A metabolic perspective of selection for fruit quality related to apple domestication and improvement.

BACKGROUND: Apple is an economically important fruit crop. Changes in metabolism accompanying human-guided evolution can be revealed using a multiomics approach. We perform genome-wide metabolic analysis of apple fruits collected from 292 wild and cultivated accessions representing various consumption types. RESULTS: We find decreased amounts of certain metabolites, including tannins, organic acids, phenolic acids, and flavonoids as the wild accessions transition to cultivated apples, while lysolipids increase in the "Golden Delicious" to "Ralls Janet" pedigree, suggesting better storage. We identify a total of 222,877 significant single-nucleotide polymorphisms that are associated with 2205 apple metabolites. Investigation of a region from 2.84 to 5.01 Mb on chromosome 16 containing co-mapping regions for tannins, organic acids, phenolic acids, and flavonoids indicates the importance of these metabolites for fruit quality and nutrition during breeding. The tannin and acidity-related genes Myb9-like and PH4 are mapped closely to fruit weight locus fw1 from 3.41 to 3.76 Mb on chromosome 15, a region under selection during domestication. Lysophosphatidylethanolamine (LPE) 18:1, which is suppressed by fatty acid desaturase-2 (FAD2), is positively correlated to fruit firmness. We find the fruit weight is negatively correlated with salicylic acid and abscisic acid levels. Further functional assays demonstrate regulation of these hormone levels by NAC-like activated by Apetala3/Pistillata (NAP) and ATP binding cassette G25 (ABCG25), respectively. CONCLUSIONS: This study provides a metabolic perspective for selection on fruit quality during domestication and improvement, which is a valuable resource for investigating mechanisms controlling apple metabolite content and quality.

Hydrogen-rich water alleviates chilling injury-induced lignification of kiwifruit by inhibiting peroxidase activity and improving antioxidant system.

BACKGROUND: Kiwifruit is prone to chilling stress and displays chilling injury (CI) such as lignification; however, the underlying physicochemical mechanism remains largely unknown. Here, the changes in levels of quality attributes, lignin biosynthesis, antioxidant system and sugars were compared in kiwifruit between control and hydrogen-rich water (HRW) treatments during cold storage for 90 days at 0 °C. RESULTS: The results reveal that HRW is an effective measure for CI alleviation, as indicated by the decrease in lignification level with repressed peroxidase activity but enhanced polyphenol oxidase activity. The amelioration of membrane peroxidation was suggested by the repressed levels of H2 O2 and malondialdehyde. They were accompanied by the improvement of antioxidant system, which is supported by the enhancement of sugars including fructose and glucose. CONCLUSION: In conclusion, HRW can enhance chilling tolerance, as suggested by the alleviation of lignification through inhibiting peroxidase activity and elevating the antioxidant system to attenuate membrane peroxidation. © 2022 Society of Chemical Industry.

Abscisic acid alleviates chilling injury in cold-stored peach fruit by regulating ethylene and hydrogen peroxide metabolism.

Peach (Prunus persica (L.) Batsch) is susceptible to chilling injury under improper low-temperature storage (2°C-5°C). Previous research has shown that abscisic acid (ABA) alleviates chilling injury in fruits and vegetables, but the potential mechanism is still unclear. To explore its effectiveness and potential mechanism in alleviating chilling injury during cold storage, exogenous ABA was applied to peach fruit by immersion in 100 µmol L-1 solutions for 10 min. In our experiment, ABA alleviated chilling injury by reducing hydrogen peroxide (H2O2) content and ethylene production. In addition, ABA inhibited the expression of the ethylene synthesis-related genes PpACO1 and PpEIN2. At the same time, ABA activated the antioxidant enzymatic pathway and the ascorbate-glutathione (AsA-GSH) cycle, the transcript abundance encoding genes related to antioxidant enzyme activities also changed correspondingly. The results suggested that ABA alleviated chilling injury by scavenging excessive H2O2 by promoting antioxidant enzymes and the AsA-GSH pathway.

