3.1. SA, Ca2+ and NO recover
Ni2+-induced damage in growth
To examine the SA induced Ca2+ and NO signaling in
easing Ni2+‒induced toxicity, Anabaena PCC 7120
cells were exposed to different donors, scavengers and inhibitors of SA,
Ca2+ and NO. Nickel at its tested dose significantly
declined the growth (dry weight) of Anabaena by 31%, which is
attributed to: (i) higher Ni2+ accumulation in the
cell (Fig. 1D), (ii) reduction in photosynthetic pigments content (Table
1) and PSII performance (Fig. 2C), which subsequently decreased
photosynthetic rate (Fig. 2A), (iii) overproduction of ROS (Fig. 4),
(iv) reduction in NO3− and
NO2− uptake rate (Fig. 3A,B), and (v)
decrease in EPS content (Fig. 1B). However, under similar condition,
addition of SA, CaCl2 and SNP in the growing medium
counteracted Ni2+‒induced negative impact on DW (12,
11 and 7% respectively) of test cyanobacteria, which is in agreement
with the higher NO accumulation than in the Ni2+-
stressed cyanobacterial cells alone (Fig. 1C). The SA,
Ca2+ and NO induced progressive response on growth
have also been reported by Tiwari et al. (2018, 2019a) and Singh et al.
(2020a,b) in Anabaena PCC 7120 and mustard, respectively.
Further, in order to study the possible link between SA,
Ca2+ and NO; when Ni+SA+Ca and Ni+SA+NO treated
cyanobacteria cells were supplemented with c-PTIO (a NO scavenger) and
EGTA (a Ca2+ scavenger), respectively; a significant
reduction in DW was observed, thereby satisfying the fact of involvement
of Ca2+ and NO in signaling event. Interestingly, the
growth of c-PTIO supplemented Ni+SA+Ca treated cyanobacterial cells was
comparatively more affected even in presence of Ca2+(Fig. 1A), suggesting that NO is required for the maximal and constant
Ca2+ signaling, which corresponds to reduce NO content
in presence of c-PTIO (Fig. 1C). The report of Xu et al. (2013) have
strengthened our findings that by promoting NO release, ABA hormone
protected tall fescue plant from oxidative stress, and Singh et al.
(2020a,b) have suggested that NO is essential in Ca2+mediated signaling event in arsenic-stressed mustard seedlings.
Results for intracellular Ni2+ accumulation showed
higher Ni2+ accumulation under
Ni2+-stressed condition, which was neutralized upon
SA, CaCl2 and SNP treatment to
Ni2+-stressed cyanobacterial cells; however, in
presence of c-PTIO and EGTA, even SA was unable to limiting
Ni2+-accumulation; therefore, highest
Ni2+-accumulation was found in Ni+SA+c-PTIO+EGTA even
than that of Ni2+-stressed cyanobacterial cells alone
(Fig. 1D).