Role of brassinosteroids in alleviation of phenanthrene-cadmium co-contamination-induced photosynthetic inhibition and oxidative stress in tomato

Abstract

Heavy metal pollution often occurs together with organic contaminants. Brassinosteroids (BRs) induce plant tolerance to several abiotic stresses, including phenanthrene (PHE) and cadmium (Cd) stress. However, the role of BRs in PHE+Cd co-contamination-induced stress amelioration is unknown. Here, the interactive effects of PHE, Cd, and 24-epibrassinolide (EBR; a biologically active BR) were investigated in tomato plants. The application of Cd (100 µM) alone was more phytotoxic than PHE applied alone (100 µM); however, their combined application resulted in slightly improved photosynthetic activity and pigment content compared with Cd alone after a 40 d exposure. Accumulation of reactive oxygen species and membrane lipid peroxidation were induced by PHE and/or Cd; however, the differences in effect were insignificant between Cd and PHE+Cd. The foliar application of EBR (0.1 µM) to PHE- and/or Cd-stressed plants alleviated photosynthetic inhibition and oxidative stress by causing enhancement of the activity of the enzymes and related transcript levels of the antioxidant system, secondary metabolism, and the xenobiotic detoxification system. Additionally, PHE and/or Cd residues were significantly decreased in both the leaves and roots after application of EBR, more specifically in PHE+Cd-stressed plants when treated with EBR, indicating a possible improvement in detoxification of these pollutants. The findings thus suggest a potential interaction of EBR and PHE for Cd stress alleviation. These results advocate a positive role for EBR in reducing pollutant residues for food safety and also strengthening phytoremediation.

Fig. 1.

Interactive effects of PHE, Cd, and EBR on (A) plant biomass, (B) PHE residue, and (C) Cd content in tomato plants after 40 d of the respective treatments. The culture solution was co-contaminated with 100 µM PHE and 100 µM Cd. EBR (0.1 µM) was applied on the foliar portion. Bars of the same colour with different letters are significantly (P <0.05) different according to Tukey’s test. Data are the means, and the error bar indicates ±SD (n=8 for dry weight analysis and n=3 for pollutant residue analysis).

Fig. 2.

Interactive effects of PHE, Cd, and EBR on (A) leaf gas exchange and (B) chlorophyll fluorescence quenching parameters in tomato. All measurements were taken on the third fully expanded leaves after 40 d of the respective treatments. The culture solution was co-contaminated with 100 µM PHE and 100 µM Cd. EBR (0.1 µM) was applied on the foliar portion. Data are the means of six biological replicates (±SD). Means denoted by the same letter did not differ significantly at P < 0.05 according to Tukey’s test.

Fig. 3.

Effects of PHE, Cd, and EBR alone or in combination on the maximum photochemical efficiency of PSII (F _v/F _m) and chlorophyll contents in tomato leaves after 40 d of the respective treatments. (A) Original image of the third leaves (upper panels) and pseudocolour image of F _v/F _m (lower panels); (B) photosynthetic pigment contents under different treatments. Data are the average of three replicates and are presented as the mean ±SD. Means denoted by the same letters did not differ significantly at P < 0.05 according to Tukey’s test. Horizontal bars=1cm.

Fig. 4.

Interactive effects of PHE, Cd, and EBR on (A) O₂∙^– (upper panel) and H₂O₂ accumulation (lower panel), (B) H₂O₂ contents, and (C) MDA contents in tomato leaves after 40 d of the respective treatments. Accumulation of O₂∙^– and H₂O₂ in leaves was detected by NBT and DAB staining, respectively. Each value in the graph shows the mean with the standard deviation of three replicates. Means denoted by the same letters did not differ significantly at P < 0.05 according to Tukey’s test. Horizontal bars=2cm.

Fig. 6.

Effects of EBR on PHE and Cd detoxification-related enzyme activities, glutathione content, and related gene expression. Each value in the graph shows the mean with the standard deviation of three replicates. Means denoted by the same letters did not differ significantly at P < 0.05 according to Tukey’s test.

Fig. 7.

Interactive effects of PHE, Cd, and EBR on secondary metabolism-related (A) enzyme activities and (B) gene expression. Each value in the graph shows the mean with the standard deviation of three replicates. Means denoted by the same letters did not differ significantly at P < 0.05 according to Tukey’s test.

Fig. 5.

Interactive effects of PHE, Cd, and EBR on (A) antioxidant enzyme activities and (B) related gene expression. Each value in the graph shows the mean with the standard deviation of three replicates. Means denoted by the same letters did not differ significantly at P < 0.05, according to Tukey’s test.