PpHY5 is involved in anthocyanin coloration in the peach flesh surrounding the stone.

Red coloration around the stone (Cs) is an important trait of canned peaches (Prunus persica). In this study, an elongated hypocotyl 5 gene in peach termed PpHY5 was identified to participate in the regulation of the Cs trait. The E3 ubiquitin ligase PpCOP1 was expressed in the flesh around the stone and could interact with PpHY5. Although HY5 is known to be degraded by COP1 in darkness, the PpHY5 gene was activated in the flesh tissue surrounding the stone at the ripening stages and its expression was consistent with anthocyanin accumulation. PpHY5 was able to promote the transcription of PpMYB10.1 through interacting with its partner PpBBX10. Silencing of PpHY5 in the flesh around the stone caused a reduction in anthocyanin pigmentation, while transient overexpression of PpHY5 and PpBBX10 resulted in anthocyanin accumulation in peach fruits. Moreover, transgenic Arabidopsis seedlings overexpressing PpHY5 showed increased anthocyanin accumulation in leaves. Our results improve our understanding of the mechanisms of anthocyanin coloration in plants.

Two R2R3-MYB genes cooperatively control trichome development and cuticular wax biosynthesis in Prunus persica.

The fruit surface has an enormous impact on the external appearance and postharvest shelf-life of fruit. Here, we report two functionally redundant genes, PpMYB25 and PpMYB26, involved in regulation of fruit skin texture in peach. PpMYB25 can activate transcription of PpMYB26 and they both induce trichome development and cuticular wax accumulation, resulting in peach fruit with a fuzzy and dull appearance. By contrast, nonfunctional mutation of PpMYB25 caused by an insertional retrotransposon in the last exon in nectarine fails to activate transcription of PpMYB26, resulting in nectarine fruit with a smooth and shiny appearance due to loss of trichome initiation and decreased cuticular wax accumulation. Secondary cell wall biosynthesis in peach fruit pubescence is controlled by a transcriptional regulatory network, including the master regulator PpNAC43 and its downstream MYB transcription factors such as PpMYB42, PpMYB46 and PpMYB83. Our results show that PpMYB25 and PpMYB26 coordinately regulate fruit pubescence and cuticular wax accumulation and their simultaneous perturbation results in the origin of nectarine, which is botanically classified as a subspecies of peach.

The MADS-box gene PpPI is a key regulator of the double-flower trait in peach.

Double flower is an invaluable trait in ornamental peach, but the mechanism underlying its development remains largely unknown. Here, we report the roles of ABCE model genes in double flower development in peach. A total of nine ABCE regulatory genes, including eight MADS-box genes and one AP2/EREBP gene, were identified in the peach genome. Subcellular localization assay showed that all the ABCE proteins were localized in the nucleus. Four genes, PpAP1, PpAP3, PpSEP3, and PpPI, showed a difference in expression levels between single and double flowers. Ectopic overexpression of PpPI increased petal number in Arabidopsis, while transgenic lines overexpressing PpAP3 or PpSEP3 were morphologically similar to wild-type. Ectopic overexpression of PpAP1 resulted in a significant decrease in the number of basal leaves and caused early flowering. These results suggest that PpPI is likely crucial for double flower development in peach. In addition, double flowers have petaloid sepals and stamens, and single flower could occasionally change to be double flower by converting stamens to petals in peach, suggesting that the double-flower trait is likely to have evolved from an ancestral single-flower structure. Our results provide new insights into mechanisms underlying the double-flower trait in peach.

The sucrose transporter MdSUT4.1 participates in the regulation of fruit sugar accumulation in apple.

