Transcriptome and Metabolome Analyses Provide Insights into the Watercore Disorder on "Akibae" Pear Fruit

Metabolomics and transcriptomics analyses for characterizing the alkaloid metabolism of Chinese jujube and sour jujube fruits

Introduction

Jujube is an important economic forest tree whose fruit is rich in alkaloids. Chinese jujube (Ziziphus jujuba Mill.) and sour jujube (Ziziphus spinosa Hu.) are the two most important species of the jujube genus. However, the mechanisms underlying the synthesis and metabolism of alkaloids in jujube fruits remain poorly understood.

Methods

In this study, the fruits of Ziziphus jujuba 'Hupingzao' and Ziziphus spinosa 'Taigusuanzao' in different harvest stages were used as test materials, we first integrated widely targeted metabolomics and transcriptomics analyses to elucidate the metabolism of alkaloids of jujube fruits.

Results

In the metabolomics analysis, 44 alkaloid metabolites were identified in 4 samples, 3 of which were unique to sour jujube fruit. The differential alkaloid metabolites (DAMs) were more accumulated in sour jujube than in Chinese jujube; further, they were more accumulated in the white ripening stage than in the red stage. DAMs were annotated to 12 metabolic pathways. Additionally, transcriptomics data revealed 259 differentially expressed genes (DEGs) involved in alkaloid synthesis and metabolism. By mapping the regulatory networks of DAMs and DEGs, we screened out important metabolites and 11 candidate genes.

Discussion

This study preliminarily elucidated the molecular mechanism of jujube alkaloid synthesis. The candidate genes regulated the synthesis of key alkaloid metabolites, but the specific regulation mechanism is unclear. Taken together, our results provide insights into the metabolic networks of alkaloid synthesis in Chinese jujube and sour jujube fruits at different harvest stages, thereby providing a theoretical reference for further research on the regulatory mechanism of jujube alkaloids and their development and utilization.

A pear S1-bZIP transcription factor PpbZIP44 modulates carbohydrate metabolism, amino acid, and flavonoid accumulation in fruits

Fruit quality is defined by attributes that give value to a commodity. Flavor, texture, nutrition, and shelf life are key quality traits that ensure market value and consumer acceptance. In pear fruit, soluble sugars, organic acids, amino acids, and total flavonoids contribute to flavor and overall quality. Transcription factors (TFs) regulate the accumulation of these metabolites during development or in response to the environment. Here, we report a novel TF, PpbZIP44, as a positive regulator of primary and secondary metabolism in pear fruit. Analysis of the transient overexpression or RNAi-transformed pear fruits and stable transgenic tomato fruits under the control of the fruit-specific E8 promoter demonstrated that PpZIP44 substantially affected the contents of soluble sugar, organic acids, amino acids, and flavonoids. In E8::PpbZIP44 tomato fruit, genes involved in carbohydrate metabolism, amino acid, and flavonoids biosynthesis were significantly induced. Furthermore, in PpbZIP44 overexpression or antisense pear fruits, the expression of genes in the related pathways was significantly impacted. PpbZIP44 directly interacted with the promoter of PpSDH9 and PpProDH1 to induce their expression, thereby depleting sorbitol and proline, decreasing citrate and malate, and enhancing fructose contents. PpbZIP44 also directly bound to the PpADT and PpF3H promoters, which led to the carbon flux toward phenylalanine metabolites and enhanced phenylalanine and flavonoid contents. These findings demonstrate that PpbZIP44 mediates multimetabolism reprogramming by regulating the gene expression related to fruit quality compounds.

Watercore Pear Fruit Respiration Changed and Accumulated γ-Aminobutyric Acid (GABA) in Response to Inner Hypoxia Stress

