The shikimate pathway links metabolism of carbohydrates to biosynthesis of aromatic compounds. In a sequence of seven metabolic steps, phosphoenolpyruvate and erythrose 4-phosphate are converted to chorismate, the precursor of the aromatic amino acids and many aromatic secondary metabolites. All pathway intermediates can also be considered branch point compounds that may serve as substrates for other metabolic pathways. The shikimate pathway is found only in microorganisms and plants, never in animals. All enzymes of this pathway have been obtained in pure form from prokaryotic and eukaryotic sources and their respective DNAs have been characterized from several organisms. The cDNAs of higher plants encode proteins with amino terminal signal sequences for plastid import, suggesting that plastids are the exclusive locale for chorismate biosynthesis. In microorganisms, the shikimate pathway is regulated by feedback inhibition and by repression of the first enzyme. In higher plants, no physiological feedback inhibitor has been identified, suggesting that pathway regulation may occur exclusively at the genetic level. This difference between microorganisms and plants is reflected in the unusually large variation in the primary structures of the respective first enzymes. Several of the pathway enzymes occur in isoenzymic forms whose expression varies with changing environmental conditions and, within the plant, from organ to organ. The penultimate enzyme of the pathway is the sole target for the herbicide glyphosate. Glyphosate-tolerant transgenic plants are at the core of novel weed control systems for several crop plants.

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The shikimate pathway is often referred to as the common aromatic biosynthetic pathway, even though nature does not synthesize a11 aromatic compounds by this route. This metabolic sequence converts the primary metabolites PEP and erythrose-4-P to chorismate, the last common precursor for the three aromatic amino acids Phe, Tyr, and Trp and for p-amino and p-hydroxy benzoate (Fig. 1). The shikimate pathway is found in bacteria, fungi, and plants. In monogastric animals, Phe and Trp are essential amino acids that have to come with the diet and Tyr is directly derived from Phe. Since bacteria use in excess of 90% of their metabolic energy for protein biosynthesis, for most prokaryotes, the three aromatic amino acids represent nearly the entire output of aromatic biosynthesis, and regulatory mechanisms for shikimate pathway activity are triggered by the intracellular concentrations of Phe, Tyr, and Trp. This is not so in higher plants, in which the aromatic amino acids are the precursors for a large variety of secondary metabolites with aromatic ring structures that often make up a substantial amount of the total dry weight of a plant. Among the many aromatic secondary metabolites are flavonoids, many phytoalexins, indole acetate, alkaloids such as morphine, UV light protectants, and, most important, lignin. THE PATHWAYAlthough the enzyme-catalyzed reactions of the shikimate pathway seem to be identical for prokaryotes and eukaryotes, plants have branches off of the main pathway that so far have not been demonstrated in bacteria or fungi. In this update I will focus on the main pathway. First, I will briefly review the basic regulatory features of the shikimate pathway in bacteria and then describe what is known about these features in higher plants. Here the differences between prokaryotes and eukaryotes are rather striking. I will close with some projections into future research in this exciting field.The main trunk of the shikimate pathway consists of reactions catalyzed by seven enzymes. The best studied of these are the penultimate enzyme, the 5-enol-pyruvoyl shikimate-3-P synthase, the primary target site for the herbicide glyphosate, and the first enzyme, DAHP synthase, the enzyme that controls carbon flow into the shikimate pathway. DAHP synthase catalyzes the condensation of PEP and erythrose-4-P to yield DAHP and Pi. Even though the enzyme was discovered in Escherichia coli more than three decades ago and has been purified to electrophoretic homogeneity from a number of sources, the fine structure of DAHP, the product of the enzyme-catalyzed reaction, was not described until many years later as the structure given in Figure 2 (Garner and Herrmann, 1984). D A H P SYNTHASE OF BACTERIAThe most intensively investigated DAHP synthase has been the E. coli enzyme. This organism encodes three DAHP synthase isoenzymes, a Phe-sensitive, a Tyr-sensitive, and a Trp-sensitive activity. The three enzymes have been purified to homogeneity, their structural genes have been characterized, and their complete primary struct...

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The first enzyme of the shikimate pathway, 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase (EC 4.1.2.15), is induced by wounding potato or tomato tissue. The increase in enzyme activity is associated with elevated amounts of the enzyme as determined by immunoblots. The specific wound-induced protein synthesis is preceded by an increase in the mRNA encoding this enzyme. The induced mRNA of potato tuber, leaf, and stem tissue is translated into a precursor polypeptide that is recognized by antibodies raised against the mature enzyme from tuber plastids. Wounding also induces mRNA encoding phenylalanine ammonia-lyase (EC 4.3.1.5), a key enzyme of plant secondary metabolism. The time courses for the induction of the two enzymes are similar, suggesting coordinate regulation for the biosynthesis of primary and secondary aromatic compounds.Plants respond to wounding by increased production of compounds involved in the repair of wound damage and in defense against microbial invasion. Wound and defense responses are intimately connected, since many pathogens create, or enter through, wounds in the epidermis. Repair requires lignin and suberin (1); defense against pathogens is associated with the synthesis of phytoalexins (2), low molecular weight antimicrobial compounds (3).Wounding stimulates polysome formation and induces synthesis of proteins for several metabolic pathways (4). The initiation of wound repair is characterized by the accumulation of hydroxyproline-rich proteins in plant cell walls (5). These proteins impart wall rigidity and may provide sites for lignin deposition (6). The synthesis of these proteins in response to wounding is due to gene activation (7,8). Different transcripts accumulate in response to different stresses caused by wounding, fungal infection (9), or ethylene treatment (10). Lignification appears to be ubiquitous in wounded plants. Lignin accumulation is accompanied by the activation of enzymes of lignin biosynthesis, including phenylalanine ammonia-lyase (11, 12), cinnamate 4-hydroxylase (13), p-hydroxycinnamate-CoA ligase (14), and chorismate mutase (15). Genes encoding key enzymes of lignin biosynthesis are transcriptionally activated by wounding (8,16,17).Lignin is synthesized from phenylalanine, one end-product of the shikimate pathway (18) The plastidic potato DAHP synthase was purified to electrophoretic homogeneity (23), and antibodies against this enzyme (anti-potato DAHP synthase) were raised in rabbits (21). The 374-base pair (bp) EcoRI-Kpn I cDNA fragment encoding part of the plastidic potato DAHP synthase was prepared as described (24). cDNAs encoding the Glycine max phenylalanine ammonia-lyase (EC 4.3.1.5) and the Electrophorus electricus calmodulin (26) were gifts of L. Vodkin (University of Illinois) and A. R. Means (Baylor Medical School), respectively. Potato (Solanum tuberosum cv. Superior) and tomato (Lycopersicon esculentum cv. Rutgers) plants were grown under greenhouse conditions. Plant Wounding. Potato tubers were surface sterilized in 0.05% NaClO fo...

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