Agriculture Reference
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in plants. It also modulates the expression of many genes, the products of which might
function in stress adaptation and tolerance.
In addition to stimulating the production of ABA, the above stress conditions also
lead to the generation of reactive oxygen species (ROS), which are highly active and
toxic. ROS that have been investigated in plants include superoxide radical (O
2
−
),
hydrogen peroxide (H
2
O
2
), singlet oxygen (
1
O
2
), hydroxyl radical (OH
) and nitric
oxide (NO). H
2
O
2
, O
2
and OH
can interconvert into one another (Halliwell
2012
;
Quan et al.
2008
). Such interconversion may occur spontaneously or be catalysed by
enzymes (Van Breusegem et al.
2008
). Being highly reactive and toxic, ROS have
long been considered as compounds that damage cellular components, such as by the
destruction of nucleic acids, the oxidation of proteins and the induction of lipid per-
oxidation (Foyer and Noctor
2005
). Unfavourable environmental conditions and ABA
induce ROS production from numerous sources; an increase in ROS imposes oxida-
tive stress, which can ultimately result in cell death. Therefore, plants have evolved an
elaborate system to control cellular ROS levels (Mittler et al.
2011
). ROS act in signal
transduction to help the cell to counteract oxidative damage by initiating the expression
of certain genes and thus activating related signal transduction pathways; these findings
suggest that ROS function as important signals to activate and control various stress
responses (Dalton et al.
1999
). Not only do ROS function in stress responses, but they
are also essential for the maintenance of normal energy and metabolic fluxes, the opti-
misation of cell function, the acclimation responses activated by retrograde signalling
and the regulation of whole-plant systemic signalling pathways (Møller
2001
; Jaspers
and Kangasj￤rvi
2010
). ROS metabolism and signalling in plants have become a fron-
tier of research in basic and applied plant science, given the differential actions of ROS
depending on the cellular compartments in which they function as components of a
complex network.
Similarities between ABA and ROS include that both can be induced by various
stress conditions, both act as ubiquitous 'signalling molecules' or 'secondary mes-
sengers' in normal plant cell function, and both initiate plant stress adaptation and
resistance by the modulation of gene transcription. They also exhibit similar spa-
tial and temporal distributions, as well as dynamic changes in plants in response to
the alteration of environmental conditions. Several reviews that discuss ABA and
ROS signalling have been published (Ahmad et al.
2010
; Song et al.
2014
). In this
chapter, we focus on the turnover of ROS, their biological roles and their crosstalk
with ABA signalling, and also attempt to describe the relationship between ROS
and ABA signalling.
−
10.2 ROS Turnover in the Presence of ABA
and Stress Conditions
ROS arise in plant cells via a number of routes. It has been estimated that 1 % of
O
2
consumed by plants is diverted to produce ROS in various subcellular loca-
tions (Del Rio et al.
1992
). ROS are produced continuously as by-products of
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