Role of abscisic acid in legumes under abiotic stress - Legumes Under Environmental Stress: Yield, Improvement and Adaptations - page 152

2000). However, Sibole et al. (2003) showed that in
Medicago species there was no correlation between leaf
growth and leaf ABA. Root ABA accumulation was
not significantly different in either species under salt
stress, and this growth regulator only increased in
leaves and xylem of salt-treated Medicago citrina . The
increase in leaf ABA was correlated with decreased
leaf water content, which suggests that ABA could be
involved in osmotic adjustment by lowering water
potential. In this study it was proposed that the
higher xylem ABA accumulation found in salt-treated
M. citrina could be correlated with its retranslocation
from the leaves via the phloem to xylem (Wolf et al.,
1990). Correspondingly, the lack of response in leaf
ABA content in M. arborea plants could be related to
the observed increase in leaf water content with NaCl
(Hartung et al., 2002).
Cabot et al. (2009) observed a decrease in ABA content
in Phaseolus vulgaris pretreated with fluoridone under
salt stress. These plants had lower Na + uptake and higher
leaf Na + exclusion capacity in comparison with non-
pretreated plants. Moreover, leaf Na + exclusion was
decreased by ABA and Na + uptake showed an increment.
Leaf ABA increased in NaCl- and KCl-treated plants,
and this ABA content caused a decrease in transpiration
rates, whereas supplemental Ca 2+ and higher strength
nutrient solution caused a decrease in leaf ABA and leaf
Na + . These results evidenced the osmotic component of
salt associated with non-ion-specific increase in ABA.
The higher ABA levels observed in Phaseolus vulgaris
plants may be correlated with higher leaf Na + concen-
trations due to lower Na + exclusion or increased
root-shoot Na + translocation. However, in the legume
P. strombulifera an anionic component of salt was
observed. The high ABA levels found in Na 2 SO 4 -treated
plants at 48 days of culture may be correlated with a
failure in ion compartmentalization by these plants
when compared with NaCl-treated plants. In fact, NaCl-
treated plants accumulated levels of Na + in their leaves
similar to those of Na 2 SO 4 -treated plants but they suc-
cessfully accomplished ion compartmentalization and
osmoregulation with direct consequences for their
growth. Therefore, production of high levels of ABA in
leaves under Na 2 SO 4 treatment may be considered as
the stress signal to protect this legume against dehydra-
tion and ion toxicity caused by the sulphate anion as
well as a regulating factor controlling vegetative growth
(Devinar et al., 2013).

10.6 aBa as a regulator of
nodulation under abiotic stress

Abscisic acid is a negative regulator of nodulation, as
was first described by Phillips (1971) in Pisum sativum.
In fact, Cho and Harper (1993) demonstrated that
ABA application to roots reduced nodule number and
isoflavonoid levels in the wild-type as well as in the
super-nodulating mutant NOD1-3 in Glycine max
(soybean). Indeed, all phases of soybean nodulation
(nodule initiation, nodule development, and nitrogen
fixation) were inhibited by exogenous ABA (Bano &
Harper, 2002), suggesting general physiological tox-
icity or suppression. Furthermore, Lotus japonicus
nodule number decreased after ABA application to
roots but increased with abamine, an inhibitor of ABA
biosynthesis (Suzuki et al., 2004). ABA levels in
wild-type soybean and the super-nodulating mutant
nts382 were investigated to elucidate the possible role
of ABA in nodule development (Carroll et al., 1985a,b;
Searle et al., 2003; Nontachaiyapoom et al., 2007).
In wild-type roots, ABA levels were significantly
higher than in nts382, and increased further upon
inoculation with Bradyrhizobium , but only in the
wild-type (Caba et al., 2000). Endogenous ABA also
increased in the shoot at the onset of nodule regula-
tion, but again, only in the wild-type and not in
nts382. The ABA/cytokinin ratio has been positively
correlated with nodule suppression and autoregula-
tion, with the root ratio always being higher in
wild-type soybean compared to the super-nodulating
nts382mutant (Bano et al., 2002). Therefore, it was
suggested that ABA may be a root-induced inhibitor,
decreasing nodule numbers in wild-type but not in
super-nodulating mutants. Additionally, the mutant
latd, a nodulation-defective mutant of Medicago trun-
catula, produced nodules that were immature and
non-nitrogen fixing. This mutant is deficient in the
growth of three types of meristems - those of the pri-
mary root, lateral root and nodules. Organization and
function of both primary and lateral root meristems,
but not nodulation, were restored by ABA application.
The mutant was later found to have reduced sensi-
tivity to ABA (Liang et al., 2007). Another mutagenesis
analysis was performed in Lotus japonicus MG-20, and
the ABA-insensitive mutant Beyma was isolated. This
mutant failed to show any nodulation sensitivity to
ABA, and had a slow-growing (50-70% of wild-type