Phytic Acid Treatment Inhibits Browning and Lignification to Promote the Quality of Fresh-Cut Apples during Storage.

Browning and lignification often occur in fresh-cut apple processing, leading to quality deterioration and limiting the shelf life of products. In this study, 0.8% (v/v) phytic acid was used to improve the quality and shelf life of fresh-cut apples. From the results, the browning was inhibited by the phytic acid treatment and the browning index (BI) of the control fruit was 1.62 times that of phytic acid treatment at 2 d of storage. The lignin content in phytic acid-treated fruit significantly decreased at 2, 4, and 6 d of storage compared to the control. Phytic acid treatment also reduced H2O2 and malonaldehyde (MDA) contents, which may indicate lighter membrane damage to apples. Compared with the control, the polyphenol oxidase (PPO) and peroxidase (POD) activities decreased while superoxide dismutase (SOD) and catalase (CAT) activities increased in phytic acid-treated fruit. Consistent with the lignin content, the activities of phenylpropane metabolism-related enzymes phenylalanine ammonia-lyase (PAL), cinnamate 4-hydroxylase (C4H), and 4-coumarate: CoA ligase (4CL) were inhibited by phytic acid treatment. In conclusion, phytic acid alleviated the browning and lignification of fresh-cut apples by reducing PPO and POD activities, maintaining cell membrane integrity, and inhibiting phenylpropane metabolism.

Transcription Factor MdWRKY32 Participates in Starch-Sugar Metabolism by Binding to the MdBam5 Promoter in Apples During Postharvest Storage.

Starch degradation with fruit ripening is closely related to the aging process and flavor formation in apples. In this study, ethylene, 1-methylcyclopropene (1-MCP), and apples treated at different temperatures were used to determine the key genes of starch-sugar metabolism during storage. Compared with 4 °C storage, 20 °C storage promoted starch degradation and sugar accumulation in apples. In addition, ethylene treatment promoted starch degradation and sugar accumulation in apples, while 1-MCP treatment showed the opposite effects. The expression of MdBams indicated the crucial role of MdBam5 in starch-sugar conversion. Transient overexpression of MdBam5 significantly reduced the starch content in apples. Furthermore, MdWRKY32 directly combined the MdBam5 promoter and activated the MdBam5 expression, which may promote the starch degradation in apples. Therefore, it was concluded that MdWRKY32 may be involved in the regulation of starch-sugar metabolism in postharvest apples by activating the MdBam5 expression.

Nitric oxide alleviates chilling injury by regulating the metabolism of lipid and cell wall in cold-storage peach fruit.

Peach fruit are prone to development of chilling injury during cold storage at around 0-7 °C. Nitric oxide (NO) has been proven to alleviate chilling injury, but the mechanism was still unclear. In this study, peach fruit were immersed in a NO donor (sodium nitroprusside, SNP) solution for 10 min, then stored at 0 °C. The SNP alleviated chilling injury, including decreasing the internal browning index, malondialdehyde content, electrolyte leakage, and lipoxygenase activity, and maintaining firmness. Furthermore, SNP maintenance of fruit firmness was associated with reduction of xyloglucan endotransglycosylase/hydrolase family member gene expression and decrease of cell wall hydrolase activity, especially the activities of polygalacturonase, xyloglucan endoglycosyl transferase, cellulase, and ß-galactosidase. Meanwhile, SNP regulated the lipid metabolism by up-regulating the expression of genes encoding glycerol-3-phosphate acyltransferase, ketoacy-ACP synthase, phosphatidylinositol bisphosphate and long-chain acyl-CoA. Thus, the results of this study indicate that SNP alleviates chilling injury of post-harvest peach fruit by regulating cell wall and lipid metabolism.

Postharvest nitric oxide treatment induced the alternative oxidase pathway to enhance antioxidant capacity and chilling tolerance in peach fruit.

