Transcriptomic and Metabolic Analyses Reveal the Mechanism of Ethylene Production in Stony Hard Peach Fruit during Cold Storage.

Lei Pan,Yan Wang,Zhenhua Lu,Liang Niu,Wenfang Zeng,Junren Meng,Li Deng,Zhiqiang Wang,Guochao Cui

doi:10.3390/ijms222111308

Abstract

Stony hard (SH) peach (Prunus persica L. Batsch) fruit does not release ethylene and has very firm and crisp flesh at ripening, both on- and off-tree. Long-term cold storage can induce ethylene production and a serious risk of chilling injury in SH peach fruit; however, the regulatory mechanism underlying ethylene production in stony hard peach is relatively unclear. In this study, we analyzed the phytohormone levels, fruit firmness, transcriptome, and lipidome changes in SH peach ‘Zhongtao 9’ (CP9) during cold storage (4 °C). The expression level of the ethylene biosynthesis gene PpACS1 and the content of ethylene in SH peach fruit were found to be upregulated during cold storage. A peak in ABA release was observed before the release of ethylene and the genes involved in ABA biosynthesis and degradation, such as zeaxanthin epoxidase (ZEP) and 8’-hydroxylase (CYP707A) genes, were specifically induced in response to low temperatures. Fruit firmness decreased fairly slowly during the first 20 d of refrigeration, followed by a sharp decline. Furthermore, the expression level of genes encoding cell wall metabolic enzymes, such as polygalacturonase, pectin methylesterase, expansin, galactosidase, and β-galactosidase, were upregulated only upon refrigeration, as correlated with the decrease in fruit firmness. Lipids belonging to 23 sub-classes underwent differential rearrangement during cold storage, especially ceramide (Cer), monoglycosylceramide (CerG1), phosphatidic acid (PA), and diacyglyceride (DG), which may eventually lead to ethylene production. Exogenous PC treatment provoked a higher rate of ethylene production. We suspected that the abnormal metabolism of ABA and cell membrane lipids promotes the production of ethylene under low temperature conditions, causing the fruit to soften. In addition, ERF transcription factors also play an important role in regulating lipid, hormone, and cell wall metabolism during long-term cold storage. Overall, the results of this study give us a deeper understanding of the molecular mechanism of ethylene biosynthesis during the postharvest storage of SH peach fruit under low-temperature conditions.

Highlights

Cold stress is an important abiotic factor that has serious effects on horticultural plants, decreasing their growth, development, and postharvest maturation
We found that the increase in PpACS1 transcript abundance at least partially coincided with ethylene production in Stony hard (SH) fruit stored at low temperature; no difference was detected in PpYUCC11 expression during cold temperature storage, suggesting that the biosynthesis of ethylene at low temperatures is independent of the maturation pathway
We found that ERF may change the hormone levels, firmness, and lipid content in fruit by inducing hormone, cell wall, and lipid metabolism-related gene expression under low temperature storage

Summary

Introduction

Cold stress is an important abiotic factor that has serious effects on horticultural plants, decreasing their growth, development, and postharvest maturation. Previous studies have shown that exogenous ethylene affects fruit softening and membrane fluidity by regulating the expression of cell wall- and lipid metabolism-related genes, reducing the Cl symptoms in peach fruit [14]; the molecular mechanism of ethylene biosynthesis under cold stress and the direct relationship between endogenous ethylene level and fruit softening and lipid metabolism have not yet been explored. A total of 2914 DEGs (|Log2FC| ≥ 2; FPKM ≥ 10; p < 0.05) were identified among the following three comparison groups: L1 vs CK0, L2 vs CK0, and L2 vs L1 The number of both upregulated and downregulated genes was the highest in the L2 vs CK0 group (Figure 2B,C), indicating that with the extension of cold storage time, the fruit was affected by more serious transcriptional regulation. DEGs that responded to both short (L1) and long term (L2) cold storage were mainly involved in ‘Biosynthesis of secondary metabolite’, ‘Biotin metabolism’, ‘Anthocyanin biosynthesis’, ‘Flavonoid biosynthesis’, ‘Steroid biosynthesis’, ‘Plant hormone signal transduction’, ‘Starch and sucrose metabolism’, and ‘Fatty acid biosynthesis and metabolism’ (Figure 2E)

Changes in Transcript Abundance during Cold Storage

PC Promoted Ethylene Production in SH Peach Fruit under Low Temperature

Library Construction and Transcriptome Sequencing

RNA-Seq Data Analysis

Lipidome Analysis

Treatment with PC in SH Peach

Findings

Conclusions