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The HSP20 family is the major member of heat shock proteins that are essential components involved in plant growth, development, and stress response, but little is known about tree peony (Paeonia suffruticosa). In this study, genome-wide analysis combining HSP20 gene family expression analysis of tree peony transcriptome was conducted, and the association between SNPs of HSP20s and flower organ number-related traits was analyzed. A total of 149 members were identified in the P. ostii genome, and divided into 10 subfamilies, most of which were classified into cytoplasm or nucleus. Interestingly, their protein sequences were highly conserved, mainly containing motif 1 or 3. In addition, a total of 38 deferentially expressed HSP20s were identified from transcriptome of flower buds with 5-carpels and polycarpels of P. ostii plants at three developmental stages, among which PoHSP89 and PoHSP133 were further cloned from 271 cultivars for association analysis. It indicated that seven or 19 pairs of associated combinations were obtained with the number of carpel, petal, stamen in PoHSP89 and PoHSP133, which could explain the phenotypic variation by 1.79% to 4.06%, and 1.92% to 12.37%, respectively. It will provide a valuable basis for clarifying the phylogenetic relationship to understand their biological function within the HSP20 gene family, and the identified candidate genes and their corresponding associated SNP loci would reveal the genetic basis on floral organ number variation and be useful for molecular marker-assisted breeding of tree peony in view to improving ornamental traits and yields.

Abstract Background Flower color patterns play an important role in the evolution and subsequent diversification of flowers by attracting animal pollinators. This interaction can drive the diversity observed in angiosperms today in many plant families such as Liliaceae, Paeoniaceae, and Orchidaceae, and increased their ornamental values. However, the molecular mechanism underlying the differential distribution of anthocyanins within petals remains unclear in Paeonia. Results In this study, we used an intersectional hybrid between the section Moutan and Paeonia, hereafter named Paeonia ‘He Xie’, which has purple flowers with dark purple blotches. After Ultra-high performance liquid chromatography-diode array detector (UPLC-DAD) analysis of blotched and non-blotched parts of petals, we found the anthocyanin content in the blotched part was always higher than that in the non-blotched part. Four kinds of anthocyanins, namely cyanidin-3-O-glucoside (Cy3G), cyanidin-3,5-O-glucoside (Cy3G5G), peonidin-3-O-glucoside (Pn3G), and peonidin-3,5-O-glucoside (Pn3G5G) were detected in the blotched parts, while only Cy3G5G and Pn3G5G were detected in the non-blotched parts. This suggests that glucosyltransferases may play a vital role in the four kinds of glucosylated anthocyanins in the blotched parts. Moreover, 2433 differentially expressed genes (DEGs) were obtained from transcriptome analysis of blotched and non-blotched parts, and a key UDP-glycosyltransferase named PhUGT78A22 was identified, which could use Cy3G and Pn3G as substrates to produce Cy3G5G and Pn3G5G, respectively, in vitro. Furthermore, silencing of PhUGT78A22 reduced the content of anthocyanidin 3,5-O-diglucoside in P. ‘He Xie’. Conclusions A UDP-glycosyltransferase, PhUGT78A22, was identified in P. ‘He Xie’, and the molecular mechanism underlying differential distribution of anthocyanins within petals was elucidated. This study provides new insights on the biosynthesis of different kinds of anthocyanins within colorful petals, and helps to explain petal blotch formation, which will facilitate the cultivar breeding with respect to increasing ornamental value. Additionally, it provides a reference for understanding the molecular mechanisms responsible for precise regulation of anthocyanin biosynthesis and distribution patterns.