Reactive Oxygen Species (ROS) and ABA Signalling - Abscisic Acid: Metabolism, Transport and Signaling - page 203

for root gravitropism (Joo et al. 2001 ). The requirement for ROS in root growth
has already been established using a mutation in RbohC ( rhd2 ), in that the reduced
ROS generation in rhd2 decreased Ca 2 + channel activities in root hairs, which fur-
ther led to the inhibition of root hair growth (Foreman et al. 2003 ). The NADPH
oxidases RbohD and RbohF also regulate root growth, with the double-mutant
rbohD/rbohF showing marked reductions in root length relative to the wild type
(Kwak et al. 2003 ). ROS control the transition from cell proliferation to differen-
tiation in roots via the auxin signalling pathway, which is mediated by the basic
helix-loop-helix TF UPBEAT1 (Tsukagoshi et al. 2010 ). As highly dynamic
signalling molecules, ROS also function as long-distance auto-propagating
signals (Miller et al. 2009 ), with ROS oscillations shown to occur in root hairs
(Monshausen et al. 2007 ; Takeda et al. 2008 ).
Plant hormones have long been known to play a crucial role in the regulation
of root growth (Moubayidin et al. 2009 ; Ubeda-Tomas et al. 2008 ). Cytokinin,
auxin, brassinosteroids (BR), ethylene and ABA all play roles in the control of
meristem size and root growth. ABA acts on quiescent centre (QC) and stem
cells in order to regulate root meristem size. In addition, QC cells induce divi-
sion when ABA biosynthesis is blocked in seedlings (Zhang et al. 2010 ). ABA is
required for root growth and ABA-deficient mutants have reduced root systems
(Xiong and Zhu 2003 ); however, high concentrations of exogenous ABA inhibit
root growth. It is believed that ABA stimulates the production of ROS through
NADPH oxidases (Kwak et al. 2003 ). In Arabidopsis , both the single-mutant
rbohF and the double-mutant rbohD/rbohF are insensitive to ABA-mediated inhi-
bition of root growth (Kwak et al. 2003 ). However, the mechanisms that underlie
this process remain elusive. A recent study showed that the double mutants atr-
bohD1/F1 and atrbohD2/F2 were less sensitive to ABA suppression of root cell
elongation than WT plants. Furthermore, the double mutants showed impaired
ABA responses in roots, including ROS generation, cytosolic Ca 2 + increases
and activation of plasma membrane Ca 2 + -permeable channels, compared with
WT plants. Exogenous H 2 O 2 can activate the Ca 2 + channel activity in roots of
atrbohD1/F1 plants. In addition, exogenous application of the auxin transport
inhibitor naphthylphthalamic acid effectively promoted ABA-mediated inhibition
of root growth of the mutants relative to that of WT plants. The ABA-induced
decreases in auxin sensitivity of the root tips were more pronounced in the WT
than in atrbohD1/F1 . These findings suggest that both AtrbohD and AtrbohF
are essential for ABA-promoted ROS production in roots. ROS activate Ca 2 +
signalling and reduce auxin sensitivity of roots, thus positively regulating ABA-
inhibited primary root growth in Arabidopsis (Jiao et al. 2013 ). This suggests that
ROS not only play roles in ABA-regulated stomatal movement but also in ABA-
regulated root growth.
Besides plasma membrane NADPH oxidases, mitochondrially produced ROS
have also been proved to be important in the ABA signalling pathway in root
growth. In a study to screen the regulator in ABA-mediated Arabidopsis root
growth, a mutant overly sensitive to ABA, abo6 , showed a higher level of ROS
in the mitochondria under both ABA treatment and normal conditions. ABO6