Alteration of flesh color and enhancement of bioactive substances via the stimulation of anthocyanin biosynthesis in 'Friar' plum fruit by low temperature and the removal

Effects of appropriate low-temperature treatment on the yield and quality of pigmented potato (Solanum tuberosum L.) tubers

Temperature is one of the important environmental factors affecting plant growth, yield and quality. Moreover, appropriately low temperature is also beneficial for tuber coloration. The red potato variety Jianchuanhong, whose tuber color is susceptible to temperature, and the purple potato variety Huaxinyangyu, whose tuber color is stable, were used as experimental materials and subjected to 20 °C (control check), 15 °C and 10 °C treatments during the whole growth period. The effects of temperature treatment on the phenotype, the expression levels of structural genes related to anthocyanins and the correlations of each indicator were analyzed. The results showed that treatment at 10 °C significantly inhibited the potato plant height, and the chlorophyll content and photosynthetic parameters in the leaves were reduced, and the enzyme activities of SOD and POD were significantly increased, all indicating that the leaves were damaged. Treatment at 10 °C also affected the tuberization of Huaxinyangyu and reduced the tuberization and coloring of Jianchuanhong, while treatment at 15 °C significantly increased the stem diameter, root-to-shoot ratio, yield and content of secondary metabolites, especially anthocyanins. Similarly, the expression of structural genes were enhanced in two pigmented potatoes under low-temperature treatment conditions. In short, proper low temperature can not only increase yield but also enhance secondary metabolites production. Previous studies have not focused on the effects of appropriate low-temperature treatment during the whole growth period of potato on the changes in metabolites during tuber growth and development, these results can provide a theoretical basis and technical guidance for the selection of pigmented potatoes with better nutritional quality planting environment and the formulation of cultivation measures.

Hormonal regulation of anthocyanin biosynthesis for improved stress tolerance in plants

Due to unprecedented climate change, rapid industrialization and increasing use of agrochemicals, abiotic stress, such as drought, low temperature, high salinity and heavy metal pollution, has become an increasingly serious problem in global agriculture. Anthocyanins, an important plant pigment, are synthesized through the phenylpropanoid pathway and have a variety of physiological and ecological functions, providing multifunctional and effective protection for plants under stress. Foliar anthocyanin accumulation often occurs under abiotic stress including high light, cold, drought, salinity, nutrient deficiency and heavy metal stress, causing leaf reddening or purpling in many plant species. Anthocyanins are used as sunscreens and antioxidants to scavenge reactive oxygen species (ROS), as metal(loid) chelators to mitigate heavy metal stress, and as crucial molecules with a role in delaying leaf senescence. In addition to environmental factors, anthocyanin synthesis is affected by various endogenous factors. Plant hormones such as abscisic acid, jasmonic acid, ethylene and gibberellin have been shown to be involved in regulating anthocyanin synthesis either positively or negatively. Particularly when plants are under abiotic stress, several plant hormones can induce foliar anthocyanin synthesis to enhance plant stress resistance. In this review, we revisit the role of plant hormones in anthocyanin biosynthesis and the mechanism of plant hormone-mediated anthocyanin accumulation and abiotic stress tolerance. We conclude that enhancing anthocyanin content with plant hormones could be a prospective management strategy for improving plant stress resistance, but extensive further research is essentially needed to provide future guidance for practical crop production.

PsERF1B-PsMYB10.1-PsbHLH3 module enhances anthocyanin biosynthesis in the flesh-reddening of amber-fleshed plum (cv. Friar) fruit in response to cold storage

