Alterations of the glutathione redox state improve apical meristem structure and somatic embryo quality in white spruce (Picea glauca)

In white spruce, an improvement of somatic embryo number and quality can be achieved through experimental manipulations of the endogenous levels of reduced (GSH) and oxidized (GSSG) glutathione. An optimal protocol for embryo production included an initial application of GSH in the maturation medium, followed by replacement with GSSG during the remaining maturation period. Under these conditions, the overall embryo population more than doubled, and the percentage of fully developed embryos increased from 22% to almost 70%. These embryos showed improved post-embryonic growth and conversion frequency. Structural studies revealed remarkable differences between embryo types, especially in storage product deposition pattern and organization of the shoot apical meristem (SAM). Compared with their control counterparts, glutathione-treated embryos accumulated a larger amount of starch during the early stages of development, and more protein and lipid bodies during the second half of development. Differences were also noted in the organization of SAMs. Shoot meristems of control embryos were poorly organized and were characterized by the presence of intercellular spaces, which caused separation of the subapical cells. Glutathione-treated embryos had well-organized meristems composed of tightly packed cells which lack large vacuoles. The improved organization of the shoot apical meristems in treated embryos was ascribed to a lower production of ethylene. Differences in meristem structure between control and treated embryos were also related to the localization pattern of HBK1, a shoot apical meristem 'molecular marker' gene with preferential expression to the meristematic cells of the shoot pole. Expression of this gene, which was localized to the apical cells in control embryos, was extended to the subapical cells of treated embryos. Overall, it appears that meristem integrity and embryo quality are under the direct control of the glutathione redox state.