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.