Flesh-reddening usually occurs in the amber-fleshed plum (Prunus salicina Lindl.) fruit during cold storage but not during ambient storage direct after harvest. It is not clear how postharvest cold signal is mediated to regulate the anthocyanin biosynthesis in the forming of flesh-reddening yet. In this study, anthocyanins dramatically accumulated and ethylene produced in the 'Friar' plums during cold storage, in comparison with plums directly stored at ambient temperature. Expression of genes associated with anthocyanin biosynthesis, as well as transcription factors of PsMYB10.1, PsbHLH3, and PsERF1B were strongly stimulated to upregulated in the plums in the period of cold storage. Suppression of ethylene act with 1-methylcyclopropene greatly suppressed flesh-reddening and downregulated the expression of these genes. Transient overexpression and virus-induced gene silencing assays in plum flesh indicated that PsMYB10.1 encodes a positive regulator of anthocyanin accumulation. The transient overexpression of PsERF1B, coupled with PsMYB10.1 and PsbHLH3, could further prompt the anthocyanin biosynthesis in a tobacco leaf system. Results from yeast two-hybrid and luciferase complementation assays verified that PsERF1B directly interacted with PsMYB10.1. PsERF1B and PsMYB10.1 enhanced the activity of the promoter of PsUFGT individually, and the enhancement was prompted by the co-action of PsERF1B and PsMYB10.1. Overall, the stimulation of the PsERF1B-PsMYB10.1-PsbHLH3 module mediated cold signal in the transcriptomic supervision of the anthocyanin biosynthesis in the 'Friar' plums. The results thereby revealed the underlying mechanism of the postharvest alteration of the flesh phenotype of 'Friar' plums subjected to low temperature.

Integrative Analysis of Fruit Quality and Anthocyanin Accumulation of Plum cv. 'Cuihongli' (Prunus salicina Lindl.) and Its Bud Mutation

Fruit color is one of the quality indicators to judge the freshness of a plum. The coloring process of plum skin is valuable for research due to the high nutritional quality of anthocyanins found in plums. 'Cuihongli' (CHL) and its precocious mutant variety 'Cuihongli Red' (CHR) were used to analyze the changes of fruit quality and anthocyanin biosynthesis during plum development. The results showed that, during the development of the two plums, the total soluble solid and soluble sugar contents were highest at the mature stage, as the titratable acid trended gradually downward as the fruits of the two cultivars matured, and the CHR fruit showed higher sugar content and lower acid content. In addition, the skin of CHR turned red in color earlier than CHL. Compared with CHL, the skin of CHR had higher anthocyanin concentrations, higher activities of phenylalanine ammonia-lyase (PAL), chalcone isomerase (CHI), dihydroflavonol-4-reductase (DFR), and UDPglucose: flavonoid-3-O-glucosyltransferase (UFGT), and higher transcript levels of genes associated with anthocyanin production. In the flesh of the two cultivars, no anthocyanin content was detected. Taken together, these results suggest that the mutation exerted a major effect on anthocyanin accumulation via modification of the level of transcription; thus, CHR advances the ripening period of 'Cuihongli' plum and improves the fruit quality.

Ethylene Signal Is Involved in the Regulation of Anthocyanin Accumulation in Flesh of Postharvest Plums (Prunus salicina Lindl.)

Ethylene is positively correlated with the anthocyanin accumulation in postharvest plum fruit, but the regulation mechanism has not been fully clarified. In this work, the 'Friar' plum fruit under different storage temperatures (0, 10 and 25 °C) and treatments (100.0 μL L^-1 ethylene and 1.0 μL L^-1 1-MCP) were applied to study the relationship between anthocyanin accumulation and ethylene signal pathway. The fruits stored at 10 °C had higher ethylene production rate and more anthocyanin in flesh than those stored at 0 °C and 25 °C. Ten ethylene biosynthesis associated genes and forty-one ethylene signal transduction related genes were obtained from the previous transcriptome data. Among them, the expression levels of ethylene biosynthesis associated genes (PsACS1, PsACS4 and PsACO1), and ethylene signal transduction related genes (PsERS1s, PsETR2, PsERF1a, and PsERF12) were markedly higher in the fruits stored at 10 °C than those at 0 °C and 25 °C. Exogenous ethylene treatment enhanced while 1-MCP treatment inhibited the anthocyanin accumulation in the flesh under storage at 10 °C. In addition, exogenous ethylene treatment markedly increased the expression levels of PsACS1, PsACS4, PsACO1, PsETR2, PsERF1a, and PsERF12 in the flesh once it turning red, as well as the anthocyanin biosynthesis related genes (PsPAL, PsCHS, PsF3H, PsDRF, PsANS, PsUFGT and PsMYB10), whereas 1-MCP treatment manifested the contrary effects. Correlation analysis indicated that there was a significant positive correlation between genes expression related to ethylene signal pathway and anthocyanin biosynthesis, except for PsERF11. In conclusion, ethylene signal pathway is involved in the flesh reddening by up-regulating the anthocyanin biosynthesis related genes.

