Applications of DL-buthionine-[S,R]-sulfoximine deplete cellular glutathione and improve white spruce (Picea glauca) somatic embryo development

In white spruce (Picea glauca), an improvement of somatic embryo yield and quality can be achieved by applications of dl-buthionine-[S,R]-sulfoximine (BSO), which inhibits the biosynthesis of reduced glutathione (GSH), thereby switching the total glutathione pool towards its oxidized form (GSSG). Applications of BSO almost tripled the embryogenic output of two cell lines by increasing the number of embryos produced by 100 mg⁻¹ tissue from 65 to 154 in the (E)WS1 line and from 59 to 130 in the (E)WS2 line. This increase in embryo number was ascribed to a higher production of morphologically normal embryos with four or more cotyledons (group A embryos), at the expense of group B embryos, characterized by fewer cotyledons. The quality of the embryos produced, estimated by their post-embryonic performance, was also different between treatments. In both cell lines applications of BSO in the maturation medium increased the conversion frequency, i.e. root and shoot emergence, of group A embryos while it enhanced root emergence in group B embryos. Compared to their control counterparts, BSO-treated embryos had normal shoot apical meristems as in their zygotic counterparts. Such meristems were characterized by large apical cells and vacuolated sub-apical cells. They also lacked intercellular spaces, which were present in the apical poles of control embryos where they contributed to cell–cell separation and meristem degradation. Furthermore, storage product accumulation was also improved in the presence of BSO, with protein bodies prevailing over starch. These data show that an oxidized glutathione environment is beneficial for spruce embryo production in vitro.