Analysis of global gene expression profiles during the flowering initiation process of Lilium × formolongi

The onset of flowering is critical for the reproductive development of plants. Lilium × formolongi is a lily hybrid that flowers within a year after sowing. We successfully identified four important stages during vegetative growth and flowering initiation of L. × formolongi under long day conditions. The plant tissues from the four stages were used in a genome-wide transcriptional analysis to investigate stage-specific changes of gene expression in L. × formolongi. In total, the sequence reads of the four RNA-sequencing libraries were assembled into 52,824 unigenes, of which 37,031 (70.10%) were differentially expressed. The global expression dynamics of the differentially expressed genes were predominant in flowering induction phase I and the floral differentiation stage, but down-regulated in flowering induction phase II. Various transcription factor families relevant to flowering were elucidated, and the members of the MADS-box, SBP and CO-like transcription factor families were the most represented. There were 85 differentially expressed genes relevant to flowering. CONSTANS-LIKE, FLOWERING LOCUS T, TREHALOSE-6-PHOSPHATE SYNTHASE and SQUAMOSA PROMOTER BINDING PROTEIN-LIKE homologs were discovered and may play significant roles in the flowering induction and transition process of L. × formolongi. A putative gene regulatory network, including photoperiod, age-dependent and trehalose-6-phosphate flowering pathways, was constructed. This is the first expression dataset obtained from a transcriptome analysis of photoperiod-mediated flowering pathway in lily, and it is valuable for the exploration of the molecular mechanisms of flowering initiation and the short vegetative stage of L. × formolongi.