods (Rolla et al., 2013).

Arabidopsis vacuolar gene AVP1 enabled greater drought
and salt tolerance in transgenic peanuts simultaneously
(Qin et al., 2013). The vacuolar phosphatase protein
encoded by the gene acts as a proton pump and gen-
erates a chemical proton gradient across vacuolar
membranes. The proton pump increases the activity of
other secondary transporters such as the Na + /H + anti-
porter, which in turn helps to amplify salt tolerance in
the transgenic peanut (Blumwald, 2000; Gaxiola et al.,
2002). Abiotic stress tolerance was enhanced in trans-
genic peanuts transformed with a small GTP-binding
protein AhRab7 (AhRabG3f). The transcription level of
the gene was positively correlated with other environ-
mental stresses such as salt and cold, indicating its
involvement in the abiotic stress tolerance mechanism
(Kumar et al., 2013).

13.7.2 Chickpea
The major environmental or abiotic constraint on the
development and productivity of the chickpea is drought,
and the subsequent oxidative stress experienced by the
plant reduces yield potential by 40 to 50% (Ahmad et al.,
2005). Therefore, intense research has been directed at
mechanisms governing abiotic stress tolerance and
adaptation in chickpea. The potential effect of the phyto-
oxylipin pathway on drought tolerance of plants has
been investigated (De Domenico et al., 2012). The
expression of certain genes involved in oxylipin metab-
olism was studied, and the early activation and
overexpression of a specific lipo-oxygenase (Mt- LOX1 ),
two hydroperoxide lyases (Mt- HPL1 , Mt- HPL2 ), one
allene oxide synthase (Mt- AOS ) and oxophytodienoate
reductase (Mt- OPR ) were found to be positively corre-
lated with significant levels of oxylipin metabolites
leading to increased levels of jasmonates. These com-
pounds, in turn, play a vital role in the tolerance
mechanism of drought-tolerant chickpea varieties. The
heterologous expression of the pyrroline-5-carboxylate
synthetase ( P5CS ) gene through Agrobacterium -mediated
transformation was also shown to improve salt tolerance
in chickpea. As compared to the control plants, the
transgenic lines were able to produce higher levels of
proline, a factor that helps assuage salinity stress.

13.8 Functional genomics

Over the years, understanding the genetic basis of
drought tolerance in crop plants has been of prime
importance for researchers. To date, many drought-
responsive genes, namely LIP9 , OsNAC6 , OsLEA14a ,
OsRAB16D , OsLEA3-1 and Oshox24 in rice (Nakashima
et al., 2014) and ADF3, APB, ASR, DLP, LTP1 and UGE5 in
pigeon pea (Deeplanaik et al., 2013) have been discov-
ered through extensive genomics projects. Nonetheless,
the analysis of these drought-inducible genes and their
expression remains a significant factor in developing
crop varieties tolerant to abiotic stresses. Functional
genomics aims at discovering and characterizing the
involved genes and their corresponding proteins with
respect to their functions and interactions. Mutagenesis,
RNA interference (RNAi) and genetic interaction map-
ping are just some of the different functional genomics
approaches employed for the purpose of determining
gene functions and investigating the response of plants
to abiotic stresses at a molecular level.

13.7.3 peanut
Drought as well as salinity are the major environmental
stresses that limit worldwide peanut production
(Stansell & Pallas, 1985; Lamb et al., 1997). Since the
major peanut-producing countries, namely India, China
and the USA, are facing critical water scarcity for
peanut irrigation, research is continually being carried
out to generate drought- and salinity-tolerant peanut
phenotypes. Both Agrobacterium -mediated and biolistic
bombardment techniques have been applied for the
purpose of transforming peanut. Overexpression of the

13.8.1 Gene silencing approaches
In addition to insertional mutagenesis, another potent
tool for gene expression analysis and function in
plants is the technique of gene silencing. To knock
out or reduce the expression of genes, transfer DNA
(T-DNA) insert mutations, random mutation, RNAi-
based methods and virus-induced gene silencing (VIGS)
can be employed (Quadrana et al., 2011). A recombinant