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Fig. 3 Hypothetical scheme for the action of SA interacting with ion channels and ROS-
generating enzymes upon exposure to O
3
, finally leading to PCD in A. thaliana cells
4 Metal Toxicity
4.1 Role of Trace Elements During Environmental Oxidative
Stress Responses
Induction of cell death by oxidative stresses accompanying the generation of ROS
is often mediated by early signaling events such as Ca
2+
influx via ROS-mediated
activation of calcium channels on PM (see above sections). Our previous studies
have revealed that various phytotoxic metals such as Al
3+
and 15 rare earth ele-
ments induce an acute generation of ROS such as O
•-
by stimulating the plant
RBOH activity (Kawano et al.
2001
,
2002
,
2003
) and as a consequence, ROS
stimulates the opening of ROS-responsive calcium channels on the surface of plant
cells (Kawano et al.
2003
,
2004a
,
c
).
Copper is known to be a phytotoxic metal which induces an increase in [Ca
2+
]
c
in
cultured tobacco cells (Inoue et al.
2005
). However, the possible mechanism of acute
copper action may differ from such metals stimulating the O
•-
generation since
production of O
•-
could not be detected after addition of CuSO
4
to the plant cell
suspension culture (Kawano and Muto
2000
). On the other hand, copper may
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