Role of BABY BOOM Transcription Factor in Promoting Somatic Embryogenesis and Genetic Transformation in a Woody Magnoliid <i>Liriodendron</i>

Somatic embryogenesis (SE) is a powerful biotechnological tool widely utilized for large-scale propagation and genetic transformation. Morphogenic genes like BABY BOOM (BBM) and WUSCHEL (WUS) play crucial roles in SE and are extensively applied to improve SE-based genetic transformation. However, the transcriptome profiling and key regulatory factors of SE in the woody magnoliid Liriodendron hybrid remain unclear. Here, we depicted the time-series transcriptome profiling of SE in Liriodendron hybrid, highlighting the temporal significance of morphogenic genes like BBM in embryogenic callus and developing somatic embryos. Expression patterns were validated using qRT-PCR and transgenic lines expressing β-glucuronidase (GUS) and red fluorescent protein mCherry driven by the LhBBM promoter. Overexpression of LhBBM, both constitutive (CaMV 35S promoter) and SE-specific (Liriodendron WOX9 promoter), enhanced SE and embryonic callus induction. Conversely, CRISPR/Cas9-mediated knockout of LhBBM reduces SE efficiency without compromising callus induction. Furthermore, we developed a secondary callus induction method that minimized the heterogeneity of a transgenic callus line, confirming the sufficiency and necessity of LhBBM in SE. Notably, LhBBM significantly improved genetic transformation efficiency in Liriodendron. These findings establish LhBBM as a promising target for enhancing SE capacity and SE-based transformation efficiency, particularly in forest trees.