Two stimulatory effects of the peptidyl growth factor phytosulfokine during somatic embryogenesis in Japanese larch (Larix leptolepis Gordon)

In contrast to angiosperms, some gymnosperms form well-development suspensors during somatic embryogenesis. This feature is highly useful to studies of suspensor biology. In cell cultures of Japanese larch (Larix leptolepis Gordon), somatic embryos rarely formed when the initial cell density was lower than 0.1 ml packed cell volume (PCV) l−1. However, in the presence of phytosulfokine (PSK), a peptidyl plant growth factor, mitotic activity during somatic embryogenesis was stimulated, even when the cell density was lower than 0.1 ml PCV l−1, particularly, the development of the suspensor. Manual separation of somatic embryos into the embryo proper and the suspensor arrested further development, but when embryos proper lacking a suspensor were treated with PSK, the suspensor regenerated and development proceeded to maturity. Because no division of the suspensor cells was observed, the cells of the regenerated suspensor probably originated from basal cells of the embryo proper. In contrast, PSK treatment of suspensors lacking the embryo proper did not stimulate further development. These results indicate that PSK stimulates not only cell division of embryo proper but also development of the suspensor.