Biology Reference
In-Depth Information
Na
+
may be extruded out through the Na
+
/H
+
antiporter activities due to the availability of energy
from photosynthesis. In the absence of light due to the non-availability of energy, the Na
+
/H
+
antiporter becomes inoperative resulting in an increase of Na
+
. This leads to the accumulation of Na
+
at high concentrations in the intra-thylakoid space irreversibly inactivating O
2
-evolving machinery.
Allakhverdiev
et al
. (2001) presented evidences in favour of the existence of rapid and slow phases
in NaCl-induced inactivation of Na
+
/H
+
antiporters in
S. elongatus
PCC 7942. Light restoration of the
activity of Na
+
/H
+
antiporters constituted the slow phase. The use of uncouplers of photosynthetic
electron transport such as carbonylcyanide m-chlorophenylhydrazone and carbonylcyanide
p-trifl uoro-methoxyhydrazone prevented the restoration and recovery of PSII and PSI suggesting
that the ATP produced during photophosphorylation is important for tolerance of photosynthetic
machinery to salt stress. Pomati
et al
. (2004) studied the effect of ion channel modulating agents on
Na
+
-K
+
fl uxes of whole cells of
Cylindrospermopsis raciborskii
strain T3 and
C
.
racibosrskii
strain AWT
205 under salt stress (10 mM). An increase in extracellular pH to alkalinity was directly proportional
to the increase in cellular Na
+
levels whereas at pH (8.0) that is optimum for growth, cellular K
+
levels
predominantly increased. In presence of channel modulating agents, like lidocaine hydrochloride
(1 µM) and veratridine (100 µM), the increase of Na
+
was coupled with K
+
but in presence of channel
blockers such as aniloride (1 mM) and saxitoxin (1 µM) there was a decrease in the levels of both
ions. Na
+
/H
+
antiporter mediated H
+
accumulation caused the Na
+
effl ux, energy for which may
have been obtained through a proton motive force.
The presence of aquaporin proteins specifi c to water have been detected in all major groups
of organisms. Although it has been suggested that water effl ux from cells through the cytoplasmic
membrane takes place due to higher osmotic stress, conclusive proof in favour of this has only been
presented by Shapiguzov
et al
. (2005). The identifi cation of an aquaporin (
aqpZ
) gene in
Synechocystis
sp. strain PCC 6803 and its mutation led to prevention of the water effl ux due to osmotic stress of
the mutant cells. PCR analysis showed that such mutant cells possessed a disrupted copy of the
gene and the stress caused by 0.5 M sorbitol did not cause any shrinkage in mutant cells (due to
prevention of water loss) but in the wild-type, the cell shrinkage was half of the original level (due
to water loss). A comparison of the gene expression profi les, conducted after hyperosmotic stress
for 15, 60 and 120 min, of the wild-type and the mutant revealed that the mRNA levels of nearly
150 genes showed more than two-fold increase in the wild-type cells at 15 min. However, in the
mutant cells the expression of certain genes governing the synthesis of chaperones (
hspA
,
htpG
),
proteases (
htrA
.
clpB
,
hhoA
), glucosylglycerol phosphate synthase (
ggps
) and many other proteins
(26) of unknown function was signifi cantly affected. On the other hand, at 60 min though similar
sets of genes were induced in both wild-type and AqpZ mutant but it was less marked in the latter.
This signifi es that the disruption of
aqpZ
did not cause any direct infl uence on the expression of
these genes. It was also noted that at both 15 min and 60 min the expression of
ggps
was markedly
reduced. However, the essentiality of
aqpZ
gene for glucose metabolism during photomixotrophic
growth of
Synechocystis
sp. strain PCC 6803 has been highlighted (Akai
et al
., 2011).
Reports exist in literature on the involvement of lipids in conferring protection against salt
stress (Hufl ejt
et al
., 1990; Khamutov
et al
., 1990; Ritter and Yopp, 1993). The presence of unsaturated
fatty acids in the membrane lipids enhanced the tolerance of
Synechocystis
sp. strain PCC 6803 cells
to salt stress. Tasaka
et al
. (1996) isolated mutants by targeted mutagenesis of the genes of fatty
acid desaturases that produced low levels of unsaturated fatty acids. A comparison of desaturase
(
des)
-defi cient mutants of
Synechocystis
sp. strain PCC 6803 with that of wild-type suggested that
unsaturated fatty acids are required for protection of photosynthetic machinery from salt stress.
The cells defi cient in
desA
and
desD
genes governing Δ
12
and Δ
6
desaturases, respectively contained
