Phenylpropanoid compounds encompass a wide range of structural classes and biological functions. Limiting discussion to stress-induced phenylpropanoids eliminates few of the structural classes, because many compounds thst are constitutive in one plant species or tissue can be induced by various stresses in another species or in another tissue of the same plant (Beggs et al., 1987; Christie et al., 1994).

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Stress-Induced Phenylpropanoid Metabolism.

P is an important plant macronutrient, making up about 0.2% of a plant's dry weight. It is a component of key molecules such as nucleic acids, phospholipids, and ATP, and, consequently, plants cannot grow without a reliable supply of this nutrient. Pi is also involved in controlling key enzyme

Phosphorus Uptake by Plants: From Soil to Cell

Physiological and genetic responses of bacteria to osmotic stress.

▪ Abstract Plants respond to herbivore attack with a bewildering array of responses, broadly categorized as direct and indirect defenses, and tolerance. Plant-herbivore interactions are played out on spatial scales that include the cellular responses, well-studied in plant-pathogen interactions, as well as responses that function at whole-plant and community levels. The plant's wound response plays a central role but is frequently altered by insect-specific elicitors, giving plants the potential to optimize their defenses. In this review, we emphasize studies that advance the molecular understanding of elicited direct and indirect defenses and include verifications with insect bioassays. Large-scale transcriptional changes accompany insect-induced resistance, which is organized into specific temporal and spatial patterns and points to the existence of herbivore-specific trans-activating elements orchestrating the responses. Such organizational elements could help elucidate the molecular control over the d...

Plant responses to insect herbivory: The emerging molecular analysis

Phenolic acids are aromatic secondary plant metabolites, widely spread throughout the plant kingdom. Existing analytical methods for phenolic acids originated from interest in their biological roles as secondary metabolites and from their roles in food quality and their organoleptic properties. Recent interest in phenolic acids stems from their potential protective role, through ingestion of fruits and vegetables, against oxidative damage diseases (coronary heart disease, stroke, and cancers). High performance liquid chromatography (HPLC) as well as gas chromatography (GC) are the two separation techniques reviewed. Extraction from plant matrixes and cleavage reactions through hydrolysis (acidic, basic, and enzymatic) are discussed as are the derivatization reagents used in sample preparation for GC. Detection systems discussed include UV-Vis spectroscopy, mass spectrometry, electrochemical, and fluorometric detection. The most common tandem techniques are HPLC/UV and GC/MS, yet LC/MS is becoming more common. The masses and MS fragmentation patterns of phenolic acids are discussed and tabulated as are the UV absorption maxima.

Phenolic Acids in Foods: An Overview of Analytical Methodology

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.

The shikimate pathway.

The shikimate pathway was discovered as the biosynthetic route to the aromatic amino acids phenylalanine, tyrosine, and tryptophan through the classic studies of Bernhard Davis and David Sprinson and their collaborators. This pathway has been found only in microorganisms and plants. Phenylalanine and tryptophan are essential components of animal diets, and animals synthesize tyrosine in a single step from phenylalanine. Thus, with respect to plant specificity, the shikimate pathway is a bit more widespread than nitrogen fixation or photosynthesis but less ubiquitous than, for example, nitrogen assi milat ion . Bacteria spend >90% of their total metabolic energy on protein biosynthesis. Consequently, the bacterial shikimate pathway serves almost exclusively to synthesize the aromatic amino acids (Herrmann, 1983; Pittard, 1987). In contrast, higher plants use these amino acids not only as protein building blocks but also, and in even greater quantities, as precursors for a large number of secondary metabolites, among them plant pigments, compounds to defend against insects and other herbivores (see Dixon and Paiva, 1995, this issue), UV light protectants, and, most importantly, lignin (Bentley, 1990; Singh et al., 1991; see Whetten and Sederoff, 1995, this issue). Under normal growth conditions, 20% of the carbon fixed by plants flows through the shikimate pathway (Haslam, 1993). Globally, this amounts to ~7 x 1015 kg each year, most of it used for the synthesis of the various secondary metabolites. And the variation in shikimate pathway-derived secondary metabolites is very extensive among plant species. The secondary metabolite makeup of a plant could be used for species classification. Different plants not only synthesize different aromatic secondary metabolites but also synthesize varying amounts of them at specific times and in specific subcellular compartments. One would expect that regulation of the differential biosynthesis of sometimes very complex molecular structures might involve regulation of the supply of the precursors influencing the rate-limiting step for carbon flow through the shikimate pathway. Recent data on transgenic potatoes give some indication that this is indeed the case (Jones et al., 1995). This review gives a short overview of the shikimate pathway and briefly introduces the seven enzymes that catalyze the sequential steps of the pathway. This is followed by a discussion of some enzymes of quinate metabolism, which use shikimate pathway intermediates as substrates, thus forming branches off the main trunk. I end by discussing some regulatory features of severa1 of the enzymes.

The Shikimate Pathway: Early Steps in the Biosynthesis of Aromatic Compounds.

The shikimate pathway is often referred to as the common aromatic biosynthetic pathway, even though nature does not synthesize all 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,

The shikimate pathway as an entry to aromatic secondary metabolism.

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.

https://www.pnas.org/content/pnas/86/19/7370.full.pdf

Wounding induces the first enzyme of the shikimate pathway in Solanaceae.

13C and 31P NMR spectra of wild-type Escherichia coli showed resonances from metabolic intermediates of glycolysis and ATP formation but no detectable signals from aromatic amino acids. However, tyrosine biosynthesis from D-[l-13C]glucose was observed in cells harboring a feedback-resistant allele of aroF, the gene encoding tyrosine-sensitive 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase [7-phospho-2-keto-3-deoxy-D-arabino-heptonate D-erythrose-4-phosphate-lyase (pyruvate-phosphorylating), EC4.1.2.15], one of the isoenzymes that control carbon flow through the common aromatic biosynthetic pathway. A similar accumulation of tyrosine and phenylalanine is seen in cells carrying a multiple-copy plasmid that carries a wild-type aroF allele in addition to pheA and tyrA, the structural genes for controlling enzymes of the terminal pathways to phenylalanine and tyrosine biosynthesis. These in vivo measurements by a noninvasive probe suggest feedback inhibition as the quantitatively major mechanism controlling carbon flow in the common aromatic compound biosynthetic pathway. In strains accumulating aromatic amino acids, a transient accumulation of trehalose was detected, indicating that previously unknown changes in Escherichia coli metabolism accompany overproduction of aromatic compounds.

https://www.pnas.org/content/pnas/79/19/5828.full.pdf

Klaus M. Herrmann

Papers

The shikimate pathway.

The Shikimate Pathway: Early Steps in the Biosynthesis of Aromatic Compounds.

The shikimate pathway as an entry to aromatic secondary metabolism.

Wounding induces the first enzyme of the shikimate pathway in Solanaceae.

Biosynthesis of aromatic compounds: 13C NMR spectroscopy of whole Escherichia coli cells.