Mechanisms Underlying The Regulation Of Anthocyanin Coloration And Proanthocyanidin Synthesis In Peach

Peach tree is an important fruit crop because it not only produces edible fruits, but also serves as an ornamental tree. Peach accumulates mainly two flavonoids, anthocyanin and proanthocyanidin (PA) in fruit. The former affects fruit coloration and nutrient value, while the latter have a great negative impact on flavor. These two flavonoid compounds play a very important role in influencing the buying decisions of consumers. Red leaf coloration is an important ornamental trait. Therefore, it is important to conduct research on mechanisms of anthocyanin and PA accumulation in peach. In this study, mechanism underlying flavonoid biosynthesis was inviestigaed and the main results are listed as follows.Firstly, genetic basis of the blood-flesh trait was investigated in cv. Dahongpao, which accumulates high level of cyanidin-3-glucoside in the mesocarp at fruit mature stage. Elevation of anthocyanin levels in the flesh was correlated well with the expression of an R2R3 MYB transcription factor, PpMYB10.1. However, PpMYBl0.1 did not co-segregate with the blood-flesh trait. The blood-flesh trait was mapped to a 200-kb interval on peach Linkage Group (LG) 5. Within this interval, a gene encoding a NAC domain transcription factor (TF) was found to be highly up-regulated in blood-fleshed peaches compared with non-blood-fleshed peaches. This NAC TF, designated BLOOD (BL), acts as a heterodimer with PpNACl which shows a high level of expression in fruits at late developmental stages. We show that the heterodimer of BL and PpNACl can activate the transcription of PpMYB10.1, thereby, resulting in anthocyanin pigmentation in tobacco. Furthermore, silencing the BL gene reduces anthocyanin pigmentation in blood-fleshed peaches. The transactivation activity of the BL-PpNACl heterodimer is repressed by a SQUAMOSA promoter-binding protein-like transcription factor, PpSPL1. Low expression levels of PpMYB10.1 in fruit at early developmental stages is likely attributed to lower expression levels of PpNAC1, in addition to the presence of high levels of repressors such as PpSPL1. We present a mechanism whereby BL is the key gene for blood-flesh trait in peach, via its activation of PpMYB10.1 in maturing fruits. Partner TFs such as NAC1 are required, as are the removal of transcriptional repressors. Our study not only reveals a new mechanism underlying anthocyanin accumulation in plants, but it is also proveds a way to pro vent the negative effect of environmental factors such as weak light and bagging on fruit coloration.Secondly, red-leaf cultivar Hongyetao was selected to study the mechanism of coloration in leaf Based on comparative transcriptome analysis between red and green colored leaves, an MYB transcription regulator PpMYB 10.4 in the Gr interval was identified to regulate anthocyanin pigmentation in peach leaf. Transient expression of PpMYB10.4 in tobacco and peach leaves can induce anthocyanin accumulation. Moreover, a functional MYB gene PpMYB10.2 on linkage group 3, which is homologous to PpMYB10.4, is also expressed in both red-and green-colored leaves, but plays no role in leaf red coloration. All the results show that PpMYB10.4 is the key gene determining the red-leaf trait in peach. In addition, PpMYB10.4 and other anthocyanin-activating MYB genes in Rosaceae responsible for anthocyanin accumulation in fruit are dated to a common ancestor about 70 million years ago (MYA). However, PpMYB10.4 has diverged from these anthocyanin-activating MYBs to generate a new gene family, which regulates anthocyanin accumulation in vegetative organs such as leaves.Finally, RNA-Seq data was screened to search for PA-related MYB activitors. As a result, a MYB gene designated PpMYB7 in peach was isolated and funcitnally characterized. The peach PpMYB7 represents a new group of R2R3-MYB genes regulating PA synthesis in plants. It is able to activate transcription of PpLAR1 but not PpANR, and has a broader selection of potential bHLH partners compared with PpMYBPA1. Our study suggests a transcriptional network regulating PA synthesis in peach, with the results aiding the understanding of the functional divergence between R2R3-MYB TFs in plants.Taken together, genetic analysis and molecular biology technologies were combined to investigate the complicated regulatory network of anthocyanin biosynthesis in peach mesocarp and leaf, and to understand proanthocyanin biosynthesis in peach mesocarp. Key genes controlling anthocyanin and PA accumulation were identified in peach fruit and leaf. Our study is not only helpful for understaning of mechanisms underlying flavonoid biosynthesis in peach, but it also provides molecular tools for genetic improvement in peach breeding programs.