Expression of Chia4‐Pa chitinase genes during somatic and zygotic embryo development in Norway spruce (Picea abies): similarities and differences between gymnosperm and angiosperm class IV chitinases

The developmental pathway of somatic embryogenesis in Norway spruce involves proliferation of proembryogenic masses (PEMs), PEM‐to‐somatic embryo transition and further development of the somatic embryos. It has previously been shown that extracellular signal molecules, including arabinogalactan proteins, lipo‐chitooligosaccharides and chitinases, regulate somatic embryogenesis. The Chia4‐Pa1 gene from Norway spruce is described here. The Chia4‐Pa1 encodes a typical basic class IV chitinase, although the intron–exon organization of this gymnosperm chitinase is different from that in angiosperm class IV chitinases. The Chia4‐Pa1 belongs to a small gene family with highly similar members, and the expression pattern of Chia4‐Pa1 cannot be distinguished from that of other Chia4‐Pa members. Upon withdrawal of plant growth regulators, i.e. during a treatment that stimulates PEM‐to‐somatic embryo transition and massive programmed cell death, a significant increase in transcription and translation of Chia4‐Pa genes takes place. The expression pattern analysis revealed that Chia4‐Pa genes are expressed in a subpopulation of proliferating cells and at the base of the somatic embryo. Furthermore, in seeds, Chia4‐Pa genes are expressed in the megagametophyte in the single cell‐layered zone surrounding the corrosion cavity. Taken together these results suggest that the Chia4‐Pa expressing cells have a megagametophyte signalling function and that CHIA4‐Pa stimulates programmed cell death and promotes PEM‐to‐somatic embryo transition.