Role of abscisic acid in legumes under abiotic stress - Legumes Under Environmental Stress: Yield, Improvement and Adaptations - page 146

as drought, salinity, mineral toxicity and deficiencies,
and high or low temperatures affect plant growth,
development and final yield performance of a crop.
Salinity and drought exert a negative effect mainly by
disrupting the ionic and osmotic homeostasis of the cell,
while low temperatures exert mechanical constraints. It
has been proposed that to know the specific responses
of plants to multiple adverse environmental factors, it is
crucial to impose the stresses simultaneously and treat
each set of environmental conditions as an entirely new
stress (Mittler, 2006).
In general, abiotic stress may elicit a wide range of
cellular adaptive responses that plants have evolved.
These responses include accumulation of compatible
solutes, induction of stress proteins and acceleration
of scavenging systems for reactive oxygen species
(Zhu, 2002). One of the most important water toler-
ance responses is the leaf abscission induced by water
stress (Hamdia, 2008; Chalk & Alves, 2010). Plants
can 'sense' changes in environmental conditions by
generating non-heritable modifications reflecting the
morphological and physiological plasticity of the
plant, or by generating heritable modifications. These
responses are mediated by plant growth regulators
(phytohormones), compounds derived from plant bio-
synthetic pathways that can act either at the site of
synthesis or, following their transport, elsewhere in
the plant (Javid et al., 2011). These regulators are
essential for adaptation to abiotic stresses by mediating
physiological adaptive responses. The phytohormone
abscisic acid (ABA) plays a central role in regulating
physiological responses.
The rapid accumulation of ABA in response to stress
and the regulation of a wide range of adaptive responses
define this phytohormone as 'the stress hormone'
helping the plant's survival under stresses. Early inves-
tigations demonstrated that ABA acts as a long-distance
water stress signal in sensing impending soil drying
(Davies & Zhang, 1991). ABA synthesis is a common
and fast response of plants to abiotic stresses, triggering
ABA-inducible gene expression (Yamaguchi-Shinozaki
& Shinozaki, 2006) and causing stomatal closure,
thereby reducing water loss via transpiration and even-
tually decreasing cellular growth (Wilkinson & Davies,
2010). In water-stressed plants, ABA provokes the
rapid closure of stomata a few minutes after perception
of stress, triggering a signalling cascade in guard cells
that leads to reduction of water loss by transpiration

(Schroeder et al., 2001). In addition to these short-term
effects, modulation of the transcript levels of hundreds
of genes has been reported for Arabidopsis thaliana sub-
jected to salt (Sottosanto et al., 2004), drought and heat
(Rizhsky et al., 2004) or cold stress (Provart et al., 2003;
Vogel et al., 2005). In drying soil, the roots of plants
synthesize ABA and it is transported from dehydrated
roots to the xylem and thence to regulate stomatal
opening and leaf growth in the shoots (Zhang et al.,
1987; Zhang & Davies, 1990a, 1990b). pH and
ionic conditions in the xylem modify this mechanism
(Trejo & Davies, 1991; Bacon et al., 1998; Wilkinson
et al., 1998). In fact, Hartung et al. (2002) have shown
that pH changes play a central role in the ABA redistri-
bution in leaf tissues and control the stomata when no
significant changes in ABA levels are detected in the
xylem. On the other hand, ABA is required to regulate
the effects of nitrate on root branching, signalling
through part of the ABA response pathway comprising
the transcription factors ABA INSENSITIVE (ABI) of
Arabidopsis , ABI4 and ABI5. It has been proposed that
ABI1, ABI2 and ABI3 are not required for the lateral
root response, while these ABA signalling genes play a
central role in seed germination. This differential par-
ticipation of ABI genes indicates a branched pathway
(Signora et al., 2001). Interestingly, although auxin has
been thought to be the primary regulator of lateral root
formation, the ABA-induced inhibition of lateral root
development appears to be auxin dependent (De Smet,
2003). Thus, an ABA role in regulating root architecture
is to mediate nutritional signals in an auxin-independent
manner.
In legumes, it was shown that ABA was rapidly trans-
ported from roots to shoots and accumulated in the
apoplast of guard cells of leaves responding to drought
(Jia et al., 1996). Additionally, ABA is produced in lupins
( Lupinus cosentinii and L. angustifolius ) subjected to soil
water deficit, and accumulation of ABA in the leaf is
correlated with a reduction in stomatal conductance
(Henson, 1981; Gallardo et al., 1994). Thus, numerous
studies suggest a relevant role for this phytohormone in
legumes, and that knowledge of ABA responses would
help to improve crop productivity.
In this chapter we highlight the latest progress in
determining the role of ABA in the responses of legumes
to abiotic conditions. We then discuss recent advances
in engineering of ABA hormone-associated genes aimed
at ameliorating environmental stresses.