Expression of malic enzyme reveals subcellular carbon partitioning for storage reserve production in soybeans

Authors: Morley, Stewart; MA, FANGFANG - Donald Danforth Plant Science Center; ALAZEM, MAZEN - Donald Danforth Plant Science Center; FRANKFATER, CHERYL - Donald Danforth Plant Science Center; YI, HOCHUL - Donald Danforth Plant Science Center; BURCH-SMITH, TESSA - Donald Danforth Plant Science Center; CLEMENTE, TOM - University Of Nebraska; VEENA, VEENA - Donald Danforth Plant Science Center; NGUYEN, HANH - University Of Nebraska; Allen, Douglas - Doug

Submitted to: New Phytologist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/24/2023
Publication Date: 8/2/2023
Citation: Morley, S.A., Ma, F., Alazem, M., Frankfater, C., Yi, H., Burch-Smith, T., Clemente, T.E., Veena, V., Nguyen, H., Allen, D.K. 2023. Expression of malic enzyme reveals subcellular carbon partitioning for storage reserve production in soybeans. New Phytologist. 239(5):1834-1851. https://doi.org/10.1111/nph.18835.
DOI: https://doi.org/10.1111/nph.18835

Interpretive Summary: The market value for soybeans is derived from high quality protein and oil seed reserves. Soybeans contain approximately 40% protein by weight, the highest of any major crop, and the amino acid profile of soybean meal is the compositional standard for animal feed. Soybeans also contain approximately 20% oil by weight whereas other legumes such as peas and dry beans produce a greater percentage of starch. Based on our prior studies, we hypothesized that changing a specific step in central metabolism could enhance the levels of lipids in soybean seeds. Plants were genetically altered and characterized, indicating most notably, a greater production of oil in seeds consistent with the hypothesis and importantly did not result in a drop in protein levels. The studies are important for the production of soybeans that can meet future needs for renewable feedstocks used in biofuel production and as a source of vegetable oil and lipids for human and animal diets.

Technical Abstract: Central metabolism produces amino and fatty acids for protein and lipids that establish seed value. Biosynthesis of storage reserves occurs in multiple organelles that exchange central intermediates including two essential metabolites, malate, and pyruvate that are linked by malic enzyme. Malic enzyme can be active in multiple subcellular compartments, partitioning carbon and reducing equivalents for anabolic and catabolic requirements. Prior studies based on isotopic labeling and steady-state metabolic flux analyses indicated malic enzyme provides carbon for fatty acid biosynthesis in plants, though genetic evidence confirming this role is lacking. We hypothesized that increasing malic enzyme flux would alter carbon partitioning and result in increased lipid levels in soybeans. Homozygous transgenic soybean plants expressing Arabidopsis malic enzyme alleles, targeting the translational products to plastid or outside the plastid during seed development, were verified by transcript and enzyme activity analyses, organelle proteomics, and transient expression assays. Protein, oil, central metabolites, cofactors, and acyl-acyl carrier protein (ACPs) levels were quantified overdevelopment. Amino and fatty acid levels were altered resulting in an increase in lipids by 0.5–2% of seed biomass (i.e. 2–9% change in oil). Subcellular targeting of a single gene product in central metabolism impacts carbon and reducing equivalent partitioning for seed storage reserves in soybeans.