BACKGROUND: Sugar content is an important determinant of fruit sweetness, but details on the complex molecular mechanism underlying fruit sugar accumulation remain scarce. Here, we report the role of sucrose transporter (SUT) family in regulating fruit sugar accumulation in apple. RESULTS: Gene-tagged markers were developed to conduct candidate gene-based association study, and an SUT4 member MdSUT4.1 was found to be significantly associated with fruit sugar accumulation. MdSUT4.1 encodes a tonoplast localized protein and its expression level had a negative correlation with fruit sugar content. Overexpression of MdSUT4.1 in strawberry and apple callus had an overall negative impact on sugar accumulation, suggesting that it functions to remobilize sugar out of the vacuole. In addition, MdSUT4.1 is located on chromosomal region harboring a previously reported QTL for sugar content, suggesting that it is a candidate gene for fruit sugar accumulation in apple. CONCLUSIONS: MdSUT4.1 is involved in the regulation of fruit sugar accumulation in apple. This study is not only helpful for understanding the complex mechanism of fruit sugar accumulation, but it also provides molecular tools for genetic improvement of fruit quality in breeding programs of apple.

Analysis of sorbitol content variation in wild and cultivated apples.

BACKGROUND: Sorbitol is the major sugar alcohol in apple and its accumulation in fruit is associated with fruit sweetness. However, little is known about variation in sorbitol content in fruits of apple germplasm. In this study, we investigated sorbitol content in mature fruits of 243 apple cultivars and 20 wild relatives using high-performance liquid chromatography (HPLC). RESULTS: Sorbitol accumulation showed a significant variation in apple germplasm. Overall, cultivated fruits had significantly lower content of sorbitol than wild fruits. Fruit sorbitol concentration was significantly correlated with fruit size and acidity that are extensively domesticated traits. Hence, the variation in sorbitol accumulation between cultivated and wild fruits may be the indirect result of fruit size and acidity selection during domestication. Moreover, sorbitol content was maintained at low levels throughout fruit development, with a dramatic decrease at the middle stage. The SDH1 gene was highly expressed throughout fruit development, and its expression showed a significant correlation with fruit sorbitol concentration, suggesting its potential role in apple fruit sorbitol accumulation. CONCLUSIONS: The finding that there is a great variation in fruit sorbitol content among apple germplasm will be helpful for genetic improvement of fruit sorbitol content in apple breeding programs. © 2019 Society of Chemical Industry.

PpHYH is responsible for light-induced anthocyanin accumulation in fruit peel of Prunus persica.

Peach Prunus persica is an economically important fruit tree crop worldwide. Although the external color of fruit is an important aspect of fruit quality, the mechanisms underlying its formation remain elusive in peach. Here, we report an elongated hypocotyl 5-homolog gene PpHYH involved in the regulation of anthocyanin pigmentation in peach fruit peel. Anthocyanin accumulation in fruit peel is light-dependent in peach. PpHYH had no auto-activation activity and its transcription was induced by sunlight. PpHYH activated transcription of a cluster of three PpMYB10 genes in the present of a cofactor PpBBX4 encoding a B-BOX protein, leading to anthocyanin accumulation in the sun-exposed peel. However, the PpHYH activity was repressed by a negative regulator of PpCOP1 encoding constitutive photomorphogenesis protein 1 which accumulated in the nucleus under dark condition, resulting in failure of anthocyanin accumulation in the shaded peel. PpCOP1 was re-localized into the cytosol under light condition, in accordance with fruit peel pigmentation. Additionally, transport of anthocyanins from the cytoplasm to the vacuole was a rate-limiting step for anthocyanin accumulation in peach fruit peel. Our results reveal for the first time the HYH gene involved in the regulation of anthocyanin accumulation in fruits, and provide target genes for genetic manipulation of fruit coloration.

Two vacuolar invertase inhibitors PpINHa and PpINH3 display opposite effects on fruit sugar accumulation in peach.

