Organic acid and sugar components accumulation and flavor associated metabolites dynamic changes in yellow- and white-fleshed seedless loquats (Eriobotrya japonica)

Triploid loquats are divided into yellow- and white-fleshed cultivars. To better understand taste variations in triploid loquat fruits, we used a UPLC-ESI-QTRAP-MS/MS-based widely targeted metabolomic analysis to examine the metabolic composition of two different color fleshed triploid loquats with a sample size of 54 and external validation method within a confidence level of P＜0.05. We identified key flavor-related differentially accumulated metabolites using the variable importance in projection (VIP) value (VIP ≥ 1.0) and fold change ≥ 2 or ≤ 0.5. Furthermore, the results of the HPLC analysis showed that white-fleshed loquats had a low malic acid content. We also performed the UPLC-MS/MS system to investigate the carotenoids contents and lipidome in four triploid cultivars. In the fruits of white-fleshed varieties, the carotenoids contents were significantly downregulated, but the contents of most glycerolphospholipids were increased. Our results reveal the metabolomic changes between the yellow- and white-fleshed cultivars.

Optimization of apple fruit flavor by MdVHP1-2 via modulation of soluble sugar and organic acid accumulation

For fleshy fruits, the content and ratio of organic acids and soluble sugars are key factors for their flavor. Therefore, a better understanding of soluble sugar and organic acid accumulation in vacuoles is essential to the improvement of fruit quality. Vacuolar-type inorganic pyrophosphatase (V-PPase) has been found in various plants with crucial functions based on the hydrolysis of PPi. However, the effects of V-PPase on the soluble sugar and organic acid accumulation in apple fruit remain unclear. In this study, MdVHP1-2, a V-PPase protein in the vacuolar membrane, was identified. The results showed a positive correlation between the expression of MdVHP1-2 and the sugar/acid ratio during ripening of apple fruits. A series of transgenic analyses showed that overexpression of MdVHP1-2 significantly elevated the contents of soluble sugars and organic acids as well as the sugar/acid ratio in apple fruits and calli. Additionally, transient interference induced by MdVHP1-2 expression inhibited the accumulation of soluble sugars and organic acids in apple fruits. In summary, this study provides insight into the mechanisms by which MdVHP1-2 modulates fruit flavor through mediation of soluble sugar and organic acid accumulation, thereby facilitating improvement of the overall quality of apple and other fruits.

Chilling-induced peach flavor loss is associated with expression and DNA methylation of functional genes

INTRODUCTION

Flavor is a major contributor to consumer preference. Despite being effective at extending the fruit's commercial life, cold storage also results in a significant loss of flavor volatiles. To date, there has been few studies on the metabolic dynamics and the mechanism underlying the regulatory networks that modulate flavor loss during cold storage for fruit.

METHODS

The volatile contents were detected by Gas Chromatography-Mass Spectrometer (GC-MS). Weighted gene co-expression network analysis (WGCNA) was used to identify structure genes and transcription factors (TFs). DNA methylation was analyzed by whole-genome methylation sequencing during cold storage.

RESULTS

We generated a temporal map, over hourly to weekly timescales, for the effects of chilling on flavor volatiles by combining metabolome, transcriptome, and DNA methylome in peach fruit. Based on the big data analysis, we developed a regulatory network for volatile formation and found that a decrease in volatiles during cold storage was significantly correlated with a decrease in the expression of synthesis genes. Moreover, TFs associated with these structure genes were identified. Expression of genes involved in ethylene biosynthesis was reduced while cold tolerance pathway was activated in response to low temperature. Functions of those genes were confirmed through transgenic experiments and across peach cultivars, suggesting our dataset is a useful tool for elucidating regulatory factors that have not yet been clarified in relation to flavor and cold tolerance. Genome wide DNA methylation was induced by chilling and peaked at 7 d followed by a decline during 28 d cold storage. Reduction of gene expression was accompanied by major changes in the methylation status of their promoters, including PpACS1, PpAAT1, PpTPS3 and PpMADS2.

CONCLUSION

Our study revealed the mechanism for chilling-induced flavor loss of peach fruit through time-course transcriptome and DNA methylome analysis.

Targeting ripening regulators to develop fruit with high quality and extended shelf life

Fruit quality directly impacts fruit marketability and consumer acceptance. Breeders have focused on fruit quality traits to extend shelf life, primarily through fruit texture, but, in some cases, have neglected other qualities such as flavor and nutrition. In recent years, integrative biotechnology and consumer-minded approaches have surfaced, aiding in the development of flavorful, long-lasting fruit. Here, we discussed how specific transcription factors and hormones involved in fruit ripening can be targeted to generate high-quality fruit through traditional breeding and bioengineering. We highlight regulators that can be used to generate novel-colored fruit or biofortify fresh produce with health-promoting nutrients, such as vitamin C. Overall, we argue that addressing grower and industry needs must be balanced with consumer-based traits.

Amino acids metabolism as a source for aroma volatiles biosynthesis

Aroma volatiles are essential for plant ecological fitness and reproduction. Plants produce and use volatiles to attract pollinators and seed dispersers, repel herbivores and recruit their natural enemies, and communicate with other plants. Amino acids and their biosynthetic intermediates play key roles as precursors for the biosynthesis of plant volatiles. Different plants utilize different strategies and biosynthetic pathways to meet their specific biological needs. This review focuses on the different biosynthetic pathways that plants utilize to form amino acid-derived aroma volatiles, emphasizing their common and unique aspects and stressing the importance of the limiting enzymes residing in the primary-specialized metabolism interface. We also briefly review how biotechnology has used this interface and point to promising future directions for improving the quality of agricultural produce and the production of key volatiles.

