Expression Dynamics of WOX Homeodomain Transcription Factors During Somatic Embryogenesis in Liriodendron Hybrids

The relict woody plant genus Liriodendron contains two endangered species, Liriodendron chinense and Liriodendron tulipifera. Understanding the molecular mechanisms involved in early embryo development is important for horticultural and ecological research, particularly for the development of improved somatic embryogenesis systems. However, the specific molecular processes underlying embryogenesis in these species remain largely unexplored. To address this, we investigated expression of the WOX (WUSCHEL-related homeobox) gene family of transcription factors throughout somatic embryogenesis. We confirmed expression of eight out of 11 novel candidate LcWOX genes in L.chinense using qRT-PCR and examined spatiotemporal expression patterns of the expressed genes using stable reporter lines that had been transformed with different LcWOX promoters driving GUS expression. We observed embryo developmental stages and expression patterns that broadly correlated with those reported for Arabidopsis somatic embryogenesis. LcWUS was weakly expressed during the transition stage and was predominantly restricted to the apical meristem. LcWOX5 was specifically expressed in the root meristem and restricted to the cotyledons thereafter, and LcWOX4 expression was restricted to the vascular tissue of cotyledonary embryos. In contrast, LcWOX9 was expressed in the embryonic callus and the entire embryonic cell mass, then became restricted to the basal cells, indicating a potential role in regulating embryonic maintenance. Our findings provide insights into spatiotemporally specific WOX transcription and shed new light on potential functions of WOX genes during Liriodendron somatic embryogenesis.