The biochemistry and medical significance of the flavonoids.

Symbioses between plants and beneficial soil microorganisms like arbuscular-mycorrhizal fungi (AMF) are known to promote plant growth and help plants to cope with biotic and abiotic stresses. Profound physiological changes take place in the host plant upon root colonization by AMF affecting the interactions with a wide range of organisms below- and above-ground. Protective effects of the symbiosis against pathogens, pests, and parasitic plants have been described for many plant species, including agriculturally important crop varieties. Besides mechanisms such as improved plant nutrition and competition, experimental evidence supports a major role of plant defenses in the observed protection. During mycorrhiza establishment, modulation of plant defense responses occurs thus achieving a functional symbiosis. As a consequence of this modulation, a mild, but effective activation of the plant immune responses seems to occur, not only locally but also systemically. This activation leads to a primed state of the plant that allows a more efficient activation of defense mechanisms in response to attack by potential enemies. Here, we give an overview of the impact on interactions between mycorrhizal plants and pathogens, herbivores, and parasitic plants, and we summarize the current knowledge of the underlying mechanisms. We focus on the priming of jasmonate-regulated plant defense mechanisms that play a central role in the induction of resistance by arbuscular mycorrhizas.

Mycorrhiza-Induced Resistance and Priming of Plant Defenses

Role of soil microbes in the rhizospheres of plants growing on trace metal contaminated soils in phytoremediation.

Review on polyphenols in Theobroma cacao: changes in composition during the manufacture of chocolate and methodology for identification and quantification

Non-covalent and covalent associations of polyphenols with food macromolecules are two of the most fundamental factors affecting the quality of polyphenol-rich food products. This review therefore describes the biochemical bases of associations between polyphenols and macromolecules, that is, proteins and polysaccharides. Our intent is to provide a level of understanding that can be used to underpin future research directions. This will help to resolve existing issues that limit organoleptic and nutritional qualities of polyphenol-rich foods and drinks. It will also allow a better understanding of the functional consequences of these interactions on food/biological systems. The methods used to study non-covalent and covalent interactions are described, and the limiting factors of each method are emphasized. The biochemical mechanisms of interaction between polyphenols and macromolecules are also described. In processed food, non-covalent polyphenol/macromolecule interactions are largely due to weak associations, and result from a combination of hydrogen bonds and hydrophobic interactions. The biochemical mechanisms for covalent interactions involve oxidation of phenolic compounds, whether enzymatically mediated or not, with the formation of o-quinones or o-semi-quinones, or the cleavage of procyanidin interflavanic bonds in acid medium with the formation of carbocations. The effects of factors such as polyphenol structure, macromolecule structure, relative concentrations of both polyphenol and macromolecule, solvent composition, ionic strength, temperature, and pH are discussed.

Interactions between Polyphenols and Macromolecules: Quantification Methods and Mechanisms

Fractionation and pretreatment of orange peel by dilute acid hydrolysis

Flavonoids and related phenols are ubiquitous in land plants (Mabry and Ulubelen, 1980), and as a common element of plants they have taken their place in importance in far ranging arrays of biological systems, being integral to the function of most living systems. The study of these compounds has attracted the attention of generations of chemists. In fact, as one of the first flavonoids described, hesperidin, the primary flavanone glycoside in citrus, was first reported by Lebreton nearly 170 years ago (Lebreton, 1828). Although there has been a long history of the use of medicinal plants rich in flavonoids, our recent interest in the biological properties of the flavonoids began with the findings of Szent-Gyorgyi who found that citrus flavonoids were important to proper capillary function (Rusznyak and Szent-Gyorgyi, 1936). Although the vitamin status that was briefly attributed to flavonoids was discontinued in 1950, subsequent research has clearly shown an ever increasing list of biological systems over which flavonoids exert significant control. The pace of this research has rapidly accelerated, and recently, a much clearer understanding has emerged of the significance of these compounds, not only in plants, but also in animal systems, and ultimately pertaining to human health. This volume is based primarily on an American Chemical Society symposium held in Orlando, FL in August, 1996. Some of the material was added later to highlight new findings regarding the biological properties of flavonoids. It is not intended to be a comprehensive treatment of flavonoid research, nevertheless it is hoped that it shows new perspectives and encourages new research into this broad subject area at the cellular and molecular levels.

Flavonoids in the Living System

We extended our previous investigations of enzymatic hydrolysis of polysaccharides in orange peel by commercial cellulase and pectinase enzymes to higher, more practical concentrations of orange peel solids. High yields of saccharification could be maintained even at substrate concentrations as high as 22–23%, but the rates of solubilization and saccharification decreased 2-3-fold. We also tested the fermentability of these hydrolysates by the yeastSaccharomyces cerevisiae, which revealed the presence of inhibitory compounds. These compounds could be removed by the filtration of hydrolyzed peel. Successful fermentations of filtered hydrolysates were achieved after pH adjustment with calcium carbonate.

Production of ethanol from enzymatically hydrolyzed orange peel by the yeast Saccharomyces cerevisiae.

A high-performance liquid chromatography method was developed for the simultaneous determination of dehydroascorbic, ascorbic, malic, citric, and oxalic acids in fruits, vegetables, and beverages. Separation of these compounds was accomplished by coupling reversed-phase and organic acid columns using 2% KH2PO4 (pH 2.3) as mobile phase with a flow rate of 0.4 mL/min. Detection was performed at 215 and 260 nm using a diode array detector interfaced with portable integrators and a chromatography data system. Selected fresh fruits, vegetables, and commercial orange juices were analyzed using this method.

Simultaneous detection of dehydroascorbic, ascorbic, and some organic acids in fruits and vegetables by HPLC

Enzymatic hydrolysates of orange peel contain relatively high levels of galacturonic acid and arabinose which are not fermentable to ethanol by yeasts. We observed complete utilization of both sugars during fermentation of peel hydrolysates by the ethanologenic construct of E. coli KO11. The bacterium exhibits a novel pattern of galacturonic acid fermentation producing equimolar amounts of acetate and ethanol accompanied by carbon dioxide.

Béla S. Buslig

Papers

Fractionation and pretreatment of orange peel by dilute acid hydrolysis

Flavonoids in the Living System

Production of ethanol from enzymatically hydrolyzed orange peel by the yeast Saccharomyces cerevisiae.

Simultaneous detection of dehydroascorbic, ascorbic, and some organic acids in fruits and vegetables by HPLC

Fermentation of galacturonic acid and other sugars in orange peel hydrolysates by the ethanologenic strain of Escherichia coli