An R2R3-MYB transcription factor NtMYB78 modulates growth and development in tobacco via the phenylpropanoid biosynthesis pathway

MYB transcription factors are integral to plant growth and development. In this study, we aimed to elucidate the function of NtMYB78, an R2R3-type MYB transcription factor in tobacco (Nicotiana tabacum L.), by generating overexpression (OE) and knockout (KO) lines. Through a combination of phenotypic characterization, transcriptomic analysis, and metabolomic profiling, we investigated the regulatory mechanisms by which NtMYB78 influences tobacco growth and development. The findings indicated that NtMYB78 was predominantly expressed in roots and was localized to the nucleus. The OE lines exhibited a marked increase in root length during both the seedling and mature stages. Additionally, these lines demonstrated significantly higher SPAD values, chlorophyll content, and photosynthetic rates during the seedling stage. Conversely, the KO lines manifested phenotypes that were diametrically opposed to those observed in the OE lines. Transcriptome analysis demonstrated that, in roots, the OE lines exhibited upregulation of genes associated with the motor protein pathway and downregulation of genes involved in the phenylpropanoid biosynthesis pathway. Conversely, the KO lines showed a marked downregulation of genes linked to motor protein and carbohydrate metabolism. In shoots, the OE lines upregulated genes involved in the phenylpropanoid biosynthesis and photosynthetic carbon fixation pathways, whereas the KO lines downregulated genes associated with photosynthesis-related pathways. Metabolomic analysis revealed a significant increase in spermine content within the roots of OE lines, potentially facilitating root elongation. Integrated analysis further demonstrated that NtMYB78 enhances "source-sink" resource allocation in tobacco by activating phenylpropanoid biosynthesis and carbon fixation pathways in shoots, while concurrently inhibiting phenylpropanoid synthesis in roots to reduce lignin deposition. This study elucidates the molecular mechanisms by which NtMYB78 modulates root growth and photosynthetic capacity in tobacco, offering a novel molecular target for the genetic improvement of tobacco.