Methyl jasmonate counteracts cadmium toxicity in water spinach plant by adjusting growth, physiology and redox regulation

Abstract

Increased cadmium (Cd) level in foods due to anthropogenic activities is a serious concern to public health. This study investigated the efficacy of exogenous methyl jasmonate (MeJA) application to mitigate adverse effects of Cd toxicity in water spinach plant. The seeds (cv. Gimakolmi) were primed with MeJA (2.5 and 5 µM) and grown under two levels of Cd (10 and 20 µM CdCl₂) with or without the respected levels of MeJA solutions under the hydroponic system. The experiment was set in a completely randomized design with three replications maintaining seven growth conditions: (1) Control, (2) Cd₁₀, (3) Cd₂₀, (4) Cd₁₀MJ_2.5, (5) Cd₁₀MJ₅, (6) Cd₂₀MJ_2.5 and (7) Cd₂₀MJ₅. Cd-stress significantly hindered growth and photosynthesis; induced oxidative damage accumulating higher malondialdehyde (MDA) and H₂O₂ contents; enhanced activities of antioxidative enzymes and increased Cd uptake in water spinach plant. The treatments Cd₁₀MJ₅ and Cd₂₀MJ₅ stimulated plant growth by increasing total dry mass (66% and 38%) and rate of photosynthesis (51% and 55%) of water spinach under two levels of Cd stress, respectively. Application of 5 µM MeJA considerably reduced leaf MDA (32% and 17% compared to Cd₁₀ and Cd₂₀, respectively) and H₂O₂ contents (49 and 42%) and enhanced the activities of superoxide dismutase (71% and 6%), catalase (120% and 61%) and peroxidase (57% and 65%) enzymes with reduced uptake of total Cd (38% and 45%) in water spinach plant. Conclusively, 5 µM MeJA effectively mitigated Cd toxicity in water spinach plant and can be adopted in Cd-contaminated areas with further field trials.

Keywords: Antioxidants; Cd tolerance; Gas exchange; Jasmonate; Leafy vegetable; Oxidative stress.

Fig. 1

28-Day-old water spinach plants grown in different growth conditions (Control, Cd₁₀ = 10 µM Cd, Cd₂₀ = 20 µM Cd, Cd₁₀MJ_2.5 = 10 µM Cd + 2.5 µM MeJA, Cd₁₀MJ₅ = 10 µM Cd + 5 µM MeJA, Cd₂₀MJ_2.5 = 20 µM Cd + 2.5 µM MeJA, Cd₂₀MJ₅ = 20 µM Cd + 5 µM MeJA).

Fig. 2

(A) Photosynthesis rate (A), (B) stomatal conductance (g_s), (C) transpiration rate (E), and (D) leaf greenness (SPAD value) of 28 days old hydroponically grown water spinach plants at different growth conditions. The vertical bars represent SEM (n = 3). Treatment means with different letters imply significant at 5% levels of probability. Treatment description: Cd₁₀ = 10 µM Cd, Cd₂₀ = 20 µM Cd, Cd₁₀MJ_2.5 = 10 µM Cd + 2.5 µM MeJA, Cd₁₀MJ₅ = 10 µM Cd + 5 µM MeJA, Cd₂₀MJ_2.5 = 20 µM Cd + 2.5 µM MeJA, Cd₂₀MJ₅ = 20 µM Cd + 5 µM MeJA.

Fig. 3

Chlorophyll a (Chl a), Chlorophyll b (Chl b), Total Chlorophyll (Total Chl) and Total carotenoids (Total Car) contents in the leaf of 28 days old hydroponically grown water spinach plants at different growth conditions. The vertical bars represent SEM (n = 3). Treatment means with different letters imply significant at 5% levels of probability. Treatment description: Cd₁₀ = 10 µM Cd; Cd₂₀ = 20 µM Cd; Cd₁₀MJ_2.5 = 10 µM Cd + 2.5 µM MeJA, Cd₁₀MJ₅ = 10 µM Cd + 5 µM MeJA; Cd₂₀MJ_2.5 = 20 µM Cd + 2.5 µM MeJA, Cd₂₀MJ₅ = 20 µM Cd + 5 µM MeJA.

Fig. 4

Hydrogen peroxide (H₂O₂) and lipid peroxidation (MDA content) in root and leaf of 28 days old hydroponically grown water spinach plants at different growth conditions. The vertical bars represent SEM (n = 3). Treatment means with different letters imply significant at 5% levels of probability. Treatment description: Cd₁₀ = 10 µM Cd, Cd₂₀ = 20 µM Cd, Cd₁₀MJ_2.5 = 10 µM Cd + 2.5 µM MeJA, Cd₁₀MJ₅ = 10 µM Cd + 5 µM MeJA, Cd₂₀MJ_2.5 = 20 µM Cd + 2.5 µM MeJA, Cd₂₀MJ₅ = 20 µM Cd + 5 µM MeJA.

Fig. 5

Total Antioxidant Capacity (TAC) and Proline content in root and leaf of 28 days old hydroponically grown water spinach plants at different growth conditions. The vertical bars represent SEM (n = 3). Treatment means with different letters imply significant at 5% levels of probability. Treatment description: Cd₁₀ = 10 µM Cd; Cd₂₀ = 20 µM Cd; Cd₁₀MJ_2.5 = 10 µM Cd + 2.5 µM MeJA, Cd₁₀MJ₅ = 10 µM Cd + 5 µM MeJA; Cd₂₀MJ_2.5 = 20 µM Cd + 2.5 µM MeJA, Cd₂₀MJ₅ = 20 µM Cd + 5 µM MeJA.

