Enhanced UV-B Radiation in Potato Stems and Leaves Promotes the Accumulation of Anthocyanins in Tubers

Abstract

Enhanced ultraviolet-B (UV-B) radiation promotes anthocyanin biosynthesis in leaves, flowers and fruits of plants. However, the effects and underlying mechanisms of enhanced UV-B radiation on the accumulation of anthocyanins in the tubers of potatoes (Solanum tuberosum L.) remain unclear. Herein, reciprocal grafting experiments were first conducted using colored and uncolored potatoes, demonstrating that the anthocyanins in potato tubers were synthesized in situ, and not transported from the leaves to the tubers. Furthermore, the enhanced UV-B radiation (2.5 kJ·m^-2·d^-1) on potato stems and leaves significantly increased the contents of total anthocyanin and monomeric pelargonidin and peonidin in the red-fleshed potato '21-1' tubers, compared to the untreated control. A comparative transcriptomic analysis showed that there were 2139 differentially expressed genes (DEGs) under UV-B treatment in comparison to the control, including 1724 up-regulated and 415 down-regulated genes. The anthocyanin-related enzymatic genes in the tubers such as PAL, C4H, 4CL, CHS, CHI, F3H, F3'5'H, ANS, UFGTs, and GSTs were up-regulated under UV-B treatment, except for a down-regulated F3'H. A known anthocyanin-related transcription factor StbHLH1 also showed a significantly higher expression level under UV-B treatment. Moreover, six differentially expressed MYB transcription factors were remarkably correlated to almost all anthocyanin-related enzymatic genes. Additionally, a DEGs enrichment analysis suggested that jasmonic acid might be a potential UV-B signaling molecule involved in the UV-B-induced tuber biosynthesis of anthocyanin. These results indicated that enhanced UV-B radiation in potato stems and leaves induced anthocyanin accumulation in the tubers by regulating the enzymatic genes and transcription factors involved in anthocyanin biosynthesis. This study provides novel insights into the mechanisms of enhanced UV-B radiation that regulate the anthocyanin biosynthesis in potato tubers.

Keywords: UV-B radiation; anthocyanin; in situ synthesis; potato; transcriptome.

Figure 1

The tuber photographs of six potato clones used for reciprocal grafting. (A) A red potato clone ‘21-1’ with red skin and red flesh. (B) A white potato clone ‘21-3′ with pale yellow skin and white flesh. (C) A purple potato clone ‘1417-5′ with purple skin and purple flesh. (D) A white potato clone ‘15D1′ with white skin and white flesh. (E) A purple potato cultivar ‘HJG’ with purple skin and purple flesh. (F) A white potato cultivar ‘LS6′ with white skin and white flesh.

Figure 2

The effects of enhanced UV-B radiation on anthocyanin’s accumulation in tubers. (A) The photographs of red-fleshed potato clones ‘21-1’ after 20, 40, 60, and 80 days of enhanced UV-B and natural sunlight (CK). The line represents 2 cm. (B) TAC of red-fleshed potato clones ‘21-1’ after 20, 40, 60, and 80 days of enhanced UV-B and CK. (C) The proportion of pelargonidin (Pg) and peonidin (Pn) after 80 days of UV-B treatment and in untreated tubers. (D) MAC after 80 days of UV-B treatment and in untreated tubers. The bars in (B,D) represent mean ± SD, and the lowercase letters represent significant differences (t-test, n = 3, p < 0.05).

Figure 3

Differential expression analysis of genes between UVB and CK tubers via RNA-seq. (A) Principal component analysis (PCA) showed differences between UVB and CK tubers. (B) Volcano plot illustrated the differentially expressed genes (DEGs) in UVB and CK tubers. (C) Heatmap and hierarchical cluster analysis of the DEGs between UVB and CK tubers. The Log₂(FPKM + 1) values were row-scaled and displayed according to the color code. The red and blue colors represent the highest (up-regulation) and lowest (down-regulation) expression levels, respectively.

Figure 4

Gene ontology (GO) and Kyoto encyclopedia of genes and genomes (KEGG) enrichment analysis of DEGs of UVB and CK tubers. (A,B) GO analysis of up-regulated and down-regulated DEGs. Top 20 GO terms are displayed for each. (C,D) KEGG analysis of up-regulated and down-regulated DEGs. All KEGG pathways are shown. The dot size and color indicate the number of genes, and the p-value, respectively.

Figure 5

Differentially expressed transcription factor (TF)-encoding genes between UVB and CK tubers. (A) The number of TF DEGs. (B) The top 20 TF-encoding genes with the greatest fold change between UVB and CK tubers. The “Soltu.DM.” was abbreviated for every gene name in the x-axis.

Figure 6

Differentially expressed MYB genes between UVB and CK tubers. (A) Heatmap and hierarchical cluster showing 26 MYB DEGs. Each column represents a sample, and each row represents a DEG. The numbers in boxes refer to FPKM. The Log₂(FPKM + 1) values were row-scaled and displayed according to their color code. The red and blue colors represent the highest and lowest expression levels, respectively. (B) Pearson correlation analysis of 26 MYBs and 30 anthocyanin biosynthetic genes. Pearson correlation coefficient values were displayed according to the color code. The red and blue colors represent a positive and negative correlation, respectively. The asterisks in boxes show the p-value, * p < 0.05, ** p < 0.01, and *** p < 0.001. Gene names (“Soltu.DM.” was abbreviated) with a small black dot represent R3-MYB, the rest belong to R2R3-MYB.

Figure 7

Identification of hormone-related and UV-B-related genes between UV-B and CK tubers. (A) Heatmap showing 44 DEGs in ethylene-activated signaling pathway (GO:0009873). (B) Heatmap showing 8 DEGs in JA biosynthetic process (GO:0009695). (C) Heatmap showing 26 UV-B-related genes in potato tubers, including 15 UVR8, 2 COP1, 1 HY5, and 8 BBX. Genes with an underline represent a DEG, the rest display no significant difference.

Figure 8

Expression analysis of 14 selected genes between UVB and CK tubers. (A) Histogram showing the relative expression levels of 14 selected genes. The error bars indicate the standard errors of three biological and three technical replicates. (B) Scatter plots show linear regression and R² between RNA-Seq and qRT-PCR data.