Soluble sugars are an important determinant of fruit taste, but their accumulation mechanisms remain elusive. In this study, we report two vacuolar invertase inhibitor genes involved in sugar accumulation in peach, PpINHa and PpINH3. Transient overexpression of PpINH3 in peach fruits resulted in an increase in sugar content, while the opposite trend was detected for PpINHa. Unexpectedly, PpINH3 and PpINHa both had no physical interaction with vacuolar invertase (VIN). Moreover, the PpVIN genes had no or extremely low expression in fruits at the ripening stage. These results suggested that the regulatory role of PpINHa and PpINH3 in sugar accumulation is unlikely due to their interaction with PpVINs. Additionally, overexpression of PpINHa and PpINH3 had an impact on transcription of genes related to fruit sugar metabolism and transport, which is likely responsible for their regulatory role in fruit sugar accumulation. Altogether, these results indicated an important role of PpINHs in fruit accumulation in peach. Our study provides new insights into molecular mechanisms underlying sugar accumulation, which could be useful for genetic improvement of fruit taste in breeding programs of peach and other fruit crops.

Potential Association of Reactive Oxygen Species With Male Sterility in Peach.

Male sterility is an important agronomic trait for hybrid vigor utilization and hybrid seed production, but its underlying mechanisms remain to be uncovered. Here, we investigated the mechanisms of male sterility in peach using a combined cytology, physiology, and molecular approach. Cytological features of male sterility include deformed microspores and tapetum cells along with absence of pollen grains. Microspores had smaller nucleus at the mononuclear stage and were compressed into belts and subsequently disappeared in the anther cavity, whereas tapetum cells were swollen and vacuolated, with a delayed degradation to flowering time. Male sterile anthers had an ROS burst and lower levels of major antioxidants, which may cause abnormal development of microspores and tapetum, leading to male sterility in peach. In addition, the male sterility appears to be cytoplasmic in peach, which could be due to sequence variation in the mitochondrial genome. Our results are helpful for further investigation of the genetic mechanisms underlying male sterility in peach.

Unraveling a genetic roadmap for improved taste in the domesticated apple.

Although taste is an important aspect of fruit quality, an understanding of its genetic control remains elusive in apple and other fruit crops. In this study, we conducted genomic sequence analysis of 497 Malus accessions and revealed erosion of genetic diversity caused by apple breeding and possible independent domestication events of dessert and cider apples. Signatures of selection for fruit acidity and size, but not for fruit sugar content, were detected during the processes of both domestication and improvement. Furthermore, we found that single mutations in major genes affecting fruit taste, including Ma1, MdTDT, and MdSOT2, dramatically decrease malate, citrate, and sorbitol accumulation, respectively, and correspond to important domestication events. Interestingly, Ma1 was identified to have pleiotropic effects on both organic acid content and sugar:acid ratio, suggesting that it plays a vital role in determining fruit taste. Fruit taste is unlikely to have been negatively affected by linkage drag associated with selection for larger fruit that resulted from the pyramiding of multiple genes with minor effects on fruit size. Collectively, our study provides new insights into the genetic basis of fruit quality and its evolutionary roadmap during apple domestication, pinpointing several candidate genes for genetic manipulation of fruit taste in apple.

Development of a fast and efficient root transgenic system for functional genomics and genetic engineering in peach.

Peach is an economically import fruit crop worldwide, and serves as a model species of the Rosaceae family as well. However, peach functional genomics studies are severely hampered due to its recalcitrance to regeneration and stable transformation. Here, we report a fast and efficient Agrobacterium rhizogenes-mediated transformation system in peach. Various explants, including leaf, hypocotyl and shoot, were all able to induce transgenic hairy roots, with a transformation efficiency of over 50% for hypocotyl. Composite plants were generated by infecting shoots with A. rhizogenes to induce transgenic adventitious hairy roots. The composite plant system was successfully used to validate function of an anthocyanin-related regulatory gene PpMYB10.1 in transgenic hairy roots, and two downstream genes, PpUFGT and PpGST, were strongly activated. Our stable and reproductive A. rhizogenes-mediated transformation system provides an avenue for gene function assay, genetic engineering, and investigation of root-rhizosphere microorganism interaction in peach.