Role of abscisic acid in legumes under abiotic stress - Legumes Under Environmental Stress: Yield, Improvement and Adaptations - page 153

MG-20) and near-constitutive wilty phenotype asso-
ciated with its inability to regulate stomatal opening
(Biswas et al., 2009). In the absence of exogenous
ABA, Beyma did not show alteration of nodule
number control, and the number and patterning of
nodules formed in the mutant were similar to MG-20.
As MG-20 showed that application of ABA on one
side of the split root inhibited nodulation locally but
not systemically, it was proposed that ABA is not
directly involved in autoregulation of nodulation.
Similarly, ABA insensitivity as well as a hypernodu-
lating phenotype was exhibited by Medicago truncatula
seedlings overexpressing ABI -1 gene from Arabidopsis ,
which suppresses the ABA signalling pathway (Ding
et al., 2009). In addition, the mutant enhanced
nitrogen fixation 1 (enf1) showed lower endogenous
ABA concentration in comparison with wild-type
Lotus japonicus seedlings. This mutant enf1 not only
showed increased root nodule number but also
enhanced nitrogen fixation (Tominaga et al., 2009).
Ding et al. (2008) suggested that the effect of ABA
inhibition on nodulation was linked to the abolition
of both Nod factor-induced calcium spiking and gene
expression to coordinate Nod factor and cytokinin
(CK) signaling during the regulation of nodulation.
It has been observed that cortical responses leading
to nodule primordia formation are mostly the func-
tion of CKs and auxin signalling (Oldroyd & Downie,
2008). Besides, nodule initiation is associated with
suppression of lateral root formation (Nutman,
1948), probably following changes in ABA/CK ratios
and their opposing effects on the inception of cell
division associated with nodules and lateral root
formation.
In addition, González et al. (2001) proposed that ABA
could be involved in nodule senescence by controlling
the development of the plant root system in response to
nitrogen nutrition (Gawronska et al., 2003) and its
implication in sugar- and nitrate-sensing pathways in
plants (Puppo et al., 2005).
Under saline conditions, Salah et al. (2013) demon-
strated an inverse relationship between ABA in
nodules and the growth-related parameters of Medicago
ciliaris , and with symbiotic nitrogen fixation. In
contrast, ABA levels in leaves and roots were posi-
tively correlated with both growth and symbiotic
nitrogen fixation under salt stress, suggesting comp-
lex hormonal interactions between different organs

in relation to the response to abiotic stress and to
the symbiotic relationship, which requires further
investigation.

10.7 aBa and assimilate accumulation
under abiotic stress

Abiotic stress can modify ABA accumulation, regulating
assimilate partitioning between source and different
sink tissues (Hartig & Beck, 2006; Pérez-Alfocea et al.,
2010). ABA can also trigger changes in carbohydrate
concentrations and enzyme activities involved in carbo-
hydrate biosynthesis (Kobashi et al., 1999; Taji et al.,
2002). Blöchl et al. (2005) observed an accumulation of
raffinose and stachyose in ABA-treated somatic embryos
of alfalfa; these compounds have been proposed to act
generally as compatible solutes or osmoprotectants to
allow osmotic adjustment of plant cells exposed to abi-
otic stress. Oligosaccharides have been considered as
factors involved in glass formation in cells and may act
against deterioration of macromolecular structures
during desiccation (Horbowicz & Obendorf, 1994). In
drought-stressed lupin seeds, oligosaccharides were the
predominant fraction (75-77%) of all soluble carbohy-
drates (Zalewski et al., 2001), and raffinose showed
significant increases during water deficit in Populus
euphratica (Brosche et al., 2005). Carbohydrates induced
by drought conditions and ABA treatments may serve
not only as osmoprotectants and free radical scavengers,
but also as phloem-mobile anti-senescence metabolites
(Wattana & Monica, 1999).
In legumes, the accumulation of starch is generally
associated with storage protein accumulation (Wobus &
Weber, 1999). However, in Vicia narbonensis a reduction
in starch synthesis due to antisense expression of
ADP-glucose pyrophosphorylase was accompanied by
increased total protein accumulation, possibly reflecting
an increased period of seed-filling or delayed matura-
tion (Weber et al., 2000). Besides, one set of genes,
named LEAFY COTYLEDON ( LEC ) and FUS3 , is involved
in the regulation of a variety of embryonic stages and
interact genetically with ABA-dependent regulators to
suppress vivipary at mid-embryogenesis (Raz et al.,
2002), and with several of the ABI loci to mediate ABA
sensitivity at seed maturity (Finkelstein & Gampala,
2002). Arabidopsis mutants of these regulators - i.e. fusca
3 ( fus3 ), leafy cotyledon 1 ( lec1 ) and leafy cotyledon 2