A heteromeric plastidic pyruvate kinase complex involved in seed oil biosynthesis in Arabidopsis

Glycolysis is a ubiquitous pathway thought to be essential for the production of oil in developing seeds of Arabidopsis thaliana and oil crops. Compartmentation of primary metabolism in developing embryos poses a significant challenge for testing this hypothesis and for the engineering of seed biomass production. It also raises the question whether there is a preferred route of carbon from imported photosynthate to seed oil in the embryo. Plastidic pyruvate kinase catalyzes a highly regulated, ATP-producing reaction of glycolysis. The Arabidopsis genome encodes 14 putative isoforms of pyruvate kinases. Three genes encode subunits a, beta(1), and beta(2) of plastidic pyruvate kinase. The plastid enzyme prevalent in developing seeds likely has a subunit composition of 4 alpha 4 beta(1), is most active at pH 8.0, and is inhibited by Glu. Disruption of the gene encoding the b1 subunit causes a reduction in plastidic pyruvate kinase activity and 60% reduction in seed oil content. The seed oil phenotype is fully restored by expression of the b1 subunit-encoding cDNA and partially by the b2 subunit-encoding cDNA. Therefore, the identified pyruvate kinase catalyzes a crucial step in the conversion of photosynthate into oil, suggesting a preferred plastid route from its substrate phosphoenolpyruvate to fatty acids.