Plastids are an extremely important and diverse group of organelles found in higher plants often described as the biosynthetic powerhouse of the plant cell. As such, they occur in a variety of forms with a variety of specialized functions. Best known among these are chloroplasts that occur in a variety of photosynthetic tissues and are engaged in the photosynthetic activities of plants. Non-photosynthetic plastids, however, also make a number of important contributions to the overall physiology of the plant.
These include the chromoplasts of flowers and fruit that commonly provide the characteristic yellow, orange and red colors of these organs, the amyloplasts of tubers and other storage organs that are important in the synthesis and storage of starch, and the leucoplasts of developing oilseeds involved in the synthesis of fatty acids for oil accumulation. Despite this wide range of specializations in plastid form and function, all plastids have in common a number of primary metabolic or biosynthetic processes that are vital to the plant cell and the entire plant. Besides those processes already mentioned, plastids are almost universally involved in nitrogen and sulfur assimilation, which includes the reduction of nitrite to ammonia and its
subsequent incorporation amino acids; the activation and reduction of sulfate to sulfide and its incorporation into cysteine; and finally the biosynthesis of isoprenoids and aromatic amino acids. All of these biosynthetic processes require a supply of metabolic energy and reduced carbon. Unlike the highly specialized chloroplasts that are capable of providing their own energy (ATP, NADPH) and reduced carbon intermediates, non-photosynthetic plastids must rely, either directly or indirectly, on the cytosolic compartment for their carbon and energy requirements. Equally important, it is now known that plastids have their own sets of enzymes of both the glycolytic and oxidative pentose phosphate pathways which can provide a variety of key intermediates including the energy (ATP), reducing power (NADH & NADPH) and carbohydrate required for nitrogen and sulfur assimilation, and the biosynthesis of fatty acids, starch and amino acids.
Although much is known about the individual metabolic and biosynthetic activities of plastids, relatively little is known about how plastids integrate, regulate and coordinate these processes, many of which occur at the same time and in the same plastid. Such information is crucial as we look towards the molecular genetic improvement of crop plants for nutritional, industrial and environmental purposes (e.g. the enhanced performance of soybean for the emerging bio-diesel industry). Research in my laboratory emphasizes the characterization and manipulation of the metabolic interactions that occur in the various functions of plastids. For this purpose, two model plastid systems are currently being used. These are the largely mixed function “autotrophic/heterotrophic” plastids from developing soybean embryos and the non-photosynthetic plastids from germinating pea roots.
Research in my laboratory emphasizes the characterization and manipulation of the metabolic interactions that occur in the various functions of plastids. For this purpose, two model plastid systems are currently being used. These are the largely mixed function “autotrophic/heterotrophic” plastids from developing soybean embryos and the non-photosynthetic plastids from germinating pea roots.
Specific or emerging projects in my laboratory include the following:
- Characterization of the profiles of lipid, protein, starch and oligosaccharide accumulation in developing soybean somatic embryos
- Development of methods for the isolation of plastids from developing soybean somatic embryos
- Characterization of the metabolic capabilities of plastids from developing soybean somatic embryos
- Metabolic interactions in the function of pea root plastids
Research in the Sparace Lab is funded by a grant by the United Soybean Board to S.A.Sparace