Effects of Pollen Sources on Fruit Set and Fruit Characteristics of 'Fengtangli' Plum (Prunus salicina Lindl.) Based on Microscopic and Transcriptomic Analysis

Adequate yield and fruit quality are required in commercial plum production. The pollen source has been shown to influence fruit set and fruit characteristics. In this study, 'Siyueli', 'Fenghuangli' and 'Yinhongli' were used as pollinizers of 'Fengtangli' plum. Additionally, self-pollination, mixed pollination, and open pollination were performed. We characterized the differences in pollen tube growth, fruit set and fruit quality among pollination combinations. 'Fengtangli' flowers pollinated by 'Fenghuangli' had more pistils with pollen tubes penetrating the ovary and the highest fruit set rate, while the lowest fruit set rate was obtained from self-pollination. In self-pollinated flowers, 33% of pistils had at least one pollen tube reaching the ovary, implying that 'Fengtangli' is partially self-compatible. Pollen sources affected 'Fengtangli' fruit size, weight, pulp thickness, soluble solids, and sugar content. Transcriptome analysis of 'Siyueli'-pollinated and 'Yinhongli'-pollinated fruits revealed 2762 and 1018 differentially expressed genes (DEGs) involved in the response to different pollen sources. DEGs were enriched in plant hormone signal transduction, starch and sucrose metabolism, and MAPK signaling pathways. Our findings provide a reference for the selection of suitable pollinizers for 'Fengtangli' plum and promote future research on the metaxenia effect at the molecular level.

An LTR retrotransposon in the promoter of a PsMYB10.2 gene associated with the regulation of fruit flesh color in Japanese plum

Japanese plums exhibit wide diversity of fruit coloration. The red to black hues are caused by the accumulation of anthocyanins, while their absence results in yellow, orange or green fruits. In Prunus, MYB10 genes are determinants for anthocyanin accumulation. In peach, QTLs for red plant organ traits map in an LG3 region with three MYB10 copies (PpMYB10.1, PpMYB10.2 and PpMYB10.3). In Japanese plum the gene copy number in this region differs with respect to peach: there are at least three copies of PsMYB10.1, with the expression of one of them (PsMYB10.1a) correlating with fruit skin color. The objective of this study was to determine a possible role of LG3-PsMYB10 genes in the natural variability of the flesh color trait and to develop a molecular marker for marker-assisted selection (MAS). We explored the variability within the LG3-PsMYB10 region using long-range sequences obtained in previous studies through CRISPR-Cas9 enrichment sequencing. We found that the PsMYB10.2 gene was only expressed in red flesh fruits. Its role in promoting anthocyanin biosynthesis was validated by transient overexpression in Japanese plum fruits. The analysis of long-range sequences identified an LTR retrotransposon in the promoter of the expressed PsMYB10.2 gene that explained the trait in 93.1% of the 145 individuals analyzed. We hypothesize that the LTR retrotransposon may promote the PsMYB10.2 expression and activate the anthocyanin biosynthesis pathway. We propose for the first time the PsMYB10.2 gene as candidate for the flesh color natural variation in Japanese plum and provide a molecular marker for MAS.