Nitric oxide (NO) is an important regulator of plant response to cold stress. In this study, NO treatment delayed the development of chilling injury (CI), inhibited the increase in H2O2 content, O2- production rate and decrease in firmness of postharvest peach fruit. Meanwhile, through RNA-seq analysis, NO treatment up-regulated gene expression of PpG-6-PDH, Pp6-PGDH and PpAOX while it down-regulated the expression of PpGPI and PpHK, suggesting that the pentose phosphate respiratory pathway and cyanide-resistant respiratory pathway were promoted and the glycolysis pathway was inhibited. Furthermore, the PpAOX expression was consistent with the trend of PpPOD1/2 expression and H2O2 content, indicating that AOX may play a role in reducing oxidative damage of peach fruit by scavenging H2O2. Thus, it was concluded that NO treatment could induce the cyanide-resistant respiration pathway to enhance antioxidant ability and chilling tolerance in post-harvest peach fruit.

Salicylic acid treatment mitigates chilling injury in peach fruit by regulation of sucrose metabolism and soluble sugar content.

Peach fruit stored in the cold are susceptible to chilling injury. A pre-storage treatment with the natural hormone salicylic acid can alleviate chilling damage, although the mechanism is unclear. We found that a treatment with 1 µmol L-1 salicylic acid for 15 min prior to storage at 4 °C delayed and reduced fruit internal browning, a symptom of chilling injury. Salicylic acid had a large effect on sugar metabolism, increasing total soluble sugars via a substantial increase in sucrose content. The transcript abundance of genes related to sucrose biosynthesis and degradation was significantly regulated by salicylic acid, consistent with the changes in sucrose content. Salicylic acid treatment also increased the expression of two DREB cold stress-related proteins, transcriptional activators that regulate cold resistance pathways. The results show that salicylic acid alleviates chilling injury in peach by multiple mechanisms, including an increased content of sucrose and activation of cold response genes.

Jasmonic acid and salicylic acid induce the accumulation of sucrose and increase resistance to chilling injury in peach fruit.

BACKGROUND: Salicylic acid (SA) and jasmonic acid (JA) can both enhance resistance of chilling injury (CI) in cold-storage peach fruit, but the regulatory mechanisms involved and whether there is a coordinated regulation between them is unclear. In this study, postharvest peach fruit were treated with an aqueous SA solution for 15 min or an aqueous JA solution for 30 s before storage at 4 °C for 35 days. RESULTS: SA and JA treatments both delayed and reduced development of internal browning (a symptom of CI) and induced the accumulation of hydrogen peroxide and sucrose. The SA and JA also reduced catalase and peroxidase activities, which are involved in hydrogen peroxide generation. The SA and JA treatments significantly regulated the transcript abundance of genes related to sucrose biosynthesis and degradation consistent with the observed increase in sucrose content. CONCLUSION: These results intimate that JA and SA may be involved in coordinating the alleviation of CI via increased accumulation of sucrose. © 2021 Society of Chemical Industry.

Melatonin treatment reduces ethylene production and maintains fruit quality in apple during postharvest storage.

The effects of treatment with melatonin on ripening of 'Fuji' apples during storage at 1 °C for 56 d were investigated. The apples were harvested at the commercial ripening stage and treated with 1 mmol L-1 melatonin. Compared with the control, melatonin treated apples had significant reduced ethylene production (28 d-56 d) and weight loss (14 d-56 d) during storage (p < 0.05). Also, the melatonin treatment maintained better apple skin structure throughout storage. The reduced ethylene production was regulated by the decreased expressions of MdACO1, MdACS1, MdAP2.4 and MdERF109, based on RNA-Seq analysis, which was validated using qRT-PCR analysis. Moreover, the activity of 3 enzymes, including peroxidase (POD), superoxide dismutase (SOD) and catalase (CAT), were significantly increased in melatonin treated fruit (p < 0.05). Taken together, this study highlights the inhibitory effects of melatonin in ethylene biosynthesis and factors influencing postharvest quality in apple.

Jasmonic acid treatment alleviates chilling injury in peach fruit by promoting sugar and ethylene metabolism.

