Plant material
The SRPs were obtained from 1539-1 and 1637-2 cell lines through somatic embryogenesis. The 1539-1 and 1637-2 cell lines were initiated in nematode-resistant P. thunbergii families 39 and 37, respectively. The protocols for somatic embryogenesis in nematode-resistant P. thunbergii was conducted according to our previous group report (Sun et al. 2019). Briefly, embryogenic cells were isolated from immature cones and cultured on a maintenance medium for proliferation. Subsequently, the embryogenic cells (fresh weight 1g) were transferred to Erlenmeyer flasks (including 30 mL of liquid proliferation medium) and placed on an orbital shaker at 90 rpm for 1 week. The 2 mL suspension was transferred to a maturation medium for 10 weeks in the dark at 23 ± 1℃. After 10 weeks of culturing (without intervals), the cotyledon somatic embryos were taken from the maturation medium and transferred to the germination medium for four weeks under cool white fluorescent light (16/8 h) at 23±1℃. The post-germination SRPs were selected as the experimental samples.