Omics approaches and abiotic stress tolerance in legumes - Legumes Under Environmental Stress: Yield, Improvement and Adaptations - page 216

Salinity is also an abiotic stress factor that is detri-
mental to legume survival and rhizobium-legume
interactions, and so is soil sodicity (Chalk et al., 2010;
Shabala & Munns, 2012). Sodium chloride is also
known to increase levels of abscisic acid (ABA) in the
transpiring portion of a growing leaf. A six-fold
increase in ABA was noticed in the distal portion of the
leaf elongation zone within 10 minutes of addition of
salt, which later accumulated in the proximal portion.
The ABA increased three-fold in the growing blade
region, and stayed so for 20 minutes. This preceded a
notable decrease in both transpiration and stomatal
conductivity (Poltronieri et al., 2011). An ATP binding
cassette (ABC) transporter, AtABCG25, expressed
mainly in the roots and leaf veins and located on the
outer cell membrane (Kuromori et al., 2010), was iden-
tified to be the main player in hypersensitivity towards
ABA in germination and seedling stages in a plant
mutant. Plants that were genetically engineered to
upregulate the expression of ABA were shown to
experience decreased water loss from isolated leaves
and also had a higher leaf temperature as compared to
normal plants.
Biotechnological approaches have many advan-
tages over conventional crop breeding. A number of
applications were identified by Drehr et al. (2003)
where marker-aided techniques offered substantial
cost saving as compared to conventional methods.
This is because conventional methods require the
breeder to rely on observed differences that give a
clue to the underlying gene (Bruce, 2012). This not
only is extremely time consuming, but ever since the
advent of molecular diagnostic screening, is also inef-
ficient. Biotechnology-based tools allow the quick,
reliable and efficient identification of desired genes.
Furthermore, given that biotechnological tools reduce
the breeding time as compared to conventional tech-
niques, it can also be expected that new varieties
would be developed and introduced to the market at a
much faster pace (Farre et al., 2010; Anthony &
Ferroni, 2012).
Conventional breeding methods and improved
management strategies may help alleviate some
constraints, but it is of extreme importance that
biotechnological approaches be applied in order to
tackle these stresses and help sustain agriculture
globally.

13.2 Omics: solutions to abiotic
stress in legumes?

Biotechnological approaches like tissue culture and
genetic transformation can help tackle hindrances that
cannot be addressed through classical plant breeding
due to sexual incompatibility or limited natural sources
of resistance (Urano et al., 2010; Syrenne et al., 2012).
The use of biotechnology may seem to be an excellent
solution, but it requires deep knowledge of the target
plant (legumes in this case) and the mechanisms that
are stimulated or repressed on exposure to stress. The
response to stress should be identifiable from the plant
level all the way down to the molecular level. Three
properties of the stress determine the intensity of the
plant's response: type, length and severity. The plant's
response to any stress is highly governed by its genome;
therefore efforts are being made to understand the
responses at a molecular level (Bray, 2004). Until
recently research was focused on model plants, notably
Arabidopsis , instead of legume plants due to the count-
less advantages of the former. Its growth, development,
flowering and seed production characteristics are similar
to those of higher plants, including many legumes.
Furthermore, it has a very short generation time, with
seeds germinating and maturing to form more seeds
within only 6 weeks. The most beneficial property of
this model plant is its small genome, only 125 Mb,
which allows easy manipulation (Koorneef & Meinke,
2010). Since there are many similarities between the
model plant and legumes, the knowledge obtained
about stress responses has been applied to legume
research as well. Nevertheless, the physiological differ-
ences between the two plants cannot be ignored.
Legumes have a large genome size, and some also
show polyploidy (Anderson et al., 2005). This hampers
research; therefore, in order to understand legume
biology better, model systems have been developed to
study the genetics behind nodule formation and other
vital processes like resistance, potential tolerance and
susceptibility to various stresses. Much of the research
on legumes is carried out on model plant systems, most
commonly Medicago truncatula and Lotus japonicus ,
because of their small, diploid genome, short life cycle,
autogamous nature and abundant seed production
(Handberg & Stougaard, 1992; Cook, 1999). The use of
these models has greatly increased the genetic as well as