Quinate, Focal Point of Upstream Metabolism Leading to Lignin

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9728619 Jensen and Bonner Quinic acid is one of the most abundant natural products in the biosphere. It is accumulated in some tissues of woody plants, e.g., newly forming needles of conifers, in amounts equaling up to 10% of the total dry weight. Quinic acid is exceedingly active as an intermediate which possesses alternative metabolic fates. In xylem-forming cells it is mainly transformed to phenylpropanoid precursors of lignin. In autotrophic cells, on the other hand, quinic acid is an effective precursor of other phenolic compounds - the most noteworthy ones being chlorogenic acid, gallic acid and protocatechuic acid. Chlorogenic acid and gallic acid confer resistance to herbivore predation and phytopathogenic microorganisms. Protocatechuic acid is catabolized to central intermediates and can serve as a carbon-source reserve during dark metabolism. Finally, quinic acid is a highly soluble, readily transported metabolite which can serve as a versatile precursor of aromatic amino acids in different tissue compartments. Loblolly pine (Pinus taeda), the experimental system, is under intensive study by others in order to understand and manipulate lignin content. Therefore, many biological and molecular-genetic resources are available to support this project, including a tissue-culture system which is lignin-inducible. The goal of this research is to elucidate the nature of the interfacing upstream metabolism which delivers L-phenylalanine to the phenylpropanoid metabolic section. It will focus upon three quinate dehydrogenase proteins: nQDH-NADP and nQDH-NAD in needle tissue and xQDH-NADP in xylem-forming cells. NQDH-NADP appears to be trifunctional, having catalytic domains which carry out the overall conversion of quinate to protocatechuate. The enzymes will be purified, used to raise specific antibodies, and characterized for physical and catalytic properties. cDNA clones corresponding to each QDH will be obtained by immunoscreening. Specific antibody will be used for immunogold EM cytoloca lization. Regulation will be examined by monitoring levels of enzyme activity, amounts of protein, and transcript abundance in response to developmental and environmental cues. The longterm objective is to understand how this entire metabolic network is differentially regulated to accomplish the dynamic alternative molecular fates of quinate. This is expected to provide ultimately a rational basis for biotechnological manipulations designed to modify lignin content by alteration of flux toward and away from quinate in different cellular compartments and in different specialized tissues.