Integrated transcriptomic and phytohormone analyses provide insight into the regulation mechanisms of petal elongation in safflower (Carthamus tinctorius L.)

The tubular petal of safflower ( Carthamus tinctorius L.) holds significant economic value as an important traditional Chinese medicine and a source of industrial dye. Petal size is a major determinant of safflower yield, however, the molecular mechanisms governing petal elongation remain poorly understood. In this study, we characterized the morphological changes in safflower disc floret petals during flowering and demonstrated that rapid petal elongation was driven primarily by cell proliferation from S1 to S2 stage, followed by cell expansion from S2 to S3 stage. Transcriptomic analysis identified 11,807 differentially expressed genes (DEGs) across four developmental stages, including 727 transcription factors (TFs). KEGG enrichment analysis highlighted the importance of phytohormone signaling pathways. Endogenous hormone quantification and exogenous hormone treatment further confirmed the essential roles of auxin and cytokinin in safflower petal elongation. Moreover, we identified four bHLH transcription factors that may integrate auxin and cytokinin signaling to regulate petal size by modulating cell division and expansion. Collectively, our findings establish a foundational framework for deciphering the regulatory network of safflower petal elongation and provide theoretical support for breeding high-yield varieties.