Fig. 6

Activities of superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD) in root and leaf of 28 days old hydroponically grown water spinach plants at different growth conditions. The vertical bars represent SEM (n = 3). Treatment means with different letters imply significant at 5% levels of probability. Treatment description: Cd₁₀ = 10 µM Cd, Cd₂₀ = 20 µM Cd, Cd₁₀MJ_2.5 = 10 µM Cd + 2.5 µM MeJA, Cd₁₀MJ₅ = 10 µM Cd + 5 µM MeJA, Cd₂₀MJ_2.5 = 20 µM Cd + 2.5 µM MeJA, Cd₂₀MJ₅ = 20 µM Cd + 5 µM MeJA.

Fig. 7

Cd contents (µg g⁻¹ DW) in roots, stems and leaves of 28 days old water spinach plant grown in seven growth conditions. Treatment means with different letters in the middle portion of the respective colored bars indicate significant differences at 5% levels of probability. Treatment description: Cd₁₀ = 10 µM Cd, Cd₂₀ = 20 µM Cd, Cd₁₀MJ_2.5 = 10 µM Cd + 2.5 µM MeJA, Cd₁₀MJ₅ = 10 µM Cd + 5 µM MeJA, Cd₂₀MJ_2.5 = 20 µM Cd + 2.5 µM MeJA, Cd₂₀MJ₅ = 20 µM Cd + 5 µM MeJA.

Fig. 8

Two-way hierarchical clustering heatmap of 40 measured traits of water spinach plants grown in several Cd treatments. The standardized stress tolerance index (STI) values were used to create the heatmap. A single column represents a trait while a single row denotes a treatment. Colors correspond to a relative scale of − 2 to 2 where darker red and blue indicate higher STI and lower STI scores, respectively. Both the traits and treatments were clustered into three groups. Traits description: r, s, l and t root, stem, leaf and total, respectively, PRO Proline, HP H₂O₂, Cd Cadmium, RL Root length, RSR Root-shoot ratio, MDA Malonaldehyde, SLA Specific leaf area, g_s stomatal conductance, E Transpiration rate, SOD Superoxide dismutase, TAC Total antioxidant capacity, POD Peroxidase, Chl Chlorophyll, CAR Total carotenoids, NoL Number of leaf, LFW Leaf fresh weight, A Rate of photosynthesis, RFW Root fresh weight, RDW Root dry weight, CAT Catalase, SFW Stem fresh weight, SDW Stem dry weight, LA Leaf area, TFW Total fresh weight, SL Shoot length, LDW Leaf dry weight, TDW Total dry weight.

Fig. 9

PCA-biplot illustrating the variability of seven growth conditions based on 40 measured traits in water spinach plant. The first and second principal components (PCs) explained 55.4% and 20.2% of the total variability, respectively. The traits’ contribution to the first two PCs is represented by the color gradients and arrow lengths. The longer and darker green arrows denote a higher contributing trait while the shorter and dark red arrows refer to lower contributing traits. Traits description: r, s, l and t root, stem, leaf and total, respectively, PRO proline, HP H₂O₂, Cd cadmium, RL root length, RSR root-shoot ratio, MDA malonaldehyde, SLA Specific leaf area, g_s stomatal conductance, E Transpiration rate, SOD Superoxide dismutase, TAC Total antioxidant capacity, POD Peroxidase, Chl Chlorophyll, CAR Total carotenoids, NoL Number of leaf, LFW Leaf fresh weight, A Rate of photosynthesis, RFW Root fresh weight, RDW Root dry weight, CAT Catalase, SFW Stem fresh weight, SDW Stem dry weight, LA Leaf area, TFW Total fresh weight, SL Shoot length, LDW Leaf dry weight, TDW Total dry weight.

Fig. 10

(A) Variance proportion (%) and (B) Eigenvalues of first 10 principal components (PCs) derived from the PCA-biplot. (C) The contribution of the first 20 traits to PC1 and (D) First 20 contributing traits to PC2. Bars above the reference lines (red dashed) in each plot are recognized as contributing characters to the respective PC.

Fig. 11

Correlation matrix of studied traits under Cd stress alone (left two panels) and Cd stress + MeJa (right panels). In all matrix plots, red and blue boxes represent positive and negative correlations, respectively, where greater color intensity indicates a higher coefficient. *, **, and *** signify significance at p < 0.05, p < 0.01, and p < 0.001, respectively. Traits description: r, s, l and t root, stem, leaf and total, respectively, PRO proline, HP H₂O₂, Cd cadmium, RL root length, RSR root-shoot ratio, MDA malonaldehyde, SLA specific leaf area, g_s stomatal conductance, E transpiration rate, SOD superoxide dismutase, TAC total antioxidant capacity, POD peroxidase, Chl chlorophyll, CAR total carotenoids, NoL number of leaf, LFW leaf fresh weight, A rate of photosynthesis, RFW root fresh weight, RDW root dry weight, CAT catalase, SFW stem fresh weight, SDW stem dry weight, LA leaf area, TFW total fresh weight, SL shoot length, LDW leaf dry weight, TDW total dry weight.