Plant materials and growth conditions
Tissue cultured seedlings of apple (Malus domestica cv ‘Royal Gala’) ‘GL3’ was utilized in this study (Dai et al., 2013). TheMdBT2-OE and MdBT2-anti transgenic plantlets were obtained in our previous work and kept in our lab (Wang et al., 2018). The WT and transgenic ‘GL3’ plantlets were grown on Murashige-Skoog (MS) medium supplemented with 0.2 mg/L GA (gibberellin), 0.6 mg/L 6-BA (6-benzylamino-purine), and 0.2 mg/L NAA (naphthylacetic acid) at 25 °C under long-day (16 h : 8 h/light : dark) conditions (Wang et al.,2018). The MdBT2-OE (pIR-MdBT2-GFP) and MdBT2-anti(pIR-MdBT2) transgenic apple calli, as well as the MdCUL3Aoverexpression apple calli, were generated previously and kept in our lab (Wang et al., 2018; Zhao et al, 2016). The WT and transgenic apple calli ‘Orin’ were cultured on MS medium containing 0.4 mg/L 6-BA and 1.5 mg/L 2,4-D (2,4-dichlorophenoxyacetic acid) at 25 °C under constant dark conditions. The N. benthamiana were maintained in growth chamber with 16 h : 8 h/light : dark in 23 °C.