Fluoride Stress-Mediated Regulation of Tricarboxylic Acid Cycle and Sugar Metabolism in Rice Seedlings in Absence and Presence of Exogenous Calcium

The aim of the current study was to investigate the regulation of tricarboxylic acid (TCA) cycle and sugar metabolism in the seedlings of the rice variety, Khitish, during fluoride stress, either in absence or presence of exogenous calcium compounds, viz., calcium hydroxide and calcium nitrate. The inhibitory effect of fluoride stress on energy production was reflected by lowered levels of pyruvic acid, citric acid and malic acid, substantial interference with the respiratory enzymes such as PDH, CS, IDH and SDH, and inhibiting the expression of the concerned genes. Seed priming with both calcium compounds elevated the organic acid levels, restored the activity of the above enzymes and expression of the corresponding genes. On the contrary, the enhanced fumarase and MDH activity and MDH gene expression, during fluoride stress, was lowered by calcium pre-treatment. The sucrose level, together with SPS and SS activity and expression of the corresponding genes were restored to higher levels, concomitant with lowered INV activity and gene expression, in presence of calcium compounds, suggesting lesser utilization of sucrose to combat fluoride stress. The efficacy of calcium compounds in conserving starch as the major food reserve, without provoking its catabolic breakdown, was established on the basis of enhanced starch content with lowered α-amylase and starch phosphorylase activity and corresponding gene expression. The present communication for the first time documented the detrimental impact of fluoride stress on TCA cycle metabolites and majority of the enzymes, and endogenous sucrose and starch accumulation, as well as highlighted the reversal of the negative effects with the exogenous application of calcium compounds.