Higher Phytohormone Contents and Weaker Phytohormone Signal Transduction Were Observed in Cold-Tolerant Cucumber

Abstract

Cucumbers (Cucumis sativus L.) originated from the South Asian subcontinent, and most of them are fragile to cold stress. In this study, we evaluated the cold tolerance of 115 cucumber accessions and screened out 10 accessions showing high resistance to cold stress. We measured and compared plant hormone contents between cold-tolerant cucumber CT90R and cold-sensitive cucumber CT57S in cold treatment. Most of the detected plant hormones showed significantly higher content in CT90R. To elucidate the role of plant hormones, we compared the leaf- and root-transcriptomes of CT90R with those of CT57S in cold stress treatment. In leaves, there were 1209 differentially expressed genes (DEGs) between CT90R and CT57S, while there were 703 in roots. These DEGs were not evenly distributed across the chromosomes and there were significant enrichments at particular positions, including qLTT6.2, a known QTL controlling cucumber cold tolerance. The GO and KEGG enrichment analysis showed that there was a significant difference in the pathway of plant hormone transductions between CT90R and CT57S in leaves. In short, genes involved in plant hormone transductions showed lower transcription levels in CT90R. In roots, the most significantly different pathway was phenylpropanoid biosynthesis. CT90R seemed to actively accumulate more monolignols by upregulating cinnamyl-alcohol dehydrogenase (CAD) genes. These results above suggest a new perspective on the regulation mechanism of cold tolerance in cucumbers.

Keywords: Cucumis sativus; RNA-seq; cold tolerance; plant hormone; transcriptome.

Figure 3

Heat map of DEGs in different samples. DEGs were identified from comparing a pair of CT57S-leaf vs. CT57S-root, CT90R-leaf vs. CT90R-root, CT57S-leaf vs. CT90R-leaf, and CT57S-root vs. CT90R-root. The color represents value of log₁₀ (FPKM).

Figure 4

Distribution of DEGs on the seven chromosomes. Distribution enrichment index = (bin DEG number/total DEG number)/(bin gene number/total gene number); bin = 5 kb. The arrows indicate significant enrichments with an index of >3. Blue lines indicate enrichment of DEGs in the leaf (CT57S-leaf vs. CT90R-leaf); orange lines indicate enrichment of DEGs in the root (CT57S-root vs. CT90R-root).

Figure 1

Symptom of chilling injury started to appear at 24 h in cold treatment. Left, the appearance of cold-sensitive CT57S. Right, the appearance of cold-tolerant CT90R.

Figure 2

Evaluation of sequencing quality by (A) Principal component analysis (PCA) and (B) Correlation heat map between samples. CT57S-L, CT57S-leaf; CT57S-L1, the first replicate of CT57S-L; CT57S-R, CT57S-root; CT57S-R1, the first replicate of CT57S-R. The numbers in the heatmap indicate R² value in the correlation analysis.

Figure 5

DEGs identified in different comparing pairs. (A,B) are statistical data of DEGs in comparing a pair of leaf vs. root. (C,D) are statistical data of DEGs in comparing a pair of CT57S vs. CT90R.

Figure 6

Cnetplot diagram showing the most significantly enriched 5 items in GO analysis (a biological process). (A) Comparing a pair of CT57S vs. CT90R in leaves. (B) Comparing a pair of CT57S vs. CT90R in roots.

Figure 7

Dot plot diagram showing the most significantly enriched 20 items in KEGG analysis. (A) Comparing a pair of CT57S vs. CT90R in leaves. (B) Comparing a pair of CT57S vs. CT90R in roots.

Figure 8

Processes of plant hormone signal transduction were significantly different between CT57S and CT90R in leaves. The process map was modified based on map ko04075.

Figure 9

Process of phenylpropanoid biosynthesis was significantly different between CT57S and CT90R in roots. The process map was modified based on the map ko00940.