Low concentrations of methyl jasmonate promote plant growth and mitigate Cd toxicity in Cosmos bipinnatus

Abstract

Cadmium (Cd) is a biologically non-essential heavy metal, a major soil pollutant, and extremely harmful to plants. The phytohormone methyl jasmonate (MeJA) plays an important role in plant heavy-metal resistance. However, the understanding of the effects of MeJA supply level on alleviating Cd toxicity in plants is limited. Here, we investigated how MeJA regulated the development of physiological processes and cell wall modification in Cosmos bipinnatus. We found that low concentrations of MeJA increased the dry weight of seedlings under 120 µM Cd stress by reducing the transport of Cd from roots to shoots. Moreover, a threshold concentration of exogenous MeJA increased the activities of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) in plant roots, the concentration of Cd in the root cell wall, and the contents of pectin and hemicellulose 1 polysaccharides, through converting Cd into pectin-bound forms. These results suggested that MeJA mitigated Cd toxicity by modulating root cell wall polysaccharide and functional group composition, especially through pectin polysaccharides binding to Cd, with effects on Cd transport capacity, specific chemical forms of Cd, and homeostatic antioxidant systems in C. bipinnatus.

Keywords: Cosmos bipinnatus; Cadmium; Cell wall; Methyl jasmonate.

Fig. 1

Flow chart for extraction of Cosmos bipinnatus cell wall and cell wall components

Fig. 2

Effect of different concentrations of methyl jasmonate (MeJA) on Cd concentration and their translocation factors (TFs) in Cosmos bipinnatus. A, Cd concentrations in roots. B, Cd concentrations in shoots. C, Cd translocation factors. The values shown are the means ± standard error (n = 3). Bars labelled with different letters were significantly different among treatments at p < 0.05. N.D., not detected; Cd, 120 µM CdCl₂; Cd + MeJA1, 120 µM CdCl₂ + 0.1 µM MeJA; Cd + MeJA2, 120 µM CdCl₂ + 1 µM MeJA; Cd + MeJA3, 120 µM CdCl₂ + 10 µM MeJA; Cd + MeJA4, 120 µM CdCl₂ + 100 µM MeJA

Fig. 3

Effect of different concentrations of methyl jasmonate (MeJA) on the subcellular distribution of Cd. A, Concentration of Cd in three subcellular fractions of shoot tissue. B, Concentration of Cd in three subcellular fractions in root tissue. The values shown are means ± standard deviations of three biological replicates. Values followed by different lowercase letters differed significantly at p < 0.05. N.S., not significant; F1, cell wall fraction; F2, organelle-containing; F3, soluble fraction; Cd, 120 µM CdCl₂; Cd + MeJA1, 120 µM CdCl₂ + 0.1 µM MeJA; Cd + MeJA2, 120 µM CdCl₂ + 1 µM MeJA; Cd + MeJA3, 120 µM CdCl₂ + 10 µM MeJA; Cd + MeJA4, 120 µM CdCl₂ + 100 µM MeJA

Fig. 4

Effect of different concentrations of methyl jasmonate (MeJA) on chemical forms of Cd. The values presented are means ± standard deviations of three biological replicates. Values followed by different lowercase letters are significantly different at a threshold of p < 0.05. N.S., not significant; F_ethanol, extraction of inorganic Cd, including nitrate/nitrite, chloride, and aminophenol Cd; Fd-H₂O, extraction of water-soluble Cd in the form of organic acid complexes and Cd (H₂PO₄)₂; F_NaCl, extraction of P-integrated Cd; F_HAC, extraction of insoluble CdHPO₄ in the form of organic acids and Cd (H₂PO₄)₂ and other Cd-phosphate complexes; F_HCl, extraction of oxalate acid-bound Cd; F_RES, Cd in residues; Cd, 120 µM CdCl₂; Cd + MeJA1, 120 µM CdCl₂ + 0.1 µM MeJA; Cd + MeJA2, 120 µM CdCl₂ + 1 µM MeJA; Cd + MeJA3, 120 µM CdCl₂ + 10 µM MeJA; Cd + MeJA4, 120 µM CdCl₂ + 100 µM MeJA

Fig. 5

Effect of different concentrations of methyl jasmonate (MeJA) on malondialdehyde (MDA) concentration, superoxide dismutase (SOD) activity, peroxidase (POD) activity, and catalase (CAT) activity in roots of Cosmos bipinnatus. A, MDA concentration. B, SOD activity of roots. C, POD activity of roots. D, CAT activity of roots. The values shown are means ± standard deviations of three biological replicates. Values followed by different lowercase letters are significantly different at a threshold of p < 0.05. Cd, 120 µM CdCl₂; Cd + MeJA1, 120 µM CdCl₂ + 0.1 µM MeJA; Cd + MeJA2, 120 µM CdCl₂ + 1 µM MeJA; Cd + MeJA3, 120 µM CdCl₂ + 10 µM MeJA; Cd + MeJA4, 120 µM CdCl₂ + 100 µM MeJA

Fig. 6

Effects of methyl jasmonate (MeJA) treatment on the content of polysaccharides in the cell wall of Cosmos bipinnatus roots. A, The content of pectin (P). B, The content of hemicellulose 1 (HC1). C, The content of hemicellulose 2 (HC2). The values shown are means ± standard deviations of three biological replicates. Values followed by different lowercase letters are significantly different at a threshold of p < 0.05. Cd, 120 µM CdCl₂; Cd + MeJA1, 120 µM CdCl₂ + 0.1 µM MeJA; Cd + MeJA2, 120 µM CdCl₂ + 1 µM MeJA; Cd + MeJA3, 120 µM CdCl₂ + 10 µM MeJA; Cd + MeJA4, 120 µM CdCl₂ + 100 µM MeJA

Fig. 7

Cd content in cell well polysaccharides treated with different methyl jasmonate (MeJA) treatments. A, Cd content in the pectin (P) fraction. B, Cd content in the hemicellulose 1 (HC1) fraction. C, Cd content in the hemicellulose 2 (HC2) fraction. D, Cd concentration in P, HC1, and HC2 fractions under Cd treatment alone. The values shown are means ± standard error (n = 3). Bars labelled with different lowercase letters indicate significant differences among treatments at a threshold of p < 0.05. N.D., not detected; Cd, 120 µM CdCl₂; Cd + MeJA1, 120 µM CdCl₂ + 0.1 µM MeJA; Cd + MeJA2, 120 µM CdCl₂ + 1 µM MeJA; Cd + MeJA3, 120 µM CdCl₂ + 10 µM MeJA; Cd + MeJA4, 120 µM CdCl₂ + 100 µM MeJA

Fig. 8

Correlation between root cell wall (RCW) polysaccharide composition and concentration of methyl jasmonate (MeJA). A, Correlation between pectin (P) content and MeJA concentration. B, Correlation between hemicellulose 1 (HC1) content and MeJA concentration. C, Correlation between hemicellulose 2 (HC2) content and MeJA concentration

Fig. 9

Fourier transform infrared (FTIR) spectra of the root cell wall (RCW) of Cosmos bipinnatus under different treatments

Fig. 10

Effect of different concentrations of methyl jasmonate (MeJA) on the Cd adsorption mechanism of Cosmos bipinnatus