Calcium sustains K+-Na+ homeostasis through regulating nitrogen metabolism in hydrogen sulfide-dependent manner in tomato roots under salinity

Abstract

The work presented here exhibits the function of calcium (Ca) and hydrogen sulfide (H2S) in the tolerance of tomato seedlings against NaCl stress. The NaCl (100 mM)-stressed seedlings supplemented with 20 mM Ca, showed a substantial increase in K+/Na+ ratio which however, exhibited a significant decrease when they did not receive Ca. Using plasma membrane inhibitors, it was observed that Ca maintained a higher K+/Na+ ratio by altering K+ and Na+ transport via regulation of Na+-H+ antiport system. However, a reverse trend was noticed when 1 mM hypotaurine (HT; H2S scavenger) was applied along with Ca which validates the role of H2S in Ca-mediated regulation of K+-Na+ homeostasis. Interactive effect of Ca and H2S also improved the activity of the enzymes involved in nitrogen (N) metabolism. Impact of enhanced activity of the enzymes of N metabolism was also reflected in increased inorganic N forms viz. nitrate, nitrite, and ammonium. The stressed seedlings supplied with Ca exhibited higher activity of antioxidant enzymes (superoxide dismutase, catalase, and peroxidase) which maintained optimum level of reactive oxygen species (ROS) which was higher in the stressed seedlings that did not receive Ca. A reduction in ROS by Ca led to reduced lipid peroxidation and electrolyte leakage than the NaCl-stressed seedlings. On the other hand, application of HT exhibited a reverse trend in these parameters even in presence of Ca. It signifies H2S-dependent functioning of Ca. However, a non-significant effect of HT on root Ca content suggests upstream functioning of Ca to H2S. Therefore, it can be postulated that Ca, through regulating N metabolism, protected the seedlings against NaCl-induced constrains in H2S-dependent manner, and Ca functions upstream of H2S in the regulation of plant adaptive mechanisms to salinity.