Transcriptome Profiling, Physiological and Biochemical Analyses Reveal Comprehensive Insights in Cadmium Stress in Brassica carinata L

Abstract

With the constant progress of urbanization and industrialization, cadmium (Cd) has emerged as one of the heavy metals that pollute soil and water. The presence of Cd has a substantial negative impact on the growth and development of both animals and plants. The allotetraploid Brasscia. carinata, an oil crop in the biofuel industry, is known to produce seeds with a high percentage of erucic acid; it is also known for its disease resistance and widespread adaptability. However, there is limited knowledge regarding the tolerance of B. carinata to Cd and its physiological responses and gene expressions under exposure to Cd. Here, we observed that the tested B. carinata exhibited a strong tolerance to Cd (1 mmol/L CdCl₂ solution) and exhibited a significant ability to accumulate Cd, particularly in its roots, with concentrations reaching up to 3000 mg/kg. Additionally, we found that the total oil content of B. carinata seeds harvested from the Cd-contaminated soil did not show a significant change, but there were noticeable alterations in certain constituents. The activities of antioxidant enzymes, including catalase (CAT), superoxide dismutase (SOD), peroxidase (POD), and ascorbate peroxidase (APX), were observed to significantly increase after treatment with different concentrations of CdCl₂ solutions (0.25 mmol/L, 0.5 mmol/L, and 1 mmol/L CdCl₂). This suggests that these antioxidant enzymes work together to enhance Cd tolerance. Comparative transcriptome analysis was conducted to identify differentially expressed genes (DEGs) in the shoots and roots of B. carinata when exposed to a 0.25 mmol/L CdCl₂ solution for 7 days. A total of 631 DEGs were found in the shoots, while 271 DEGs were found in the roots. It was observed that these selected DEGs, which responded to Cd stress, also showed differential expression after exposure to PbCl₂. This suggests that B. carinata may employ a similar molecular mechanism when tolerating these heavy metals. The functional annotation of the DEGs showed enrichment in the categories of 'inorganic ion transport and metabolism' and 'signal transduction mechanisms'. Additionally, the DEGs involved in 'tryptophan metabolism' and 'zeatin biosynthesis' pathways were found to be upregulated in both the shoots and roots of B. carinata, suggesting that the plant can enhance its tolerance to Cd by promoting the biosynthesis of plant hormones. These results highlight the strong Cd tolerance of B. carinata and its potential use as a Cd accumulator. Overall, our study provides valuable insights into the mechanisms underlying heavy metal tolerance in B. carinata.

Keywords: Brassica carinata; cadmium tolerance; physiological response; transcriptome.

Figure 1

Seedlings performance of B. carinata exposed to different concentrations of CdCl₂ solu tions. (a–d): the performance of shoots of B. carinata plants treated with 0 mmol/L (CK), 0.25 mmol/L, 0.5 mmol/L, and 1 mmol/L CdCl₂ solutions for 7 days. (e–h): the performance of young seedlings treated with 0 mmol/L (CK), 0.25 mmol/L, 0.5 mmol/L, and 1 mmol/L CdCl₂ solutions for 7 days.

Figure 2

The changes of root microstructure after 7 days of different concentrations of Cd treatment. bar = 50 μm. (A–D): The microstructure of B. carinata root tips in control group (A) and under different concentrations of Cd treatment (B–D) after PI staining.

Figure 3

The changes in Cd content and translocation factor in shoots and roots after seven days of treatment with different concentrations of CdCl₂ solution. (a): Cd content in shoots and roots. (b): translocation factor. Different letters represent the statistically different group, which is determined via multiple comparisons using Fisher’s least significant difference (LSD) method (p < 0.05). The error bar in the chart indicates the standard error (SE), and three replicates (n = 3) per sample are used to calculate the value of mean ± SE.

Figure 4

The oil content, constituents, and Cd content in seeds harvested from Cd-contaminated soil. (A): total oil content; (B): oleic acid content; (C): linoleic acid content; (D): linolenic acid content; (E): erucic acid content; (F): glucosinolates content; (G): Cd content. Different letters represent the statistically different group, which is determined via multiple comparisons using Fisher’s least significant difference (LSD) method (p < 0.05).

Figure 5

Physiological indicator changes in B. carinata seedlings under different concentrations of Cd treatment. (a): APX activity; (b): CAT activity; (c): MDA content; (d): POD activity; (e): PRO content; (f): SOD activity. Different letters represent the statistically different group determined via the LSD method (p < 0.05). The error bar in chart indicates SE, and three replicates (n = 3) per sample are prepared.

Figure 6

The expression patterns of differential genes in shoots after treatment with 0.25 mM PbCl₂ solution for 7 days. (a–e): up-regulated DEGs. (f–j): down-regulated DEGs. The error bar in chart indicates SE, and three replicates (n = 3) per sample are prepared. Different letters represent the statistically different group determined via the LSD method (p < 0.05).

Figure 7

GO enrichment of DEGs in the comparisons of CKs vs. Cds and CKr vs. Cdr. (a,b): the upregulated and downregulated DEGs in CKs vs. Cds. (c,d): the upregulated and downregulated DEGs in CKr vs. Cdr.

Figure 8

Top 20 KEGG enrichment pathways of DEGs in the comparisons of CKs vs. Cds and CKr vs. Cdr. (a,b): upregulated and downregulated DEGs in CKs vs. Cds. (c,d): upregulated and downregulated DEGs in CKr vs. Cdr. The x-axis represents the ratio of the number of enriched single genes (sample number) (based on the rich factor) to the number of annotated single genes (background number) in the path, and the y-axis represents the name of the pathway.

Figure 9

DEGs involving in the plant hormone signal transduction signalling pathway in both roots and shoots. The top-to-bottom heatmap representations correspond to the shoots and roots of B. carinata. The heat maps from left to right are CKs and Cds in shoots, and CKr and Cdr in roots.