Agriculture Reference
In-Depth Information
irreversible; the most common is carbonylation, which is a widely used marker
of protein oxidation (Møller et al.
2007
). Whatever the location of ROS synthesis
and action, ROS are likely to target proteins that include sulphur-containing amino
acids and thiol groups. A study to investigate protein carbonylation in wheat leaves
showed that its level was higher in mitochondria than in chloroplasts and peroxi-
somes; this indicates that mitochondria are more susceptible to oxidative damage
than chloroplasts and peroxisomes (Bartoli et al.
2004
). Oxidised peptides are
used as secondary ROS signalling molecules by mitochondria, chloroplasts, per-
oxisomes and possibly other organelles.
It is important to note that the efficiency of antioxidative systems determines
the steady-state level of ROS in a cell (Foyer et al.
1994
). When the equilibrium
is perturbed by various stress factors, an increase in the intracellular level of ROS
can cause significant damage to cell structures; oxidative stress is the dominate
type of stress in this situation. Therefore, it is the equilibrium between ROS gener-
ation and the rate of antioxidant scavenging that determines which response path-
way plant cells employ.
10.3.2 Oxidative Signalling
It is well established that ROS signalling is important in the regulation of plant
cell physiology and cellular responses to the environment. ROS have been
accepted as secondary messengers that participate in many biology processes,
and more and more evidence has shown that ROS signalling is intertwined with
other types of signalling. ROS have also been accepted as the 'cellular indicators
of stress', owing to their increased production upon exposure to many stresses
(Mittler
2002
). Oxidative stress can activate the expression of defence-related
genes. For example, early studies showed that ROS signalling in
Arabidopsis
stimulates antioxidative defence through upregulating the expression of antioxida-
tive genes and activating the genes that encode inducible stress proteins (Santos
et al.
1996
; Karpinski et al.
1997
). In this context, a question arises: What are the
advantages of ROS being used as signal molecules? One advantage is that they
can rapidly propagate signals along extensive distances throughout the plant. H
2
O
2
accumulates in many stress situations, and it has a relatively long life and high
permeability across membranes, which facilitate its role as a secondary messen-
ger. The dynamic nature of ROS has been proved in root hair cells, in which the
oscillation of ROS was shown to support root hair elongation; in addition, between
cells, a ROS burst due to wounding can trigger a rapid ROS signal at a rate of
8.4 centimeters per minute (Miller et al.
2009
). This indicates that ROS can act
as long-distance cell-to-cell signals. Intracellularly generated H
2
O
2
can move into
neighbouring cells (Allan and Fluhr
1997
). A study by Miller et al. (
2009
) sug-
gested that all cells along the signal pathway were stimulated to generate ROS
and that their capacity to transmit this signal in an autonomous manner enabled
transmission of a ROS signal over long distances. It is not known how the signal
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