Peach (Prunus persica L.) fruit are highly susceptible to chilling injury during cold storage, resulting in internal flesh browning and a failure to soften normally. We have examined the effect of a postharvest treatment consisting of a brief (30 s) dip in the natural plant hormone jasmonic acid, prior to storage at 4 °C. Jasmonic acid treatment reduced the severity of internal flesh browning and did not inhibit fruit softening over a 35 d storage period. Two major physiological effects of jasmonic acid on the fruit were observed, an increase in ethylene production and a prevention of the decline in soluble sugar content seen in controls. An increased soluble sugar content may have multiple benefits in resisting chilling stress, scavenging reactive oxygen species and acting to stabilize membranes. Our results show that a treatment with jasmonic acid can enhance chilling tolerance of peach fruit by regulating ethylene and sugar metabolism.

Cuticular wax composition changes of 10 apple cultivars during postharvest storage.

Cuticular wax chemicals differ among fruit cultivars and contribute to storage ability. However, wax analysis in apple cultivars, particularly during storage, has not been described. In this work, the chemicals and crystal structures of cuticular wax in 10 apple cultivars were analyzed to observe wax functions in apple during storage. Results showed that alkanes and primary alcohols decreased while fatty acids increased in stored fruits of all cultivars compared with the fruits before storage. Terpenoids, aldehydes, and phenols were observed in stored fruits but not in the fruits before storage in all cultivars except 'Red Star' fruit. The weight loss rate was significantly correlated with six components including C13 alcohol, C14 alkanes, total alkanes, total wax, C13 alkanes and C54 alkanes in 10 cultivar apple fruits during storage. Our findings indicate that the total wax, particularly alkanes, in the peel of apple fruits is essential for storage and quality control.

The Role of IP3 in NO-Enhanced Chilling Tolerance in Peach Fruit.

The role of inositol 1,4,5-trisphosphate (IP3) in nitric oxide (NO)-reduced chilling injury (CI) in peach fruit was investigated. The fruit were immersed in sodium nitroprusside (SNP) (NO donor) and neomycin (IP3 inhibitor). Results showed that chilling tolerance was enhanced upon exogenous SNP in postharvest peach fruit. Further, GABA accumulation was stimulated by SNP. The increase in protein expression and activity for enzymes in GABA biosynthesis, including glutamate decarboxylase (GAD), polyamine oxidase (PAO), and amino aldehyde dehydrogenase (AMADH), upon SNP treatment was also observed. Also, the up-regulation of Δ1-pyrroline-5-carboxylate synthetase (P5CS) and ornithine d-aminotransferase (OAT) and the down-regulation of proline dehydrogenase (PDH) were induced by SNP treatment, thereby accelating proline production. Additionally, SNP treatment elevated protein expression and activity of alternative oxidase (AOX). The above effects induced upon SNP were partly weakened by neomycin. Therefore, IP3 mediated NO-activated GABA and proline accumulation as well as AOX, thus inducing chilling tolerance in postharvest peach fruit.

Combined transcriptomic and proteomic analysis of cold stress induced sugar accumulation and heat shock proteins expression during postharvest potato tuber storage.

Plant species differ greatly in their ability to acclimatise to and survive, cold stress. Normally, potato tubers are stored at low temperatures (below 10 °C) to delay sprouting. In this research, combined transcriptomic and proteomic analysis was conducted on potato tubers stored at 15 °C, 4 °C and 0 °C to investigate the mechanism of cold responses during postharvest storage. Results showed that soluble sugars were accumulated under low temperatures, regulating by granule-bound starch synthase 1, beta-amylase, invertase inhibitor and fructokinase. In addition, fifteen heat shock proteins (Hsps), including three Hsp70s, two Hsp80s, one Hsp90, one Hsp100 and eight small Hsps, were induced by low temperatures, which may act individually or synergistically to prevent physiological or cellular damage from cold stress in postharvest potato tubers. This research provided general information of sugar accumulation and defense response in potato tuber under cold storage.

Inositol 1,4,5-Trisphosphate Mediates Nitric-Oxide-Induced Chilling Tolerance and Defense Response in Postharvest Peach Fruit.

