Flavonoids are a class of secondary plant phenolics with significant antioxidant and chelating properties. In the human diet, they are most concentrated in fruits, vegetables, wines, teas and cocoa. Their cardioprotective effects stem from the ability to inhibit lipid peroxidation, chelate redox-active metals, and attenuate other processes involving reactive oxygen species. Flavonoids occur in foods primarily as glycosides and polymers that are degraded to variable extents in the digestive tract. Although metabolism of these compounds remains elusive, enteric absorption occurs sufficiently to reduce plasma indices of oxidant status. The propensity of a flavonoid to inhibit free-radical mediated events is governed by its chemical structure. Since these compounds are based on the flavan nucleus, the number, positions, and types of substitutions influence radical scavenging and chelating activity. The diversity and multiple mechanisms of flavonoid action, together with the numerous methods of initiation, detection and measurement of oxidative processes in vitro and in vivo offer plausible explanations for existing discrepancies in structure-activity relationships. Despite some inconsistent lines of evidence, several structure-activity relationships are well established in vitro. Multiple hydroxyl groups confer upon the molecule substantial antioxidant, chelating and prooxidant activity. Methoxy groups introduce unfavorable steric effects and increase lipophilicity and membrane partitioning. A double bond and carbonyl function in the heterocycle or polymerization of the nuclear structure increases activity by affording a more stable flavonoid radical through conjugation and electron delocalization. Further investigation of the metabolism of these phytochemicals is justified to extend structure-activity relationships (SAR) to preventive and therapeutic nutritional strategies.

Flavonoid antioxidants: chemistry, metabolism and structure-activity relationships.

The induction of expression of genes for xenobiotic metabolizing enzymes in response to chemical insult is an adaptive response found in most organisms. In vertebrates, the AhR is one of several chemical/ligand-dependent intracellular receptors that can stimulate gene transcription in response to xenobiotics. The ability of the AhR to bind and be activated by a range of structurally divergent chemicals suggests that the AhR contains a rather promiscuous ligand binding site. In addition to synthetic and environmental chemicals, numerous naturally occurring dietary and endogenous AhR ligands have also been identified. In this review, we describe evidence for the structural promiscuity of AhR ligand binding and discuss the current state of knowledge with regards to the activation of the AhR signaling pathway by naturally occurring exogenous and endogenous ligands.

Activation of the aryl hydrocarbon receptor by structurally diverse exogenous and endogenous chemicals.

Potential toxicity of flavonoids and other dietary phenolics: significance for their chemopreventive and anticancer properties.

Metabolic profiling analyses were performed to determine metabolite temporal dynamics associated with the induction of acquired thermotolerance in response to heat shock and acquired freezing tolerance in response to cold shock. Low-Mr polar metabolite analyses were performed using gas chromatography-mass spectrometry. Eighty-one identified metabolites and 416 unidentified mass spectral tags, characterized by retention time indices and specific mass fragments, were monitored. Cold shock influenced metabolism far more profoundly than heat shock. The steady-state pool sizes of 143 and 311 metabolites or mass spectral tags were altered in response to heat and cold shock, respectively. Comparison of heat- and cold-shock response patterns revealed that the majority of heat-shock responses were shared with cold-shock responses, a previously unknown relationship. Coordinate increases in the pool sizes of amino acids derived from pyruvate and oxaloacetate, polyamine precursors, and compatible solutes were observed during both heat and cold shock. In addition, many of the metabolites that showed increases in response to both heat and cold shock in this study were previously unlinked with temperature stress. This investigation provides new insight into the mechanisms of plant adaptation to thermal stress at the metabolite level, reveals relationships between heat- and cold-shock responses, and highlights the roles of known signaling molecules and protectants.

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Exploring the temperature-stress metabolome of Arabidopsis.

Dietary flavonoids: effects on xenobiotic and carcinogen metabolism.

Bioflavonoids: selective substrates and inhibitors for cytochrome P450 CYP1A and CYP1B1.

