Flowering dogwood (Cornus florida) is a native tree species to the Eastern US that is susceptible to powdery mildew fungus. Genetic transformation of somatic embryogenic tissue in conjunction with gene editing are molecular methods used to breed pathogen-resistant cultivars. This research investigates the most effective germination treatment in regenerating plantlets from genetically transformed somatic embryos of flowering dogwood. From somatic embryogenic callus cultures containing the reporter transgene ß-glucuronidase (GUS), we grew tissue in liquid suspension and size fractioned using sterile metal sieves of different pore sizes. We collected pro-embryogenic masses (PEMs) from the small mesh size sieve, plated PEMS onto nylon supports using vacuum, and placed the nylon supports to semi-solid maturation media for embryo development. We chose somatic embryos elongating beyond the torpedo stage with the best quality and appearance and placed them across the four germination treatments. The germination experiments evaluated four different treatment concentrations of four different plant growth regulators (PGRs), gibberellic acid (GA3), abscisic acid (ABA), 6-benzylaminopurine (BAP), and melatonin. The environmental conditions for the first experiment included fluorescent lighting (10 μmol/m²/sec), temperature at 23 ºC (±1), and a photoperiod of 16h light:8h dark. Somatic embryos growing in germination media supplemented with GA3 (1, 2mg/L) had a higher percentage of embryos showing emergence of the apical shoot meristem between the cotyledons. The next highest percentage of somatic embryos with emerging apical meristems occurred using BAP at the concentrations 0.5 and1mg/L. The second germination experiment assessed the same treatments above except used early- stage torpedo somatic embryos and the light intensity increased to 35 μmol/m²/sec. BAP at 0.5 and 1mg/L showed the highest percentage of greening cotyledons and rooting rates. However, in 60 days, all the somatic embryos died in the GA3, ABA, melatonin, and BAP germination treatments. Although we successfully developed healthy transgenic somatic embryos, converting them into plants was a major challenge. The furthest germination stage we reached was the emerging of apical shoots, where the meristem elongates and continues to primary leaf formation. The difficulty in obtaining complete conversion to plants from these transgenic somatic embryos suggests there may be unintended impacts on growth or germination based on the location of the GUS transgene in the genome. Additionally, the original transgenic embryogenic culture was eight years old prior to the germination experiments, which could have influenced embryo conversion to plants.
