Heterotrophic plastids of seeds perform many biosynthetic reactions. Understanding their function in crop plants is crucial for seed production. Physiological functions depend on the uptake of precursors by a range of different metabolite translocators. The 2-oxoglutarate/malate translocator gene (PsOMT), which is highly expressed during pea (Pisum sativum) embryo maturation, has an important role during seed storage. PsOMT functions have been studied by antisense repression in maturing pea embryos, and were found to reduce mRNA levels and transport rates of 2-oxoglutarate and malate by 50-70%. Combined metabolite and transcript profiling revealed that OMT repression affects the conversion of carbohydrates from sucrose into amino acids and proteins, decreases seed weight and delays maturation. OMT-repressed pea embryos have increased levels of organic acids, ammonia, and higher ratios of Asn : Asp and Gln : Glu. Decreased levels of most other amino acids indicate the reduced usage of organic acids and ammonia for amino acid biosynthesis in plastids, possibly caused by substrate limitation of the plastidial glutamine synthetase/glutamine-2-oxoglutarate aminotransferase cycle. Expression of storage proteins is delayed, and mature seeds have reduced protein content. Downregulated gene expression of starch biosynthesis and plastidial glucose-6-phosphate transport in asOMT embryos reveals that decreased 2-oxoglutarate/malate transport capacity affects other pathways of central carbon metabolism. Gene expression analysis related to plastid physiology revealed that OMT repression delays differentiation of storage plastids, thereby maintaining gene expression associated with green chloroplasts. We conclude that OMT is important for protein-storing crop seeds, and is necessary for amino acid biosynthesis in pea seeds. In addition, carbon supply as mediated by OMT controls plastid differentiation during seed maturation.