The bud dormancy of woody plants is a complex process that allows plants to survive harsh environments such as cold and drought. Bud dormancy is classified as paradormancy, endodormancy, and ecodormancy , of which endodormancy is regulated by internal factors . External environmental factors, short-day and low temperature, play an essential role in endodormancy induction, and sufficient low temperature accumulation is a necessary prerequisite to bud break [3–5]. In Paeonia lactiflora, low temperature accumulation at 5℃ for nine weeks was required to ensure the re-growth of buds . The optimal temperatures for bud dormancy release in apple and sweet cherry were − 2 °C to 5.5 °C and − 2 °C to 7 °C, respectively, which was determined by temperature control experiments . Also, the buds of tree peony need to undergo 21 d chilling at 0–4℃ to ensure the following normal development at growth condition .
Furthermore, it is well known that external factors always work through internal factors during bud dormancy release, and hormones play an important role in dormancy regulation. With prolonged exposure to dormancy-inducing conditions (short-day or low temperature), the expression of growth-promoting signals gene (FLOWERING LOCUS T, FT) is inhibited, leading to reduce gibberellins (GAs) levels and increase abscisic acid (ABA) contents, and the ABA response could induce the close of plasmodesmata, thereby mediating the establishment of dormancy[9, 10]. GA and ABA not only involved in the establishment of growth cessation but also played a key role in dormancy release. During dormancy release, the reopening of plasmodesmata could restore the supply of growth-promoting signals with the increasing biosynthesis of GAs . And, the degradation of ABA is necessary for bud dormancy release in grapes, while ABA is accumulated during dormancy establishment .
Recently, some conservative regulatory factors were identified during the bud dormancy transition. An important advancement for this field was to determine the relationship between Dormancy Associated MADS-box (DAM) and bud dormancy, which was up-regulated during induction into dormancy and down-regulated during dormancy release . Since then, the DAM gene had been extensively studied in perennial woody plants, including Japanese pears , Japanese apricot , tea , kiwi fruit  et al. The recent results showed that SHORT VEGETATIVE PHASE (SVP)-like (SVL) with sequence homology to the DAM genes , played a vital role in the dormancy of poplar . SVL is a critical component in the genetic regulatory network of bud dormancy in a recent model: low-temperature decreases ABA levels and reduces SVL expression, leading to the induction of FT1 expression and GA biosynthesis, which promotes dormancy release. Without enough low-temperature accumulation, SVL directly binds to the promoters of GA2 oxidase 8 (GA2ox8) and CALLOSE SYNTHASE 1 (CALS1) and upregulates their expression, which have vital roles in growth cessation and blocking of plasmodesmata conduits, respectively. SVL also plays a critical role in the establishment of dormancy though a self-reinforcing loop along with ABA biosynthesis, activation of TCP/BRC1 (a member of TEOSINTE BRANCHED 1, CYCLOIDEA, PCF family) and repression of FT1 [18, 19].
Carbohydrates play multiple roles in plant growth and development, of which the most important aspect is to provide energy. Before oxidative phosphorylation, Embden Meyerhof Parnas (EMP), Tricarboxylic Acid (TCA), and Pentose Phosphate Pathway (PPP) are the main respiration pathways in plants. EMP starts from glucose, which is an end product of starch degradation. Additionally, maltose and fructose also involved in the EMP pathway after conversion to glucose. The anabolic metabolism of sucrose is mainly carried out by two enzymes, sucrose synthase (SUS; EC 188.8.131.52) and invertase (INV; EC184.108.40.206) . SUS reversibly catalyzes the formation of sucrose from UDP-glucose and fructose. And INV, which irreversibly decompose sucroses into hexose, can be divided into three categories: cell-wall invertase (CWIN), vacuolar invertase (VIN), and cytoplasmic invertase (CIN) [22, 23].
It had been reported that EMP, TCA, and PPP were strictly related to dormancy release. For example, the TCA cycle was enhanced, while the PPP pathway slowly decreased during apple bud sprouting . In grape, dormancy release induced by chemical and low temperature was found related to PPP, EMP, and TCA cycles[25–27]. Furthermore, carbohydrates could also act as a sugar signaling molecule. Mason et al. found that sucrose could serve as signaling molecule involving in paradormancy release .
Flavonoids are widespread secondary metabolites in plants, which mainly contain six subclasses: chalcones, flavones, flavonols, flavandiols, anthocyanins, and proanthocyanidins or condensed tannins . The pathway of flavonoid biosynthesis is quite conservative and well researched in some model plants . Some genes involved in the production of common precursors, such as chalcone synthase (CHS), chalcone isomerase (CHI), flavanone 3-hydroxylase (F3H), and flavonoid 3’-hydroxylase (F3’H), are called early biosynthetic genes (EBGs). Correspondingly, downstream genes for flavonoid biosynthesis are called late biosynthetic genes (LBGs). The pathway of flavonoid biosynthesis was affected by biotic and abiotic factors (e.g., pathogen infections, temperature, drought, plant hormones) . Moreover, recent reports had shown that flavonoids were involved in plant stress response , pollen development , color formation , etc. However, it remained unknown whether flavonoids participate in the process of dormancy release.
Tree peony (Paeonia suffruticosa Andr.) is one of the most ancient ornamental and medicinal plants in the world. As a woody plant, it must undergo a period of low temperature to ensure the sprouting and flowering in the next year. Due to the short and concentrated florescence every year, its anti-season production becomes an essential content of the tree peony industry. Until now, the primary method of anti-season production is to provide sufficient low temperature exposure alone or combining with gibberellin application. Therefore, it was of great value to understand the mechanism of chilling induced dormancy release in tree peony. Our previous study characterized the relationship between chilling accumulation and dormancy status: the physiological status of tree peony ‘Luhehong’ after 14 d chilling treatment was regarded as the transition stage from endodormancy to endodormancy release, and that after 21 d chilling treatment was defined as dormancy release, and after 28 d chilling as a state of ecodormancy . GA pathway plays a crucial role in endodormancy release induced by chilling . And the activity of PPP pathway also increased during the process, suggesting that it played a role in dormancy release of tree peony . As known, traits are more closely related to metabolites, which may provide a new perspective for the understanding of dormancy transition in tree peony.
Here, metabolic changes of tree peony buds during the chilling induced dormancy transition were analyzed. Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis showed that differential metabolites were involved in various metabolic pathways such as carbon metabolism, secondary metabolite synthesis, and hormone metabolism. It was revealed that starch degradation and EMP activity were enhanced during dormancy release. Interestingly, flavonoid anabolism was also activated by chilling accumulation, and its increasement might in return promote flower bud development. Furthermore, the variations of plant hormones (abscisic acid, jasmonic acid, and indole-3-acetic acid) during the dormancy transition were also evaluated in this research. Significantly, the roles of flavonoids were firstly discussed during the dormancy transition in perennial plants. All results would provide valuable information for the molecular mechanism of dormancy transition in tree peony.