Background: Tree peony (Paeonia section Moutan DC.) seeds, renowned for their rich content of unsaturated fatty acids, hold significant medicinal and nutritional value. Despite their potential, their yield is relatively low, which constrains economic gains and health benefits. Brassinosteroids (BRs), emerging as potent plant growth regulators, have demonstrated efficacy in boosting plant productivity. Methods: This study employed Paeonia ostii ‘Fengdan’ and administered either water (control group, CKs) or 0.05 mg/L BRs through foliar application during the seed enrichment and conversion phase to assess the effects of BR treatment on seed yield. Results: The BR treatment notably enhanced the protein, starch, and sugar content and yield of P. ostii ‘Fengdan’ seeds, surpassing those of the CKs. Transcriptome profiling identified a plethora of differentially expressed genes in P. ostii ‘Fengdan’ in response to BR treatment, which are implicated in biological processes associated with nutrient reservoir activity. KEGG analysis highlighted the pivotal role of starch and sucrose metabolism in the BR treatment response. WGCNA revealed key gene expression modules that correlate with physiological traits in P. ostii ‘Fengdan’. Furthermore, the quantitative real-time PCR (qRT-PCR) validation of key genes in this pathway revealed that BR treatment enhances yield through a dual mechanism: accelerating the seed-filling process and optimizing developmental timing for earlier maturation. Conclusions: Collectively, these findings offer inaugural comprehensive genomic resources delineating the transcriptional regulatory mechanisms of BRs in P. ostii ‘Fengdan’ seeds.

Tree peony is an important horticultural plant with both ornamental and oil value. The tree peony genome and databases were used to search for calmodulin family genes to explore their function in the pollination of tree peony. The CaM gene family was identified, and then the basic protein characteristics of the family members, such as gene structure, isoelectric point, molecular weight, subcellular localization, and conserved protein domain, were analyzed. The expression levels of these genes in the pistil tissue of Paeonia ostii ‘Fengdan’ at different developmental stages after pollination were also analyzed. Further, qRT-PCR was used to detect the expression levels of six PsCaMs during the development process of the pistil under bee pollination conditions. The results showed that there were six CaM family members located on three chromosomes and one non-chromosome. There were a large number of hormone response and stress response elements on the gene promoter of this family. During the development of pistil tissue after pollination, CaM family gene expression showed a trend of first increasing and then decreasing, which may be related to its function during pollination. The purpose of this study is to identify the gene characteristics and expression patterns of the CaM family during pollination, and to lay a foundation for the functional study of the CaM family in tree peony pollination.