Changes in the Primary Metabolites of ‘Fengtang’ Plums during Storage Detected by Widely Targeted Metabolomics

Plums are one of the most popular stone fruits worldwide owing to their high nutritional value. After harvest, plum fruit quality and flavor change during storage; however, little is known about the changes in metabolites during this period. A comprehensive comparison of primary metabolites in ‘Fengtang’ plum fruits during storage is performed using widely targeted primary metabolomics. A total of 272 primary metabolites were identified by means of ultra-performance liquid chromatography and tandem mass spectrometry (UPLC-MS/MS) in the plums at different storage periods. There was a significant increase in the relative amounts of twenty-eight lipids, twenty amino acids and their derivatives, thirteen organic acids, ten saccharides and alcohols, six nucleotides and their derivatives, and two vitamins. A Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis of differential metabolites revealed that glucosinolate biosynthesis, starch and sucrose metabolism, ascorbate and aldarate metabolism, lysine degradation, and other metabolic pathways were significantly enriched; therefore, changes in these metabolic pathways may be key to the quality and flavor change in ‘Fengtang’ plum fruits during storage. Our results provide a theoretical foundation and technical support to evaluate ‘Fengtang’ plum fruit quality.

Cold stress regulates accumulation of flavonoids and terpenoids in plants by phytohormone, transcription process, functional enzyme, and epigenetics

Plants make different defense mechanisms in response to different environmental stresses. One common way is to produce secondary metabolites. Temperature is the main environmental factor that regulates plant secondary metabolites, especially flavonoids and terpenoids. Stress caused by temperature decreasing to 4-10 °C is conducive to the accumulation of flavonoids and terpenoids. However, the accumulation mechanism under cold stress still lacks a systematic explanation. In this review, we summarize three aspects of cold stress promoting the accumulation of flavonoids and terpenoids in plants, that is, by affecting (1) the content of endogenous plant hormones, especially jasmonic acid and abscisic acid; (2) the expression level and activity of important transcription factors, such as bHLH and MYB families. This aspect also includes post-translational modification of transcription factors caused by cold stress; (3) key enzyme genes expression and activity in the biosynthesis pathway, in addition, the rate-limiting enzyme and glycosyltransferases genes are responsive to cold stress. The systematic understanding of cold stress regulates flavonoids, and terpenoids will contribute to the future research of genetic engineering breeding, metabolism regulation, glycosyltransferases mining, and plant synthetic biology.

Identification of microRNAs and their target genes related to the accumulation of anthocyanin in purple potato tubers (Solanum tuberosum)

MicroRNAs (miRNAs) are types of endogenous non-coding small RNAs found in eukaryotes that are 18-25 nucleotides long. miRNAs are considered to be key regulatory factors of the expression of target mRNA. The roles of miRNAs involved in the regulation of anthocyanin accumulation in pigmented potatoes have not been systematically reported. In this study, the differentially expressed miRNAs and their target genes involved in the accumulation of anthocyanin during different developmental stages in purple potato (Solanum tuberosum L.) were identified using small RNA (sRNA) and degradome sequencing. A total of 275 differentially expressed miRNAs were identified in the sRNA libraries. A total of 69,387,200 raw reads were obtained from three degradome libraries. The anthocyanin responsive miRNA-mRNA modules were analyzed, and 37 miRNAs and 23 target genes were obtained. Different miRNAs regulate the key enzymes of anthocyanin synthesis in purple potato. The structural genes included phenylalanine ammonia lyase, chalcone isomerase, flavanone 3-hydroxylase, and anthocyanidin 3-O-glucosyltransferase. The regulatory genes included WD40, MYB, and SPL9. stu-miR172e-5p_L-1R-1, stu-miR828a, stu-miR29b-4-5p, stu-miR8019-5p_L-4R-3, stu-miR396b-5p, stu-miR5303f_L-7R + 2, stu-miR7997a_L-3, stu-miR7997b_L-3, stu-miR7997c_L + 3R-5_2ss2TA3AG, stu-miR156f-5p_L + 1, stu-miR156a, stu-miR156a_R-1, stu-miR156e, stu-miR858, stu-miR5021, stu-miR828 and their target genes were validated by qRT-PCR. They play important roles in the coloration and accumulation of purple potatoes. These results provide new insights into the biosynthesis of anthocyanins in pigmented potatoes.

