STRESS RESPONSES - Cyanobacteria - page 617

monounsaturated fatty acids but not polyunsaturated fatty acids. A comparison of desA - / desD - cells
of Synechocystis sp. strain PCC 6803 to sodium-induced toxicity revealed that the recovery of these
cells from salt stress is rather slow when compared to wild-type cells. Allakhverdiev et al . (1999)
demonstrated that genetic engineering of the unsaturation of fatty acids in membrane lipids alters
the tolerance of Synechocystis sp. strain PCC 6803 to salt stress. This is supported by inhibitory
studies. Light reversal of NaCl-induced stress was eliminated by the presence of lincomycin,
an inhibitor of photosynthesis. Likewise, the presence of N,N'-dicyclohexylcarbodiimide, an
inhibitor of H + -ATPase, enhanced the NaCl-induced toxicity suggesting the involvement of Na + /
H + antiporter in conferring salt tolerance. The presence of high degree of unsaturated fatty acids in
the membrane lipids contributes to enhanced fl uidity of membrane. The desA - and desD - defi cient
cells of Synechocystis sp. strain PCC 6803 exhibited more sensitive Na + /H + antiporter to salt stress
with a slow recovery than the wild-type cells. Transformation of S. elongatus PCC 7942 with desA
gene from Synechocystis sp. strain PCC 6803 resulted in the synthesis of di-unsaturated fatty acids
(Sakamoto et al ., 1994) which otherwise normally produced only saturated and monounsaturated
fatty acids in its membrane lipids (Murata and Wada, 1995). Transformants of S. elongatus PCC 7942
with desA gene (Δ 12 desaturase) exhibited differences over wild-type with reference to fatty acid
synthesis (Allakhverdiev et al ., 2001). In wild-type cells, the synthesis of 16:0 (49%) and 16.1 (41%)
is most abundant followed by low levels of 18:0, 18:1Δ 9 and 18:1Δ 12 . On the other hand, in DesA +
cells, the synthesis of 16:2Δ 9,12 appeared (up to 15%) suggesting that certain amounts of 16:1Δ 9 has
been desaturated. Likewise 18:2Δ 9,12 also appeared accounting for 3% of the total fatty acids. The
PSII of the DesA + cells of S. elongatus PCC 7942 appeared to be more resistant to salt stress. Light
restoration of PSII activity occurred within 3 h to about 50% of the original level which remained
constant for the next 10 h. The possible sites affected by the unsaturation of fatty acids in membrane
lipids included water channels, K + (Na + ) channels, Na + /H + antiporter system and H + -ATPase(s) all
of which are located on plasma membrane. These observations constitute direct evidences for the
protection and maintenance of photosynthetic machinery under salt stress.
The inhibition in the repair of photodamaged PSII of Synechocystis sp. strain PCC 6803 has been
identifi ed by Allakhverdiev et al . (2002) to be due to suppression in the transcription and translation
of psbA genes. An examination of the synergistic effects of light (250 to 2,000 µE m -2 s -1 ) and salt
stress (0.5 M) revealed that approximately 60% of the light-inducible genes were strongly inhibited
by salt stress of which 20% were moderately suppressed. A proportion of 20% of the light-inducible
genes were enhanced by 0.5 M NaCl. The levels of D1 protein that declined in high light intensity
were returned to normal levels due to a repair in weak light as well as in low salt medium but in
presence of high concentration of NaCl (1.0 M) there was no increase in the level of D1 protein. DNA
microarray analysis revealed complete inhibition by NaCl (1.0 M) of light-induced transcripts of
all light-inducible genes. These results have been found to be consistent with the results obtained
with [S 35 ]-Met incorporation studies. However, the light-inducibility of some light-inducible genes
was enhanced to some extent in low concentrations of NaCl (0.5 M). Allakhverdiev and Murata
(2008) summarized recent progress on the effects of salt stress on photosynthesis in cyanobacteria
and suggested that the damage caused to PSII and PSI occurs through rapid and slow changes. The
fi rst rapid changes are associated with a decrease of photosynthetic activity occuring within the fi rst
few min causing the effl ux of water through water channels and infl ux of Na + into the cells. The
slow changes that occur probably in few hours are associated with dissociation of extrinsic proteins
from PSI and PSII. When salt-stressed (0.8 M NaCl) Spirulina platensis was incubated either in dark
or under different light intensities the fi rst phase was found to be independent of light that was
characterized by a rapid decrease in PSII activity (20-50% in the fi rst 15 min) followed by a subsequent