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observed in few seconds) and long lasting gradual increase (spending hours),
similarly to the tobacco model. Among six respiratory burst oxidase homologs
(atrbohs) coding for plant NADPH oxidase, AtrbohD was shown to be the only
gene responsive to Al. In the Al-treated cells, the induced O
•-
generation, At-
rbohD expression and cell death were all inhibited by NADPH oxidase inhibitor
and SOD. According to the gene expression and oxidative burst profiles and effect
of inhibitors, it is likely that the acute burst of O
•-
generation could be catalyzed
by pre-existing enzyme and slower response could be the consequence of long-
lasting gene expression.
In Arabidopsis thaliana, six genes for RBOHs as well characterized AtrbohA-
F are known (Ogasawara et al.
2008
). Among Atrbohs, AtrbohA, B and C were
shown to be expressed only in the roots, especially in the elongating regions. Atr-
bohE is reportedly expressed in seeds and roots. AtrbohD and F are known to be
expressed systemically in the plants. As the expression of systemically distributable
AtrbohD was shown to be rapid and long-lasting (1 min-24 h), we expect that Al
response has similarity with SAR-inducing stimuli (Kunihiro et al.
2011
).
4.4 Involvement of SA Signal Transduction in Aluminum
Action in Arabidopsis thaliana
In order to examine the involvement of SA signaling pathways in the cells of
Arabidopsis thaliana, the cell lines lacking SA synthesis (transgenic NahG cell
line and sid2 mutant cell line) and the cell lines with altered SA-dependent sig-
naling factors (npr1 mutant cell line and transgenic NRP1-overexpressing cell line,
NPR-Ox) were used for comparison (Kunihiro et al.
2011
). As shown in Fig.
6
,no
Fig. 6 Involvement of SA biosynthesis and signaling pathway in Al-induced expression of
Atrbohs. a Suspension-cultured cells of Arabidopsis thaliana with ecotype Columbia background
namely wild type (cell line, Col-0), mutants (cell lines, sid2 and npr1) and transgenic cell lines
(cell lines, NahG, over-expressing bacterial SA hydroxylase; NPR-Ox, over-expressing NPR1
gene) were treated with 0.1 mM AlCl
3
for 3 h. b Al-induced expression of AtrbohD at different
time points (0-60 min and 2-24 h). c Inhibition of Al-dependent expression of AtrbohD by
1 mM
diphenyleneiodonium
chloride
(DPI)
and
5,000 units/ml
SOD
(examined
3 h
after
treatment with Al). Data adapted from Kunihiro et al.
2011
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