Shikimate accumulation

Figure 13. Glyphosate effect on anthraquinone production and shikimate accumulation in G. mollugo cell cultures ( 84j. Cells were harvested 10 days after inoculation with 2 cm of packed cells into 25 mL medium containing glyphosate. Anthraquinone content of the cells was corrected for the amount present in the inoculum.

The ability of glyphosate to block phenylalanine synthesis, resulting in shikimate accumulation, makes this herbicide a useful tool for examining control of secondary metabolism. 

The effects of glyphosate on phenolic compound production are two-fold 1) accumulation of phenolic compounds that are derivatives of aromatic amino acids is reduced and 2) pools of phenolic compounds derived from constituents of the shikimate pathway prior to 5-enolpyruvylshikimate-3-phosphate become larger. Assays that do not distinguish between effects on these two groups, such as that for hydroxyphenolics of Singleton and Rossi (18), can lead to equivocal and difficult to interpret results (e.g. 3-5). 

The observations that flavonols are not involved in the fertilization process in certain species, and that this function can be completed using other compounds, suggest that flavonols only affect fertility indirectly. There are various examples of cross-talk between branch pathways of phenylpropanoid metabolism, or the shikimate pathway. The absence of flavonols in maize and Petunia could affect the accumulation of other compounds that are more specifically required for male fertility. Thus, differences between species in terms of flavonoid... 

Specific Control of Phytoalexin Accumulation by "Metabolite Shunting" of Biosynthetic Pathways. Graham and coworkers (personal communication), at the Monsanto Laboratories, St. Louis, have developed techniques to selectively shunt defensive metabolites, particularly of the shikimic acid cycle. Through various techniques, certain compounds are applied to plant aerial or root parts, and these compounds have the property of inducing specific accumulations of secondary metabolites. The directions of these accumulations are under known enzymic control (48), and the regulation of these enzymes is achieved by selecting appropriate inducers. Such inducers seem to provide a novel approach to the control of insects by magnifying the ability of plants to produce and concentrate antiherbivory compounds. 

All carbons are derived from either erythrose 4-phosphate (light purple) or phosphoenolpyruvate (pink). Note that the NAD+ required as a cofactor in step (3) is released unchanged it may be transiently reduced to NADH during the reaction, with formation of an oxidized reaction intermediate. Step (6) is competitively inhibited by glyphosate (COO—CH2—NH—CH2—PO ), the active ingredient in the widely used herbicide Roundup. The herbicide is relatively nontoxic to mammals, which lack this biosynthetic pathway. The chemical names quinate, shikimate, and chorismate are derived from the names of plants in which these intermediates have been found to accumulate. 

Quinic acid, a compound accumulated by many green plants, can be formed by reduction of 3-dehy-droquinate (Eq. 25-2) in both plants and bacteria. Quinic acid can be converted into useful industrial products such as benzoquinone and hydroquinone, and its production by bacteria provides a convenient route to these compounds.168 In the main shikimate pathway 3-dehydroquinate is dehydrated to 3-dehydroshikimate (Eq. 25-3). The latter can be dehydrated... 

Shick, J. M., Romaine-Lioud, S., I crrier-Pages, C., and Gattuso, J. P., Ultraviolet-B radiation stimulates shikimate pathway-dependent accumulation of mycosporine-like amino acids in the coral Stylophora pistillata despite decreases in its population of symbiotic dinoflagellates, Limnol. Oceanogr., 44, 1667, 1999. 

Alfalfa plants, derived from tissue culture and tolerant to Roundup, were evaluated to investigate their mechanism of resistance. No difference was found in the level of shikimate-3-biphosphate which accumulated in both resistant and susceptible lines upon treatment with glyphosate. 

When such strains as E. coli 83-24, which are blocked after shikimic acid, were grown on minimal medium plus aromatic supplement, they accumulated 400-800 mg. of shikimic acid per liter, together with variable amounts of shikimate 5-phosphate. Since no mutants that are blocked between shikimic acid and its phosphorylated form were found, it was considered that the phosphate ester is not on the main path of biosynthesis. As will be pointed out later, enzymic studies showed that shikimate 5-phosphate is actually an intermediate between shikimate and the aromatic compounds. It would appear, therefore, that the block in such strains as E. coli 83-24 is probably immediately after shikimate 5-phosphate. With filtrates from this organism, methods were developed for the isolation of pure shikimate and for its stepwise degradation. ... 

Davis (184) selected a large number of mutants of Escherichia coli requiring two or more aromatic amino acids for growth, and then tested a large number of substances to see if any could relieve the growth inhibition. Success was attained with shikimic acid (215, 268), at that time a relatively obscure natural product. This indicated either that shikimic acid (structure diagram 1) was a true aromatic precursor or that it could readily be transformed into a true precursor. The likelihood that shikimic acid was a true precursor was increased when other mutants were found to accumulate shikimic acid in the medium, from which it could be isolated (184, 185). 

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