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Plastids are major organelles found in plants and function to manufacture and help store many of the chemical compounds needed by the cell. Chloroplasts are the most well known and are found in photosynthetic tissues of the plant. They contain some yellow / orange carotenoids and the green pigment chlorophyll used in photosynthesis. Photosynthesis produces much of the ATP and reduced nucleotides like NADPH that are required for a variety of plastid functions. Non photosynthetic plastids also make a number of important contributions to the physiology of the plant. Chromoplasts synthesize and store pigments which are responsible for the coloration of fruits and flowers. These red, yellow and orange pigments are important to pollination and dispersal of the plant. Leucoplasts are white or colorless plastids which store starch, lipids and proteins and are generally found in roots and seeds. Leucoplasts also have biosynthesis functions to produce fatty acids, amino acids, and terpenes. Amyloplasts of tubers are non-pigmented plastids responsible for starch synthesis and storage. Amyloplasts also convert the stored starch granules back to sugar when the plant needs energy.

Soy Bean Pod ready for primary culture

All plastids have functions which are vital to the cell and the plant and are known as the “biosynthetic powerhouse” of the plant cell. Plastids are the only organelles which function to synthesize fatty acids and make their own membrane lipids by esterification of fatty acids to glycerol. They also export fatty acids for the synthesis of triacylglycerols as the storage form of fatty acids. Throughout the various developmental stages, the plastids will transition from the synthesis of the fatty acids for membrane lipids to the storage lipids.

The plastids also function to convert components from the oxidized nitrate and sulfate forms to the reduced forms ammonia and sulfide. This process is associated with synthesis of amino acids by the incorporation of ammonia and sulfur into amino acids. All plastids have functions related to carbohydrate metabolism, can synthesize starch, and have other biosynthetic functions such as isoprenoid and aromatic amino acid biosynthesis.

DHAP shuttle mechanism of intraplastidic ATP

In addition to photosynthesis, other specialized and overlapping functions are glycolysis and the pentose phosphate pathway. It is now known that plastids contain a complete set of enzymes related to these metabolic pathways. These processes provide ATP for energy, NADH & NADPH for reducing power, and carbon precursors for various metabolic pathways in the cell. All of these processes are very well characterized individually. What has not been examined is how these processes interact or compete for the same resources provided within the plastid. All these processes require energy, a carbon source, nucleotides such as NADH and NADPH, and other components. We are interested in how an organelle which has so many life-essential functions can somehow regulate the pool of metabolites and decide how to designate these components to a specific function.

Diagram for pathways for resources used by soybean plastids

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. Soybean seed is currently the main feedstock used for the productin of biodiesel fuel in the United States. There is considerable interest in producing a seed with higher oil content which can better meet future demands of the emerging biodiesel industry. To acheive this goal, detailed information about the biochemistry and metabolic regulation of the accumulation of starch, oil and protein in teh developing soybean seed is required.

Yan He at the greenhouse working with the soybean plants

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:

Research by the Sparace Lab is funded by a grant from the United Soybean Board

Research in the Sparace Lab is funded by a grant by the United Soybean Board to S.A.Sparace