Transcriptome profiling during double-flower development provides insight into stamen petaloid in cultivated <i>Lilium</i>

Asiatic hybrid lilies (Lilium spp.), as the biggest cultivar groups of the lily variety are an ornamental plant with elegant floral patterns and bright colors. We discovered a double-flower Asiatic hybrid cultivar 'Annemarie's Dream' whose stamens convert into petaloid stamens in varying degrees. Double-flower is a significant ornamental trait of the flower organ. However, the molecular mechanism of stamen petaloid formation has not been widely studied in Asiatic hybrid lily. Therefore, we used RNA-seq to contrast transcriptomes of stamen, petaloid stamen and inner tepal at two developmental stages. In total, 190,488 unigenes were obtained and 37,549 differentially expressed genes (DEGs) were identified. We focused on DEGs involved in phytohormone signaling and transcription factor regulation, especially the MADS-box genes (A-class gene LiAP1; B-class gene LiPI; C-class gene LiAG; E-class gene LiAGL6, LiSEP3). Furthermore, we performed weight gene co-expression network analysis (WGCNA) and identified two co-expressed MADS-box homeotic genes (LiAG, CL14315.Contig2_All; LiAP1, CL10060.Contig3_All) as hubs. We also found that CL3014.Contig2_All (PYL) and CL5627.Contig1_All (GID2) as phytohormone-related genes may participate in the regulation of the stamen petaloid during double-flower development. In summary, our findings provide an insight into the molecular regulatory network underlying stamen petaloid and thereby offering a theoretical basis for double-flower breeding in Lilium.