Weighted gene coexpression correlation network analysis reveals a potential molecular regulatory mechanism of anthocyanin accumulation under different storage temperatures in 'Friar' plum

BACKGROUND

Flesh is prone to accumulate more anthocyanin in postharvest 'Friar' plum (Prunus salicina Lindl.) fruit stored at an intermediate temperature. However, little is known about the molecular mechanism of anthocyanin accumulation regulated by storage temperature in postharvest plum fruit.

RESULTS

To reveal the potential molecular regulation mechanism of anthocyanin accumulation in postharvest 'Friar' plum fruit stored at different temperatures (0 °C, 10 °C and 25 °C), the fruit quality, metabolite profile and transcriptome of its flesh were investigated. Compared to the plum fruit stored at 0 °C and 25 °C, the fruit stored at 10 °C showed lower fruit firmness after 14 days and reduced the soluble solids content after 21 days of storage. The metabolite analysis indicated that the fruit stored at 10 °C had higher contents of anthocyanins (pelargonidin-3-O-glucoside, cyanidin-3-O-glucoside, cyanidin-3-O-rutinoside and quercetin-3-O-rutinose), quercetin and sucrose in the flesh. According to the results of weighted gene coexpression correlation network analysis (WGCNA), the turquoise module was positively correlated with the content of anthocyanin components, and flavanone 3-hydroxylase (F3H) and chalcone synthase (CHS) were considered hub genes. Moreover, MYB family transcription factor APL (APL), MYB10 transcription factor (MYB10), ethylene-responsive transcription factor WIN1 (WIN1), basic leucine zipper 43-like (bZIP43) and transcription factor bHLH111-like isoform X2 (bHLH111) were closely related to these hub genes. Further qRT-PCR analysis verified that these transcription factors were specifically more highly expressed in plum flesh stored at 10 °C, and their expression profiles were significantly positively correlated with the structural genes of anthocyanin synthesis as well as the content of anthocyanin components. In addition, the sucrose biosynthesis-associated gene sucrose synthase (SS) was upregulated at 10 °C, which was also closely related to the anthocyanin content of plum fruit stored at 10 °C.

CONCLUSIONS

The present results suggest that the transcription factors APL, MYB10, WIN1, bZIP43 and bHLH111 may participate in the accumulation of anthocyanin in 'Friar' plum flesh during intermediate storage temperatures by regulating the expression of anthocyanin biosynthetic structural genes. In addition, the SS gene may play a role in anthocyanin accumulation in plum flesh by regulating sucrose biosynthesis.

Activation of PsMYB10.2 Transcription Causes Anthocyanin Accumulation in Flesh of the Red-Fleshed Mutant of 'Sanyueli' (Prunus salicina Lindl.)

Plum is one of the most important stone fruits in the world and anthocyanin-rich plums are increasingly popular due to their health-promoting potential. In this study, we investigated the mechanisms of anthocyanin accumulation in the flesh of the red-fleshed mutant of the yellow-fleshed plum 'Sanyueli'. RNA-Seq and qRT-PCR showed that anthocyanin biosynthetic genes and the transcription factor PsMYB10.2 were upregulated in the flesh of the mutant. Functional testing in tobacco leaves indicated that PsMYB10.2 was an anthocyanin pathway activator and can activate the promoter of the anthocyanin biosynthetic genes PsUFGT and PsGST. The role of PsMYB10.2 in anthocyanin accumulation in the flesh of plum was further confirmed by virus-induced gene silencing. These results provide information for further elucidating the underlying mechanisms of anthocyanin accumulation in the flesh of plum and for the breeding of new red-fleshed plum cultivars.

Effect of benzothiadiazole treatment on quality and anthocyanin biosynthesis in plum fruit during storage at ambient temperature

BACKGROUND

Plums tend to experience a reduction in fruit quality due to ripening and they deteriorate quickly during storage at room temperature. Benzothiadiazole (BTH) is a plant elicitor capable of inducing disease resistance in many crops. In this study, the effect of BTH treatment on fruit ripening, fruit quality, and anthocyanin biosynthesis in 'Taoxingli' plum was investigated.

