Brassinosteroid Biosynthetic Gene CmDWF4 Regulates Bud Outgrowth in Chrysanthemum morifolium

Brassinosteroids (BRs), a class of steroid phytohormones, play a critical role in plant growth and development. The DWF4 gene encodes a cytochrome P450 enzyme (CYP90B1), which is considered a rate-limiting enzyme in BR biosynthesis. Here, we identified a homologous gene of DWF4 in chrysanthemum, CmDWF4. This gene was predicted to encode 491 amino acid residues with a molecular weight of 56.2 kDa and an isoelectric point (pI) of 9.10. Overexpression of CmDWF4 in chrysanthemum was found to significantly increase growth rate, number, and length of lateral buds. Transcriptome analysis showed that multiple xyloglucan endotransglycosylase/hydrolase (XTH) family encoding genes associated with cell wall modification were up-regulated in CmDWF4-overexpressing lines. qRT-PCR assay confirmed the up-regulation of CmXTH6, CmXTH23, and CmXTH28 in CmDWF4-overexpression line. Overall, this work establishes a mechanism by which BR biosynthetic gene CmDWF4 promotes lateral bud outgrowth in chrysanthemum, possibly through regulating cell elongation and expansion.