Storage protein gene expression in zygotic and somatic embryos of interior spruce

Abstract

Storage protein gene expression was compared between zygotic and somatic embryos of interior spruce (Picea glauca/engelmanii complex). Somatic embryos, grown on medium containing 40 μM or 10 μM abscisic acid (ABA), and zygotic embryos accumulated IIS legumin, 7S vicilin and 2S albumin storage proteins. Zygotic embryos displayed a rapid, transient period of storage protein accumulation, while somatic embryos differentiated on 40 μM ABA displayed a more prolonged, gradual accumulation, with some accumulation still evident after 9 weeks of maturation. Somatic embryos on 10 μM ABA accumulated storage proteins initially, but these were rapidly degraded as the embryos germinated precociously. Legumin, albumin and vicilin transcripts were detectable in torpedo stage zygotic and somatic embryos, and increased during embryo development. Transcript levels in zygotic embryos increased during cotyledon development, but following maximum dry weight accumulation and moisture loss, transcripts declined rapidly to low levels. In contrast, somatic embryos on 40 μM ABA had high transcript levels for a prolonged period. These levels were still present after 9 weeks of maturation. A decline in storage protein transcripts similar to zygotic embryos was apparent following a mild drying treatment. These results suggest that a decline in storage protein transcripts is stimulated by embryo drying during the later stages of conifer embryogenesis. Low levels of storage protein transcripts also appeared in somatic embryos on 10 μM ABA, but declined during precocious germination. Osmotic stress induced storage protein and storage protein transcript accumulation. This could be partially inhibited by inclusion of the ABA biosynthetic inhibitor, fluridone. However, endogenous ABA levels did not differ significantly between embryos cultured in the presence or absence of fluridone.