We conducted a controlled experiment to test the plant vigor and the plant stress hypotheses. The two hypotheses associate plant physiological conditions to insect feeding mode and performance. We exposed tomato, Lycopersicon esculentum ,t o different types of growing conditions: optimal (vigorous plants), resource based stress (water and/or nutrient deficit), and physical stress (punched hole in terminal leaflets). Plant performance, foliar nutritional value for insects and chemical defenses were analyzed after 14 d. These plants were offered to insects belonging to distinct feeding guilds: the silverleaf whitefly, Bemisia argentifolii, a phloem feeder; the leafminer, Liriomyza trifolii; and the corn earworm, Heliothis zea, a leaf chewing caterpillar. The experimental conditions generated a gradient of plant growth in the following order: optimal (vigorous)control =hole punchedno fertilizerno waterno water and no fertilizer. The last two treatments resulted in plants with poor nutritional value (based on %water, C/N, %N) and higher levels of defensive compounds (i.e., peroxidase and total phenolics) compared with control and the vigorous plants. Hole-punching neither affected plant growth nor any of the phytochemicals measured. In a choice experiment adult whitefly ovipositioning was not affected by either vigor or punching but was reduced on the other plants (P0.01). Leafminer feeding and oviposition and corn earworm larval growth rates were higher on the vigorous plants and lower on the punched, no fertilizer, no water, and no water and no fertilizer host plants (P0.01). Regardless of insect species or bioassay method, the results in the tomato system support the plant vigor hypothesis that predicts positive association between insect performance and plant growth. The results contradict the plant stress hypothesis that rank stressed plants as better hosts for insects. The mechanisms involved are a combination of poor nutritional value and chemical defenses. We demonstrate a negative association between plant growth and chemical defense. However, induced response triggered by hole-punching was not cost effective to the plants.

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Suitability of stressed and vigorous plants to various insect herbivores

Some elicitors of plant defensive systems can induce biochemical changes that enable the plant to reduce disease incidence; however, little is known about the effect of these induced responses on insect herbivores. We approached this problem using exogenous field applications of several abiotic elicitors of defensive systems in tomatoes (Lycopersicon esculentum), and evaluated the ability of the elicitors [benzo(1,2,3)thiadiazole-7-carbothioic acid (S)-methyl ester (BTH, Actigard); Probenazole; chitosan; salicylic acid; KeyPlex 350; KeyPlex DP2; and KeyPlex DP3] to reduce pest densities and to provide cross-resistance against various insect herbivores and pathogens. Only BTH provided cross-resistance and significantly reduced the incidence of bacterial spot (Xanthomonas campestris pv. vesicatoria), early blight (Alternaria solani), leaf mold (Fulvia fulva), and leafminer larval densities (Liriomyza spp.). The effects on leafminer larval densities were more pronounced during the early stages of plant development. A trend of reduced densities of whiteflies (Bemisia argentifolii) and powdery mildew (Oidium sp.), although not significant, was also found on the BTH-treated plants. Other elicitors had no significant effect on insect populations, but Probenazole and KeyPlex 350 significantly reduced bacterial spot and early blight incidence. The antiherbivore effects of BTH on leafminers was confirmed in a laboratory two-choice experiment. Adult leafminers preferred untreated plants to the BTH-treated tomatoes as ovipositioning host plants, generally corresponding with larval performance. BTH induced high levels of pathogenesis-related proteins in tomato plants including peroxidase, lysozymes, chitinase, and β-1,3-glucanases. The possible cross-resistance role of these proteins is discussed. The demonstration that exogenous induction of plant defensive systems in the field can result in lower damage caused by various pathogens and insects, supports the hypothesis that plant defensive systems may be general.

