Polyamine profiles and biosynthesis in somatic embryo development and comparison of germinating somatic and zygotic embryos of Norway spruce

Abstract

The polyamine (PA) contents and activities of PA biosynthetic enzymes in Norway spruce somatic embryos [Picea abies L. (Karst.), genotype AFO 541] were studied in relation to anatomical changes during their development, from proliferation to germination, and changes in these variables associated with the germination of mature somatic and zygotic embryos were compared. Activities of PA biosynthetic enzymes steadily increased during the development of somatic embryos, from embryogenic suspensor mass until early cotyledonary stages. In these stages, the spermidine (Spd) level was significantly higher than the putrescine (Put) level, and the increases coincided with the sharp increases in S-adenosylmethionine decarboxylase activity in the embryos. The biosynthetic enzyme activity subsequently declined in mature cotyledonary embryos, accompanied by sharp reductions in PA contents, especially in cellular Put contents in embryos from 6 weeks old through the desiccation phase (although the spermine level significantly increased during the desiccation phase), resulting in a shift in the Spd/Put ratio from ca. 2 in early cotyledonary embryos to around 10 after 3 weeks of desiccation. In mature zygotic embryos, Spd contents were twofold lower, but Put levels were higher, than in mature somatic embryos, hence their Spd/Put ratio was substantially lower (ca. 2, in both embryos and megagametophytes). In addition, the PA synthesis activity profiles in the embryos differed (ornithine decarboxylase and arginine decarboxylase activities predominating in mature somatic and zygotic embryos, respectively). The start of germination was associated with a rise in PA biosynthetic activity in the embryos of both origins, which was accompanied by a marked increase in Put contents in somatic embryos, resulting in the decline of Spd/Put ratio to about 2, similar to the ratio in mature and germinating zygotic embryos. The accumulation of high levels of PAs in somatic embryos may be causally linked to their lower germinability than in zygotic embryos.