A Transcriptomic Study on the Toxic Effects of Iodide (I-) Wet Deposition on Pepper (Capsicum annuum) Leaves

Abstract

Radioactive iodine (¹²⁹I), released into the environment from human nuclear activities, poses significant health risks to the biosphere due to its long half-life and mobility. This study investigates the toxic effects of wet-deposited iodine on the growth of chili pepper seedlings (Capsicum annuum L.) under soil cultivation conditions. Using sodium iodide (NaI) as the exposure agent, transcriptomic analysis was conducted to evaluate the molecular responses of chili pepper leaves to iodine at concentrations of 2, 4, and 8 ppm. The study identified 2440 and 1543 differentially expressed genes (DEGs) in leaves exposed to 2 ppm vs. 4 ppm iodine and 2 ppm vs. 8 ppm iodine, respectively. GO enrichment analysis showed that DEGs at 4 ppm were significantly associated with protein-chromophore linkage, extracellular region, and iron ion binding, while those at 8 ppm were enriched in defense response, cell wall components, and iron ion binding. Iodine stress disrupted key pathways associated with photosynthesis, antioxidant defense, and cuticle biosynthesis. In particular, the downregulation of key genes related to protein-chromophore binding, lipid metabolism, and cell wall organization indicated reduced photosynthetic efficiency and weakened stress resistance. This study provides molecular-level insights into the ecological risks of iodine stress in plants and offers a scientific basis for managing iodine contamination and breeding iodine-tolerant chili pepper cultivars.

Keywords: Capsicum annuum L.; differential gene expression; functional enrichment analysis; toxic effects; transcriptomics; wet-deposited iodine.

Figure 1

A conceptual diagram of the wet deposition iodine exposure experiment.

Figure 2

Statistics of upregulated and downregulated differentially expressed genes. Note: The iodine exposure concentrations for the PEYP-1-1, PEYP-1-2, and PEYP-1-3 treatment groups were 2 ppm, 4 ppm, and 8 ppm, respectively. The orange bars represent upregulated genes, while the green bars represent downregulated genes.

Figure 3

Volcano maps of differential gene expression. Note: (a) PEYP-1-1 vs. PEYP-1-2; (b) PEYP-1-1 vs. PEYP-1-3. The iodine exposure concentrations for the PEYP-1-1, PEYP-1-2, and PEYP-1-3 treatment groups were 2 ppm, 4 ppm, and 8 ppm, respectively. The horizontal axis represents the log2 (Fold Change), illustrating the difference in fold change, and the vertical axis represents the −log10 (FDR), illustrating the false discovery rate. Among them, the red dot represents the upregulated gene, the gray dot represents the gene with no significant difference, and the blue dot represents the downregulated gene.

Figure 4

A bar chart of GO functional enrichment analysis for each treatment. Note: (a) PEYP-1-1 vs. PEYP-1-2; (b) PEYP-1-1 vs. PEYP-1-3. The iodine exposure concentrations for the PEYP-1-1, PEYP-1-2, and PEYP-1-3 treatment groups were 2 ppm, 4 ppm, and 8 ppm, respectively. In the Molecular Function category, OA, AOPD, WIOROMO correspond to oxidoreductase activity, acting on paired donors, with incorporation of or reduction in molecular oxygen, respectively; ET, TEWTCETPOPA correspond to electron transporter and transferring electrons within the cyclic electron transport pathway of photosynthesis activity, respectively; TA and TA, GOTAG correspond to: transferase activity, transferring acyl groups other than amino-acyl groups.

Figure 4

A bar chart of GO functional enrichment analysis for each treatment. Note: (a) PEYP-1-1 vs. PEYP-1-2; (b) PEYP-1-1 vs. PEYP-1-3. The iodine exposure concentrations for the PEYP-1-1, PEYP-1-2, and PEYP-1-3 treatment groups were 2 ppm, 4 ppm, and 8 ppm, respectively. In the Molecular Function category, OA, AOPD, WIOROMO correspond to oxidoreductase activity, acting on paired donors, with incorporation of or reduction in molecular oxygen, respectively; ET, TEWTCETPOPA correspond to electron transporter and transferring electrons within the cyclic electron transport pathway of photosynthesis activity, respectively; TA and TA, GOTAG correspond to: transferase activity, transferring acyl groups other than amino-acyl groups.

Figure 5

KEGG pathway enrichment classification of genes. Note: (a) PEYP-1-1 vs. PEYP-1-2; (b) PEYP-1-1 vs. PEYP-1-3. The iodine exposure concentrations for the PEYP-1-1, PEYP-1-2 and PEYP-1-3 treatment groups were 2 ppm, 4 ppm, and 8 ppm, respectively.

Figure 6

An enrichment bubble map of KEGG pathway for each treatment differential gene. Note: (a) PEYP-1-1 vs. PEYP-1-2; (b) PEYP-1-1 vs. PEYP-1-3. The iodine exposure concentrations for the PEYP-1-1, PEYP-1-2, and PEYP-1-3 treatment groups were 2 ppm, 4 ppm, and 8 ppm, respectively.

Figure 7

The basic overview of the photosynthesis-antenna protein pathway in PEYP-1-1 vs. PEYP-1-2 and PEYP-1-1 vs. PEYP-1-3. Note: The iodine exposure concentrations for the PEYP-1-1, PEYP-1-2, and PEYP-1-3 treatment groups were 2 ppm, 4 ppm, and 8 ppm, respectively.

Figure 8

The basic diagram of the biosynthetic pathway of keratin, threonine, and wax. Note: (a) Biosynthetic pathway of keratin and threonine; (b) wax biosynthetic pathway. PEYP-1-1 was the trace iodine control group of leaves, PEYP-1-2 was the moderate iodine exposure group of leaves, and PEYP-1-3 was the excessive iodine exposure group of leaves.