Mutations in a plastid-localized elongation factor G alter early stages of plastid development in Arabidopsis thaliana

We have identified Arabidopsis thaliana mutants in a gene that encodes a plastid-targeted elongation factor G (SCO1) that is essential for plastid development during embryogenesis since two T-DNA insertion mutations in the coding sequence (sco1-2 and sco1-3) result in an embryo-lethal phenotype. In addition, a point mutation allele (sco1-1) and an allele with a T-DNA insertion in the promoter (sco1-4) of SCO1 display conditional seedling-lethal phenotypes. Seedlings of these alleles exhibit cotyledon and hypocotyl albinism due to improper chloroplast development, and normally die shortly after germination. However, when germinated on media supplemented with sucrose, the mutant plants can produce photosynthetically-active green leaves from the apical meristem.The developmental stage-specific phenotype of the conditional-lethal sco1 alleles reveals differences in chloroplast formation during seedling germination compared to chloroplast differentiation in cells derived from the shoot apical meristem. Our identification of embryo-lethal mutant alleles in the Arabidopsis elongation factor G indicates that SCO1 is essential for plant growth, consistent with its predicted role in chloroplast protein translation.In oilseed plants such as Arabidopsis (Arabidopsis thaliana) and rapeseed (Brassica napus), developing embryos are green and cells in these embryos develop functional chloroplasts [1]. The green embryos are capable of photosynthesis and have been shown to fix carbon crucial to the biosynthesis of seed storage oils [2-4]. In experiments with cultured rapeseed embryos and siliques, light was found to increase embryo growth-rates, which correlated both with improved carbon sequestration and with its utilization in seed oil synthesis [5]. These effects were largely negated by inhibition of photosynthesis, and their studies indicated that it is the reductant and/or ATP produced by photosynthesis in green embryos that is important for normal embryo growth and seed develop