Agriculture Reference
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encodes a DEXH box RNA helicase that is localised in the mitochondria and is
required for the splicing of several genes that encode components of complex I
(He et al.
2012
).
Glutathione peroxidases (GPX) fulfil important functions in oxidative signal-
ling and protect against the adverse effects of excessive oxidation.
Arabidopsis
gpx1
,
gpx4
,
gpx6
,
gpx7
and
gpx8
mutants had a significantly greater lateral root
density (LRD) than the wild type. Conversely, the
gpx2
and
gpx3
mutants had
significantly lower LRD values than WT. Auxin increased LRD and synthetic str-
igolactone GR24 and ABA decreased LRD in all genotypes. These findings dem-
onstrate the importance of redox controls mediated by AtGPX in the control of
root architecture (Passaia et al.
2014
).
The biological roles of ROS in the ABA-regulated root growth signalling path-
way remain largely unknown. However, secondary messenger calcium has been
proved to be important in Pro-rich extensin-like receptor kinase 4 (PERK4)-
regulated root growth in ABA signalling (Bai et al.
2009
). It is possible that
changes in Ca
2
+
might be an element common to the signalling pathways down-
stream of both ROS and ABA.
10.4.4 ROS-dependent Cell Death
Being highly reactive, most ROS can induce oxidative stress, which affects the
growth and development of the whole plant. When the equilibrium between anti-
oxidative systems and the production of ROS is disturbed by various stress factors,
an intense increase in the levels of ROS at the intracellular level can cause sig-
nificant damage to cell structures. Increased ROS induces oxidative stress, which
ultimately results in cell death.
There are two forms of cell death: programmed cell death (PCD) and necro-
sis; they are important features of plant development. ROS participate in both
of these, with the difference between them being that PCD is genetically regu-
lated, but necrosis is induced by severe and persistent trauma and is considered
to not be genetically regulated. Experiments that involved cultured soyabean
cells provided the first evidence that ROS act as signals that initiate plant cell
death; namely, a short pulse of H
2
O
2
was sufficient to induce it (Pennell and
Lamb
1997
). The precise ROS signal transduction pathway in cell death remains
somewhat unclear. However, it is known that H
2
O
2
-induced cell death needs
time for the transcription and translation of certain genes, and exogenous H
2
O
2
-
induced cell death shows concentration- and time-dependent features (Desikan
et al.
1998
). Exogenous H
2
O
2
applied at a concentration over 5 mM initiates cell
death in
Arabidopsis
suspension cultures; this concentration is higher than that
which stimulates the expression of defence genes (Desikan et al.
1998
; Levine
et al.
1994
). These different responses to different levels of H
2
O
2
clearly suggest
the complexity of the signal transduction mechanisms for ROS that exist in plant
cells. How can cells discriminate and adopt one of these pathways? What are
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