Linalool synthesis related PpTPS1 and PpTPS3 are activated by transcription factor PpERF61 whose expression is associated with DNA methylation during peach fruit ripening

The monoterpene linalool is a major contributor to flavor of multiple fruit species. Although great progress has been made in identifying genes related to linalool formation, transcriptional regulation for the pathway remains largely unknown. As a super transcription factor family, roles of AP2/ERF in regulating linalool production have not been elucidated. Peach linalool is catalyzed by terpene synthases PpTPS1 and PpTPS3. Here, we observed that expression of PpERF61 correlated with these two PpTPSs during fruit ripening by transcriptome co-expression analysis. Dual-luciferase assay and EMSA results indicated that PpERF61 activated the PpTPS1 and PpTPS3 transcription by binding to the DRE/CRT motif in their promoters. Transient overexpressing PpERF61 in peach fruit significantly increased PpTPS1 and PpTPS3 expression and linalool content. Further study revealed significant correlation between PpERF61 transcripts and linalool contents across 30 peach cultivars. Besides transcriptional regulation, accumulated linalool was associated with DNA demethylation of PpERF61 during peach fruit ripening. In addition, interactions between PpERF61 and PpbHLH1 were evaluated, indicating these two transcription factors were associated with linalool production during peach fruit ripening. Overall, our results revealed a new insight into the regulation of linalool synthesis in fruit.

The role of hydrogen water in delaying ripening of banana fruit during postharvest storage

Experiments were conducted to identify the role of hydrogen water (HW) in banana fruit ripening. Banana fruit soaked with 0.8 ppm HW showed longer ripening than control fruit. HW treatment significantly reduced ethylene production and respiratory rate, and inhibited the expressions of ethylene synthesis- and signaling-related genes. Similarly, HW treatment inhibited the down-regulation of chlorophylls binding proteins and delayed the increase of chromaticity a*, b* and L* in banana peel. Furthermore, HW-treated peel exhibited lower expressions of cell wall degradation-related genes and higher levels of fruit firmness, pectin, hemicellulose and lignin. In addition, HW-treated pulp exhibited higher levels of starch, lower level of total soluble solids (TSS) and lower expression of flavor-related genes. Microstructural observation further confirmed that HW treatment delayed the degradations of starch and cell walls. Those results indicated that HW treatment delayed banana ripening via the role of ethylene in relation to degreening, flavor and softening.

Functional divergence of principal alcohol o-acyltransferase for biosynthesis of volatile acetate esters among tomato wild species (Solanum Sect. Lycopersicon)

Volatile esters are the chemicals that have multiple physiological functions including plant defense responses and reproduction. From a human perspective, the esters largely contribute to the fruity aroma of freshy fruits. Composition of volatile esters show a significant diversity among the wild tomato species (Solanum sect. Lycopersicon). To address the basis for this divergence, here we conducted functional analysis of a gene encoding major alcohol o-acyltransferase (AAT1) that catalyzes volatile ester formation. Although AAT1 transcripts were highly expressed in the ripe fruits of all the wild species examined, their enzymatic properties significantly differed due to amino acid sequence variations. Notably, AAT1s from S. pennellii showed the highest ability to produce acetate esters whereas AAT1s from S. neorickii, S. chmielewskii and S. habrochaites had the lowest activities. Further, screenings using domain-swapped or point-mutated AAT1s allowed us to identify Met/Thr352 as one of the critical residues related to the transferase activity with acetyl-CoA. This finding is potentially applied to aroma engineering in which a site-directed mutagenesis at this position in alcohol o-acyltransferases could enable to manipulate volatile ester levels in ripe fruits.

Identification, introgression, and validation of fruit volatile QTLs from a red-fruited wild tomato species

Volatile organic compounds (VOCs) are major determinants of fruit flavor, a primary objective in tomato breeding. A recombinant inbred line (RIL) population consisting of 169 lines derived from a cross between Solanum lycopersicum and a red-fruited wild tomato species Solanum pimpinellifolium accession (SP) was characterized for VOCs in three different seasons. Correlation and hierarchical cluster analyses were performed on the 52 VOCs identified, providing a tool for the putative assignation of individual compounds to metabolic pathways. Quantitative trait locus (QTL) analysis, based on a genetic linkage map comprising 297 single nucleotide polymorphisms (SNPs), revealed 102 QTLs (75% not described previously) corresponding to 39 different VOCs. The SP alleles exerted a positive effect on most of the underlying apocarotenoid volatile QTLs-regarded as desirable for liking tomato-indicating that alleles inherited from SP are a valuable resource for flavor breeding. An introgression line (IL) population developed from the same parental genotypes provided 12 ILs carrying a single SP introgression and covering 85 VOC QTLs, which were characterized at three locations. The results showed that almost half of the QTLs previously identified in the RILs maintained their effect in an IL form, reinforcing the value of these QTLs for flavor/aroma breeding in cultivated tomato.