Watercore is a physiological disorder which often occurs on the pear fruit and the excessive accumulation of sorbitol in fruit intercellular space is considered to be an important cause of watercore. Our previous studies found that the metabolic disorder of sugars may lead to hypoxia stress and disturb respiration, resulting in aggravated fruit rot and the formation of bitter substances. However, the further changes of respiration and the fruit response mechanism are not well understood. A comprehensive transcriptome analysis of ‘Akibae’ pear watercore fruit was performed in this study. The transcriptome results revealed the hypoxia stress significantly induced the expression of several key enzymes in the TCA cycle and may lead to the accumulation of succinic acid in watercore fruit. The glycolytic pathway was also significantly enhanced in watercore fruit. Moreover, the γ-aminobutyric acid (GABA) synthesis related genes, glutamate decarboxylase (GAD) genes and polyamine oxidase (PAO) genes, which associated with the GABA shunt and the polyamine degradation pathway were significantly upregulated. In addition, the PpGAD1 transcript level increased significantly along with the increase of GAD activity and GABA content in the watercore fruit. Above all, these findings suggested that the hypoxic response was marked by a significant increase of the hypoxia-inducible metabolites succinic acid and GABA and that PpGAD1 may play a key role in response to watercore by controlling the GABA synthesis.

A sorbitol transporter gene plays specific role in the occurrence of watercore by modulating the level of intercellular sorbitol in pear

Watercore is a physiological disorder which often occurs on the fruits of pear and closely related to the influence of environment factors, such as high temperature. The excessive accumulation of sorbitol in fruit intercellular space is considered to be an important cause of watercore. Sorbitol transporter (SOT) is the key translocation protein of sorbitol and our previous study found the PpSOT3 expression was significantly decreased in high temperature induced-watercore pear fruit by transcriptome method. How PpSOT3 regulates the occurrence of watercore in pear remains unclear. The present study found that PpSOT3 had different expression pattern in watercore-susceptible (Akibae and Hosui) and watercore-resistant (Aikansui) pear cultivars of young fruit and mature fruit. Moreover, the accumulation of intercellular sorbitol in watercore fruit was significantly higher than that in healthy fruit, and the expression of PpSOT3 was significantly inhibited. After the treatment of sugar transport inhibitor (para-chloromercuribenzenesulphonic acid, PCMBS), the fruit pulp occurred water-soaking and the expression of PpSOT3 and the intercellular sorbitol content was significantly decreased and increased, respectively. Subcellular localization showed that PpSOT3 was located in plasma membrane. Both transient overexpression and RNAi assays suggested PpSOT3 had the function of sorbitol uptake. Taken together, these results demonstrate that PpSOT3 was a PCMBS-sensitive sorbitol transporter and played an important role in the occurrence of watercore by modulating the level of intercellular sorbitol content.

Residue Analysis and the Effect of Preharvest Forchlorfenuron (CPPU) Application on On-Tree Quality Maintenance of Ripe Fruit in "Feizixiao" Litchi (Litchi chinensis Sonn.)

Litchi is a highly perishable fruit. Ripe litchi fruit loses quality quickly as they hang on tree, giving a very short hanging life and thus harvest period. This study attempted to explore the roles of cytokinin in regulating fruit ripening and senescence of litchi and examine the possibility of applying cytokinin in "on-tree storage" of the fruit. Exogenous cytokinin, forchlorfenuron (CPPU), was applied at 20 mg L^-1 7 weeks after full bloom on litchi (Litchi chinensis cv. Feizixiao) fruit clusters. Color parameters, chlorophylls, anthocyanins, fruit and fruit part weights, total soluble solutes (TSSs), soluble sugars, organic acids, non-anthocyanin flavonoids, ethanol, and also CPPU residue in fruit were traced. CPPU residue was higher but decreased faster in the pericarp than in the aril, where it maintained < 10 μg kg^-1. CPPU had no significant effect on fruit weight but tended to increase pericarp weight. The treatment suppressed chlorophyll loss and anthocyanin accumulation in the pericarp, increased non-anthocyanin flavonoids in the aril, but had no significant effects on non-anthocyanin flavonoids in the pericarp and total sugar and organic acids in the aril. As the commercially ripe fruit hanged on tree, TSSs, total sugar, and sucrose decreased with ethanol and acetic acid accumulation in the aril. CPPU significantly suppressed the loss of sucrose and total sugar and the accumulation of ethanol and acetic acid in the aril and inhibited malondialdehyde accumulation in the pericarp of the overripe fruit. Soluble invertase, alcohol dehydrogenase, and pyruvate decarboxylase (PDC) activity and gene expression in the aril were downregulated by CPPU. The results suggest that cytokinin partially suppresses the ripening process in litchi and is effective to slow quality loss in the overripe fruit on tree.