The basic helix-loop-helix (bHLH) transcription factor family is the second-largest transcription factor in plants. Members of this family are involved in the process of growth and development, secondary metabolic biosynthesis, signal transduction, and plant resistance. Loropetalum chinense var. rubrum is a critical woody plant with higher ornamental and economic values, which used as ornamental architecture and traditional Chinese herbal medicine plant. However, the bHLH transcription factor in the L. chinense var. rubrum (Loropetalum chinense var. Rubrum) have yet to be systematically demonstrated, and their role in anthocyanin biosynthesis remains secret. Here, we identified 165 potential LcbHLHs genes by two methods, and they were unequally distributed on chromosomes 1 to 12 of the L. chinense genome. Based on a phylogenetic comparison with proteins from Arabidopsis, these bHLH proteins were categorized into 21 subfamilies. Most LcbHLHs in a given subfamily had similar gene structures and conserved motifs. The gene ontology annotation and cis-elements predicted that the LcbHLHs had many molecular functions and were involved in plant growth processes, including flavonoid and anthocyanin biosynthetic processes, etc. Transcriptomic analysis revealed different expression patterns among different tissues and cultivars in L. chinense. Many LcbHLHs were expressed in leaves, and only a few genes were highly expressed in flowers. Six LcbHLH genes differentially expressed between species and periods in color variation, which may play a key role in anthocyanin synthesis. These further functional predictions of LcbHLHs were established by quantitative real-time PCR analysis and protein-protein interaction network. This study identified the six potential bHLH genes related to anthocyanin in L. chinense and created a solid framework for future research on the function and evolution of bHLHs.

The Loropetalum chinense and Loropetalum chinense var. rubrum are typical and traditional ornamental and Chinese herbal medicine in Asia. However, more information is needed on the mechanisms underlying its flower coloring. Here, we profiled the flavonoid metabolome, full-length sequencing, and transcriptome analysis to investigate the flavonoid biosynthesis and global transcriptome changes among different petal coloring cultivars of L. chinense and L. chinense var. rubrum. The total anthocyanins and phenotypic of the petal were highly consistent with the petal color. Moreover, a total of 207 flavonoid components were identified. Of these, 12 flavonoid components were considered significantly different expression compounds among the four samples. Meanwhile, the first reference full-length transcriptome of L. chinense var. rubrum was being built, which had 171,783 high-quality non-redundant transcripts with correcting with next-generation sequencing (NGS). Among them, 52,851 transcripts were annotated in the seven database of NR, KOG, GO, NT, Pfam, Swiss-Port and KEGG. Combined with NGS analysis, the DETs involved in flavonoids and anthocyanins contributed greatest to the flower-coloring. Additionally, the different expressed of eight LcDFRs and four LcANS genes were positively correlated with flavonoid biosynthesis, and the four LcBZ1 and one Lc3Mat1 were positively correlated with the content of seven anthocyanins revealed by coupling with metabolomics and transcriptomics analysis. Together these results were used to mine candidate genes by analyzing flower coloring changes in a comprehensive metabolic and transcriptomic level in L. chinense and L. chinense var. rubrum.