Castasterone and Citric Acid Supplementation Alleviates Cadmium Toxicity by Modifying Antioxidants and Organic Acids in Brassica juncea

Abstract

Brassinosteroids are known to manage various stresses including heavy metals in plants and castasterone is a bioactive form of brassinosteroids. Citric acid, a known metal chelator which is an intermediate of tricarboxylic cycle, is involved in various environmental adaptations. The present study was conducted to evaluate the synergistic role of citric acid and castasterone on various physiological and biochemical responses in Brassica juncea seedlings under cadmium stress. B. juncea seedlings were subjected to two levels of cadmium (0 and 0.6 mM) and citric acid (0 and 0.6 mM), and four levels of castasterone (0, 0.01, 1, and 100 nM). Exposure of cadmium induced phytotoxicity by elevating hydrogen peroxide (H2O2) and superoxide anion ( $${\text{O}}_{2}^{{. - }}$$ ) generation, decreasing photosynthetic pigment concentration, altering total carbohydrate content, and inducing stomatal closure. Seed priming with castasterone (at highest level) along with citric acid was most effective in ameliorating cadmium-induced toxicity which was manifested via reduced H2O2 and $${\text{O}}_{2}^{{. - }}$$ levels by 30.71 and 12.12%, respectively. The photosynthetic pigments, namely chlorophyll and carotenoid content, were improved by 47.17 and 34.06%, respectively, whereas total carbohydrates and polyphenolic contents were increased by 32.59 and 42.44%, respectively. Additionally, endogenous low-molecular weight organic acids were altered in response to cadmium toxicity, which were also increased with the castasterone and citric acid treatments. The results suggest that binary treatment of citric acid and castasterone may be more effective than individual treatments in alleviating cadmium-induced physiological damage through antioxidants and organic acids.