Identification and analysis of the light-responsive characteristics of the bHLH transcription factor gene family in Chrysanthemum morifolium

The basic helix-loop-helix (bHLH) transcription factor (TF) family members are expressed across eukaryotic species, playing vital roles in growth and developmental processes. Transcriptome analysis in the current study revealed that the shade-tolerant `Nannongxuefeng' variation of cut chrysanthemum contains 14 low-light stress-responsive bHLH family members. Subcellular localization studies show ten bHLH TFs are expressed in the nucleus, whereas the remaining four are extracellular proteins. These bHLH proteins included two conserved domains and a primary region with a highly conserved His15-Glu19-Arg23 sequence associated with target DNA binding. Residues 33 and 64 in the α-helical region were identified as hydrophobic leucine residues. Multiple conserved amino acids were evident in these bHLH proteins, which possess E-box binding functions and 20 sites with >85 % sequence conservation consistent with the characteristics typical of bHLH domains. Three enduring patterns associated with the bHLH protein family have been identified. The 14 genes were readily categorized into five groups. The bHLH2, bHLH12, and bHLH8 genes were promptly activated in response to a white and red light mixture. Except for bHLH4, all members of this family were downregulated in response to a combination of white and blue light, demonstrating a distinct pattern. Minimal expression of bHLH1-5, 9, 11, and 14 was observed in the presence of light. Except for bHLH4 and bHLH7, all members of the bHLH family are controlled by the circadian rhythm. These thorough examinations of the chrysanthemum bHLH TF family offer a basis for future investigations to explore the expression of these genes in more detail or unravel the molecular factors that influence plant responses to low light conditions.