RESULTS

The results showed that BTH treatment could accelerate fruit ripening without affecting the incidence of fruit decay or the shelf life. Benzothiadiazole treatment improved the quality and consumer acceptability of 'Taoxingli' plums during storage by increasing the sweetness, red color formation, and the concentration of healthy antioxidant compounds. The BTH treatment could also effectively promote the biosynthesis of anthocyanin by enhancing the enzyme activities of phenylalanine ammonia-lyase (PAL), dihydroflavonol 4-reductase (DFR), anthocyanidin synthase (ANS), and uridine diphosphate flavonoid 3-O-glucosyltransferase (UFGT) and up-regulating the gene expressions of PsPAL, PsCHI, PsDFR, PsANS, and PsUFGT during storage.

CONCLUSION

Benzothiadiazole treatment could be a potential postharvest technology for improving fruit quality and consumer acceptability in harvested plum fruit. © 2020 Society of Chemical Industry.

Effect of Storage on the Nutritional Quality of Queen Garnet Plum

Due to high perishability, plums are harvested at an early stage of maturity to extend postharvest storage life. Storage time and temperature can significantly affect the phytochemical and sugar composition of plums, altering their palatability and nutritional quality. In this study, variations in physiochemical properties (total soluble solids (TSS), titratable acidity (TA), color (chroma and hue angle)), phytochemical composition (total phenolic content (TPC), total anthocyanin content (TAC), and carotenoids), and sugars in three different tissues of the Queen Garnet plum (QGP) during storage at two common domestic storage temperatures (4 and 23 °C) were evaluated. There was an increase (p > 0.05) in TSS and a reduction (p < 0.05) in TA of the outer flesh at 23 °C. Chroma values of all the tissues reduced (p < 0.05) at 23 °C. At 4 °C, chroma values fluctuated between storage days. The TAC of the peel was the highest (p < 0.05) among the different tissues and continued to increase up to 10 days of storage at 23 °C (3-fold increase). At 4 °C, the highest (p < 0.05) TAC (peel) was observed after 14 days of storage (1.2-fold increase). TPC showed similar results. The highest (p < 0.05) TPC was recorded in the peel after 10 days of storage at 23 °C (2.3-fold increase) and after 14 days of storage at 4 °C (1.3-fold increase), respectively. Total carotenoids in the flesh samples at both storage temperatures were reduced (p < 0.05) after 14 days. Total sugars also decreased during storage. The results of the present study clearly showed that common domestic storage conditions can improve the nutritional quality of plums by increasing the content of bioactive anthocyanins and other phenolic compounds. However, the increase in phytochemicals needs to be counterbalanced with the decrease in total sugars and TA potentially affecting the sensory attributes of the plums.

Metabolite Changes during Postharvest Storage: Effects on Fruit Quality Traits

Metabolic changes occurring in ripe or senescent fruits during postharvest storage lead to a general deterioration in quality attributes, including decreased flavor and ‘off-aroma’ compound generation. As a consequence, measures to reduce economic losses have to be taken by the fruit industry and have mostly consisted of storage at cold temperatures and the use of controlled atmospheres or ripening inhibitors. However, the biochemical pathways and molecular mechanisms underlying fruit senescence in commercial storage conditions are still poorly understood. In this sense, metabolomic platforms, enabling the profiling of key metabolites responsible for organoleptic and health-promoting traits, such as volatiles, sugars, acids, polyphenols and carotenoids, can be a powerful tool for further understanding the biochemical basis of postharvest physiology and have the potential to play a critical role in the identification of the pathways affected by fruit senescence. Here, we provide an overview of the metabolic changes during postharvest storage, with special attention to key metabolites related to fruit quality. The potential use of metabolomic approaches to yield metabolic markers useful for chemical phenotyping or even storage and marketing decisions is highlighted.