STRESS RESPONSES - Cyanobacteria - page 627

Studies on trehalose accumulation in cyanobacterial cells in response to salt stress and its
biosynthesis in salt-stressed cyanobacterial cells are available. Trehalose biosynthesis in cyanobacteria
occurs through the TreY-TreZ pathway. S . platensis tolerates salt concentrations up to 750 mM that
is 1.5 times more than the concentration of sea water (Giovanna et al ., 1983; Vonshak et al ., 1988;
Warr et al ., 1985; Zeng and Vonshak, 1998). Warr et al . (1985) reported that in addition to trehalose,
S . platensis showed the accumulation of GG when exposed to salt stress. Trehalose accumulation along
with sucrose in response to desiccating conditions has been reported by Higo et al . (2006) in Anabaena
sp. strain PCC 7120. The expression of this gene in response to drought stress was determined using
a DNA microarray technique (Katoh et al ., 2004; Higo et al ., 2006; Yoshimura et al ., 2006). Ohmori et
al . (2009) observed that trehalose accumulation inside the cells of S . platensis was dependent on the
presence of NaCl and its transport inside the cells because inhibition of the transport of Na + due to
the addition of amiloride and monesin resulted in a decrease of trehalose. The synthesis of trehalose
coincided with the increase of TreZ (or Mth), the enzyme that produces trehalose from maltooligosyl
trehalose. The expression of the gene mth in response to salt stress and the increase of cellular Mth
levels were noted in this organism. A comparative study of trehalose accumulation in a terrestrial
cyanobacterium Nostoc commune (both cultures as well as naturally occurring colonies) in response
to desiccation and salt stress revealed the existence of a correlation between loss of hydration and
the level of accumulation of trehalose. This coincided with the loss of photosynthetic O 2 evolution of
the cells in the dry state. As the colonies were rehydrated the photosynthetic O 2 evolution resumed
with no detectable trehalose levels in the cells. Likewise, the presence of low concentrations of
sodium chloride (0.2 M; non-ionic osmotic stress) inhibited photosynthetic O 2 evolution and induced
the accumulation of trehalose. Thus trehalose accumulation seems to be an important feature in
N . commune during desiccation (Sakamoto et al ., 2009).
The presence of mycosporine-like amino acids in unicellular cyanobacteria growing in a gypsum
crust at the bottom of a hypersaline saltern pond has been demonstrated in concentrations as high
as 98 mM. Signifi cantly, two mycosprine-like amino acids with an absorption spectrum at 332 nm
and 362 nm were present inside the cells. When the upper layers of the crusts were diluted, the
mycosprine-like amino acids were rapidly leached out directly proportional to the dilution stress
(Oren, 1997).
iii) Na + /H + antiporters : The Na + /H + antiporters are necessary to maintain ion homeostasis and
regulate the exchange of Na + and H + across the membrane. So these will be helpful in overcoming
salt stress by causing Na + effl ux in exchange for H + thereby decreasing intracellular Na + ions. An
electrochemical gradient of protons established by the respiratory chain or H + -translocating ATPase
drives this process. Thus the Na + /H + antiporters play a very important role in Na + extrusion, pH
homeostasis, cell volume regulation and establishment of electrical potential of Na + (Padan and
Schuldiner, 1994, 1996). The Na + /H + antiporter of E . coli is a high capacity Na + extrusion transporter
responsible for the salt tolerance of the cells at alkaline pH (Padan and Schuldiner, 1994; Padan et al.,
2001). In cyanobacteria, the mechanism for salt tolerance is suggested to be due to the active effl ux of
Na + and the accumulation of K + (Reed and Stewart, 1985; Richtie, 1992). The active export of Na + across
the membrane is suggested to be due to the action of primary Na + -ATPase (Richtie, 1992) because
the generation of substantial proton motive force is prevented due to adverse transmembrane pH
gradient at alkaline pH. At this juncture, it may be noted that the optimum growth of cyanobacteria
generally occurs at pH values ranging from 7.5 to 11.0.
Na + /H + antiporters are generally classifi ed as members of monovalent cation:proton antiporter
(CPA) superfamily. These are subdivided into the Na + -transporting carboxylic acid decarboxylase