1. Introduction
By 2050, the human population is projected to reach ∼9.7 billion, which
is concomitant with increasing food demand in future. To overcome this
challenge, the World Health Organization has suggested doubling the
global food production. Thus, to make the balance between production and
demand, suitable farming should be practiced for intensive agricultural
production. Great pressure mounts on agricultural land to grow
high-yielding varieties of crops on the account of continuous inputs of
chemical fertilizers that leaches into the ground water reaching the
water bodies. These chemical fertilizers are huge source for several
heavy metals, apart from this, release of industrial wastes in aquatic
systems, and several natural and anthropogenic activities are also some
sources. Nickel (Ni2+) is one of the heavy metal
released into agro-ecosystems such as rice field, through effluents,
from Ni-Cd batteries, iron, steel, electroplating industries, sewage
sludge, burning of fossil fuels etc. (Nnorom and Osibanjo, 2009), which
disturbs the soil equilibrium. Nickel is an essential metal; therefore,
its lower concentrations is essential for the activities of some enzymes
(Muyssen et al., 2004); however, at higher concentration this metal is
fatal for the growth of plants/ cyanobacteria where it reduces
photosynthetic pigments and lipid contents, interferes with electron
transport chain, photosynthetic activity and antioxidant defense system
(Muyssen et al., 2004; Boisvert et al., 2007; Martínez-Ruiz and
Martínez-Jerónimo, 2015; Jahan et al., 2020). It also produces reactive
oxygen species (ROS), which damages macromolecules like carbohydrate,
lipids, DNA, proteins and causes cell death (Prasad et al., 2005;
Boisvert et al., 2007; Balaji et al., 2013; Jahan et al., 2020).
Since few decades, the focus is being shifted to utilize the protectants
like signaling molecules, nutrient management, metabolites,
phytohormones to manage the negative impact of toxicants like
Ni2+ to improve the plant/ cyanobacterial growth (Chen
et al., 2013; Tiwari et al., 2018; Batista et al., 2019; Tiwari et al.,
2019a,b; Singh et al., 2018a,b; Singh et al., 2020a,b). Plant hormones
are necessary for plant-environment communication; salicylic acid (SA)
is one of them which are naturally produced by plants (Akula and
Ravishankar, 2011). The SA ameliorates toxicity symptoms by adjusting
enzymatic and non-enzymatic defense systems, promotes important
physiological processes like enzymes of defense compounds, nitrogen
metabolism, osmolytes by stimulating glycinebetaine and proline, under
stressful situations (Belkadhi et al., 2014; Ma et al., 2017; Batista et
al., 2019; Rai et al., 2020). SA reduces toxicity by modulating
carbohydrate metabolism, leaf gas exchange, improving photosynthetic
rate, RubisCO activity, ATP synthesis and maintains
Na+/K+ ratio in plants (Lee et al.,
2014; Ghassemi-Golezani and Lotfi, 2015; Batista et al., 2019). However,
the impact of SA in cyanobacteria is rarely studied.
Calcium (Ca2+) and nitric oxide (NO), on the other
hand are the backbone of plant signaling events in unstressed and
stressed conditions. From earlier studies it has been cleared that
Ca2+ plays essential role in regulation of growth, and
different physiological processes of plants like stabilizes membrane
structures by bonding with phospholipid bilayer, declines ROS levels,
enhances key antioxidant enzymes, improves RubisCO activity and
CO2 fixation (Singh et al., 2018a,b; Tiwari et al.,
2019a; Verma et al., 2018; Singh et al., 2020a,b), being an essential
part of catalytic site of water oxidation
(Mn4Ca1OxCly)
of PSII, it helps in the assemblage of water oxidizing complex (OEC) and
directly regulates the photosynthetic activity (Andréasson et al., 1995;
Bartlett et al. 2008; Najafpour et al., 2012; Kalaji et al., 2014; Singh
et al., 2018b; Singh et al., 2020a) under stressful environment. NO is
also known for improvement in growth by regulating different
physiological events like growth, photosynthetic pigments,
photosynthetic activities, antioxidants defence system and reduces ROS
levels produced by metals/ metalloids either by gene expression through
signaling in a molecular cascade or by neutralizing excessive ROS
through its free radical functioning (Zhang et al., 2008; Peto 2011;
Chen et al., 2013; Xu et al., 2013; Sun et al., 2018; Tiwari et al.,
2018; Singh et al., 2020a; Verma et al., 2020). According to a report,
NO alleviate salinity stress by promoting photosynthetic rate in
eggplant either by improving PSII quantum yield or by splitting excess
energy via improving antenna molecules and carotenoid content (Wu et
al., 2013).
In paddy fields, about 20-30 kg N ha−1season−1 is being fixed by Anabaena , which
reveals its importance in improving the paddy yield (Chaurasia and Apte,
2011). Anabaena is the microscopic photosynthesizing organism
thus, are the primary producers that sustain aquatic food web
(Martínez-Ruiz and Martínez-Jerónimo, 2015); therefore, the toxicity
produced at this level is estimated to affect the consumers and higher
trophic levels. Recently, due to increasing involvement of human
activities in the aquatic ecosystem, these nitrogen fixing cyanobacteria
are facing enormous challenges especially in terms of metal toxicity,
which leads to decline in soil fertility and consequently the paddy
productivity. From the above literature, it is clear that SA,
Ca2+ and NO plays important role in regulating
different processes in plants and cyanobacteria under fluctuating
environmental condition; however, their cumulative effect in
orchestrating cyanobacterial responses against different environmental
cues is not established. Therefore, in order to understand the
interaction and interrelation of SA, Ca2+ and NO in
the regulation of Ni2+-induced toxicity inAnabaena PCC 7120, various physiological, and biochemical
approaches in Anabaena cells suffering from
Ni2+-toxicity, has been investigated. Further, to find
out the probable mechanism(s) in regulating SA induced signaling
(Ca2+ and NO mediated) in Anabaena PCC 7120
suffering from Ni2+-toxicity the key components i.e.
growth and growth regulating processes: intracellular
Ni2+ accumulation, exopolysaccharaides, photosynthetic
pigments, PSII photochemistry, nitrogen metabolism status, ROS level and
antioxidants defense system were assessed in the present investigation.