First published online June 29, 2006
Journal of Experimental Biology 209, 2804-2810 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02309
Nitric oxide and cnidarian bleaching: an eviction notice mediates breakdown of a symbiosis
Santiago Perez* and
Virginia Weis
Department of Zoology, Oregon State University, Corvallis, OR 97331,
USA
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Fig. 1. Heat stress induces fluorescence of the NO-sensitive probe DAF-FM in A.
pallida. (A) Optical cross-section through a tentacle of symbiotic A.
pallida incubated at an ambient temperature of 25°C (left) and after
24 h of heat stress at 33°C (right), with inset showing expelled host
cells with algae. The DAF-FM (510-530 nm) NOdependent fluorescent signal is
colored yellow; algal autofluorescence is colored red. (B) Quantification of
relative fluorescence intensity of NO-sensitive DAF-FM (510-530 nm) in
tentacles as a function of heat stress (N=3 anemones per treatment;
bars represent means + s.d.; t-test, P=0.0015).
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Fig. 2. The NOS inhibitor L-aminoguanidine (LAG) inhibits both
lipopolysaccharide (LPS) and heat stress-induced NO production in A.
pallida. (A) Gray-scale rendering of confocal images of DAF-FM-loaded
tentacles of aposymbiotic anemones incubated for 5 h in 1 µg
ml-1 LPS alone (left) or in LPS with 10 mmol l-1 LAG
(right). (B) Quantification of DAF-FM fluorescence as a function of heat
stress alone and in the presence of 10 mmol l-1 LAG (N=3
anemones per treatment; bars represent means + s.d.). Heat-stressed anemones
showed a significantly greater (*) DAF-FM fluorescence than control
or heat-stressed anemones incubated in LAG (ANOVA, P=0.006; Tukey
HSD, P=0.021).
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Fig. 3. DCMU increases NO production in symbiotic A. pallida.
Quantification of DAF-FM fluorescence of tentacles from aposymbiotic and
symbiotic anemones after a 24 h incubation in 50 µmol l-1 DCMU
(N=3 anemones per treatment; bars represent means + s.d.;
t-test, P=0.0041).
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Fig. 4. Nitric oxide mediates bleaching in A. pallida. (A) Bleaching (%
expulsion; bars represent means + s.d.; N=3 anemones per treatment)
of anemones incubated for 24 h at 25°C with the NO donor SNP (1 mmol
l-1) with (filled bars) or without (open bars) the NO scavenger
cPTIO (1 mmol l-1). Control anemones (without SNP) released <1%
of their algae. Incubation with SNP resulted in a significant increase in
bleaching (t-test, P=0.021) while co-incubation with cPTIO
significantly decreased this effect (t-test, P=0.0368). (B)
Bleaching (% expulsion; bars represent means + s.d.; N=3 anemones per
treatment) of anemones incubated for 24 h at 25°C (control) or 33°C,
with or without cPTIO (20 mmol l-1). There was a significant
difference between the two heat-stress treatments (t-test,
P=0.0176). (C) NO-dependent DAF-FM fluorescence in control,
heatstressed or SNP-treated anemones with or without 20 mmol l-1
cPTIO. Bars sharing the same letter are not significantly different (two-way
ANOVA, P<0.001; Tukey HSD, P<0.05).
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Fig. 5. Proposed model for the role of nitric oxide during cnidarian bleaching. The
host cell responds to algal-derived ROS (including superoxide and hydrogen
peroxide) by producing NO through signaling leading to the upregulation of
NOS. This signaling could involve the transcription factor NF B, an
important mediator of NOS transcription in other systems. The reaction of
superoxide with NO produces the reactive nitrogen species peroxynitrite
(ONOO-), with additive deleterious effects leading to cell death
and bleaching.
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© The Company of Biologists Ltd 2006