The function of inositol 1,4,5-trisphosphate (IP3) on nitric oxide (NO)-induced chilling tolerance and defense response in postharvest peach fruit was explored. The postharvest fruit were treated with sodium nitroprusside (SNP, exogenous NO donor), cPTIO (NO scavenger), and neomycin (IP3 inhibitor). It turned out that SNP treatment mitigated chilling injury (CI) and stimulated NO accumulation in postharvest peach fruit. Further, SNP enhanced phosphoinositide-specific phospholipase C (PI-PLC) activity and, thereby, stimulated IP3 prodution. SNP also upregulated the activity and expression of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), ascorbate peroxidase (APX), glutathione S-transferase (GST), and glutathione reductase (GR). In addition, SNP enhanced the expression of small ubiquitin-like modifier (SUMO) and methionine sulfoxide reductase (MSR) and weakened the activity and expression of lipoxygenase (LOX) and phospholipase D (PLD). These above impacts stimulated by SNP treatment were blocked by the addition of cPTIO and neomycin. Overall, IP3 was involved in NO-enhanced chilling tolerance and defense response in postharvest peach fruit.

UV-C treatment promotes quality of early ripening apple fruit by regulating malate metabolizing genes during postharvest storage.

Early ripening apples are usually used for fresh marketing because of short storage life, although they are with high acid and low sugar contents. Understanding the malate metabolism in fleshy fruit and underpinning process during ripening is crucial for particular crop improvement where acidity is a concern for direct consumption or further processing. In this research, a traditional Chinese apple cultivar 'Hongyu', which belongs to early ripening apple cultivar, were freshly harvested at commercial maturity stage (120 Days after full bloom) and used for different storage temperature (4°C, 20°C) and UV-C treatment (following storage at 20°C after treatment). Simple sugars (glucose, sucrose, and fructose) and organic acids (malic, and oxalic) were assessed after 14 d of storage. Compared to fruits stored at 20°C, the malate content in fruits stored at 4°C significantly higher, while it was decreased significantly in UV-C treated fruits stored at 20°C after 14 d of storage. The sugar content was almost similar throughout the UV-C-treated fruits and fruits stored at different temperature. The higher ratios of total sugars to total organic acids in UV-C treated fruits after 14 d suggest that UV-C treatment has the potential to improve the taste of early ripening apple cultivars. Considering the significant difference in malate the samples at 14 d of storage were subjected for RNA-seq analysis. Transcriptome analysis revealed that the phenomena underlying this change were governed by metabolism of malate by the regulation of NADP-malic enzyme (NADP-ME) and phosphoenolpyruvate carboxylase kinase (PEPCK) in apple during postharvest storage. This transcriptome profiling results have specified the transcript regulation of malate metabolism and lead to possible taste improvement without affecting the other fruit quality attributes.

Effects of (S)-Carvone and Gibberellin on Sugar Accumulation in Potatoes during Low Temperature Storage.

Potato tubers (Solanum tuberosum L.) are usually stored at low temperature, which can suppress sprouting and control the occurrence of diseases. However, low temperatures lead potatoes to easily suffer from cold-induced sweetening (CIS), which has a negative effect on food processing. The aim of this research was to investigate potential treatments on controlling CIS in potatoes during postharvest storage. "Atlantic" potatoes were treated with gibberellin and (S)-carvone, respectively, and stored at 4 °C for 90 days. The results showed that gibberellin can significantly accelerate sprouting and sugar accumulation by regulating expressions of ADP-glucose pyrophosphorylase (AGPase), granule-bound starch synthase (GBSS), ß-amylase (BAM1/2), UDP-glucose pyrophosphorylase (UGPase) and invertase inhibitor (INH1/2) genes. The opposite effects were found in the (S)-carvone treatment group, where CIS was inhibited by modulation of the expressions of GBSS and INH1/2 genes. In summary, gibberellin treatment can promote sugar accumulation while (S)-carvone treatment has some effects on alleviating sugar accumulation. Thus, (S)-carvone can be considered as a potential inhibitor of some of the sugars which are vital in controlling CIS in potatoes. However, the chemical concentration, treatment time, and also the treatment method needs to be optimized before industrial application.