Green flowers are not an adaptive trait in natural plants due to the challenge for pollinators to discriminate from leaves, but they are valuable in horticulture. The molecular mechanisms of green petals remain unclear. Tree peony (Paeonia suffruticosa) is a globally cultivated ornamental plant and considered the 'King of Flowers' in China. The P. suffruticosa 'Lv Mu Yin Yu (LMYY)' cultivar with green petals could be utilized as a representative model for understanding petal-specific chlorophyll (Chl) accumulation and color formation. Identify the key genes related to Chl metabolism and understand the molecular mechanism of petal color changes. The petal color parameter was analyzed at five developmental stages using a Chroma Spectrophotometer, and Chl and anthocyanin accumulation patterns were examined. Based on comparative transcriptomes, differentially expressed genes (DEGs) were identified, among which three were functionally characterized through overexpression in tobacco plants or silencing in 'LMYY' petals. During flower development and blooming, flower color changed from green to pale pink, consistent with the Chl and anthocyanin levels. The level of Chl demonstrated a similar pattern with petal epidermal cell striation density. The DEGs responsible for chlorophyll and anthocyanin metabolism were characterized through a comparative transcriptome analysis of flower petals over three critical developmental stages. The key chlorophyllase (PsCLH1) and light-harvesting chlorophyll a/b binding protein 1 (PsLhcb1) and PsLhcb5 influenced the Chl accumulation and the greenness of 'LMYY' petals. PsCLH1, PsLhcb1, and PsLhcb5 were critical in accumulating the Chl and maintaining the petal greenness. Flower color changes from green to pale pink were regulated by the homeostasis of Chl degradation and anthocyanin biosynthesis. This study offers insights into underlying molecular mechanisms in the green petal and a strategy for germplasm innovation.

Great progress has been made in our understanding of floral organ identity determination and its regulatory network in many species; however, the quantitative genetic basis of floral organ number variation is far less well understood for species-specific traits from the perspective of population variation. Here, using a tree peony (Paeonia suffruticosa Andrews, Paeoniaceae) cultivar population as a model, the phenotypic polymorphism and genetic variation based on genome-wide association studies (GWAS) and expression quantitative trait locus (eQTL) analysis were analyzed. Based on 24 phenotypic traits of 271 representative cultivars, the transcript profiles of 119 cultivars were obtained, which indicated abundant genetic variation in tree peony. In total, 86 GWAS-related cis-eQTLs and 3188 trans-eQTL gene pairs were found to be associated with the numbers of petals, stamens, and carpels. In addition, 19 floral organ number-related hub genes with 121 cis-eQTLs were obtained by weighted gene co-expression network analysis, among which five hub genes belonging to the ABCE genes of the MADS-box family and their spatial–temporal co-expression and regulatory network were constructed. These results not only help our understanding of the genetic basis of floral organ number variation during domestication, but also pave the way to studying the quantitative genetics and evolution of flower organ number and their regulatory network within populations.

Carbohydrate accumulation during seed development directly influences the oil yield and quality of oilseed plants. To clarify the metabolic and molecular mechanisms underlying this process, we examined seed morphology, metabolome, and transcriptome profiles of Paeonia ostii, a representative oil tree peony, using molecular biology, bioinformatics, and GC-MS techniques. Seeds expanded rapidly and reached their maximum size at 60 days after pollination, coinciding with increased starch staining intensity. Carbohydrate metabolic patterns indicated the conversion of monosaccharides such as glucose, fructose, and inositol into disaccharides like sucrose and into polysaccharides, including starch, raffinose, cellulose, and hemicellulose. Differentially accumulated carbohydrates and associated genes were enriched in the starch and sucrose metabolism and ABC transporter pathways. We constructed a potential regulatory network comprising genes encoding sugar transporters (SWEET, SUS), glycosyl hydrolases, and transcription factors (NF-Y, MYB, LBD, Dof, and B3), which likely play essential roles in carbohydrate deposition and seed development. Therefore, this study clarifies the metabolic and molecular processes governing carbohydrate accumulation in developing seeds and provides a basis for breeding high-yield, high-quality oil tree peony varieties.