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Elicitors of Plant Defensive Systems Reduce Insect Densities and Disease Incidence

Our laboratory found that silverleaf whitefly (SLW; Bemisia argentifolii Bellows & Perring) feeding alters host plant physiology and chemistry. The SLW induces a number of host plant defenses, including pathogenesis-related (PR) protein accumulation (e.g., chitinases, b-1,3-glucanases, peroxidases, chitosanases, etc.). Induction of the PR proteins by SLW feeding occurs in various plant species and varieties. The extent and type of induction is dependent on a number of factors that include host plant growing conditions, the length of time the host plant is exposed to SLW feeding, the plant variety, and SLW population densities. The appearance of PR proteins correlates well with reduced infestations of conspecific insect herbivore competitors. Greenhouse and field experiments in which herbivore competitors (cabbage looper, Trichoplusia ni; leaf miner, Liromyza trifolii) were placed on plants previously exposed to SLW feeding demonstrated behavioral differences (oviposition, feeding preferences) and reduced survival rates and development times of these insects. The interaction was asymmetrical, i.e., SLW infestations of plants previously exposed to leaf miners had little or no effect on SLW behavior (oviposition). Induction of plant-defensive proteins by SLW feeding was both local (at the feeding site) and systemic (uninfested leaves distant to the feeding site). There are interactions between diseases such as tomato mottle virus (ToMoV; a geminivirus) and the host plant and SLW. PR proteins were induced in tomato plants infected with ToMoV much as they were via non-viruliferous SLW feeding. The presence of ToMoV in tomato plants significantly increased the number of eggs produced by SLW females. Experiments using tomato plants, powdery mildew (PM), and tobacco mosaic virus (TMV) show that whitefly infestations can affect plant pathogen relationships but the effects vary among pathogen types. Enzyme analyses prior to pathogen inoculation showed that whitefly treatment significantly increased the activities of foliar chitinase and peroxidase. Evaluation of pathogen growth 3 weeks after inoculation showed that whitefly feeding significantly reduced the incidence of PM. However, TMV levels evaluated by ELISA were not significantly affected by whitefly feeding. Six weeks after inoculation with pathogens, the chitinase and peroxidase activities were still elevated in plants initially fed on by whiteflies but continuing pathogen infection had no effect on these enzymes. The possibility that geminivirus infection and/or SLW infestations isolate the host plant for the selected reproduction of the virus and the insect is discussed. Multitrophic cascade effects may contribute to the successful eruptive appearance of SLW on various crops, ranking them as a major pest. They may explain the general observation that when SLW infest a host plant there are few if any competing insect herbivores and pathogens found in the host. However, the results indicate that certain SLW-virus relationships could be mutualistic. Arch. Insect Biochem. Physiol. 51:151-169, 2002. Published 2002 Wiley-Liss, Inc. †

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Multitrophic interactions of the silverleaf whitefly, host plants, competing herbivores, and phytopathogens.

The role of induced responses of tomato, Lycopersicon esculentum, in interspecific interactions between two polyphagous herbivores, the silver- leaf whitefly, Bemisia argentifolii (WF), and the vegetable leafminer, Lirio- myza trifolii (LM), was characterized in laboratory and field experiments. Feeding by LMs and WFs induced local and systemic production of puta- tive defensive proteins, i.e., chitinases, peroxidases, B-1,3-glucanases, and lysozymes. The magnitude of the induction for each defensive protein varied between species. Unlike WFs, LMs caused a 33% local reduction in total foliar protein content. In a whole-plant choice experiment, adult LM feeding, oviposition, and larval survival were reduced by 47.7%, 30.7%, and 26.5%, respectively, for the WF-infested host compared with the controls. Early WF infestations also had negative systemic (plant-mediated) effects on LMs. Adult LMs preferred leaves from control plants to leaves of plants that had been previously infested with WFs; no reciprocal effect of LMs on WFs were found. Feeding by Helicoverpa zea larvae, which has been shown previously to affect LM performance, had no effect on WF survival and development. LM natural population dynamics were monitored on WF-preinfested and control plants in a field experiment. WF-infested plants were less suitable for LM development with an overall 41% reduction in LM population density. These results demonstrate asymmetric direct and plant-mediated interspecific interactions between generalist herbivores feeding simultaneously on the

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Hamed Doostdar

Papers

Bioflavonoids: selective substrates and inhibitors for cytochrome P450 CYP1A and CYP1B1.

Suitability of stressed and vigorous plants to various insect herbivores

Elicitors of Plant Defensive Systems Reduce Insect Densities and Disease Incidence

Multitrophic interactions of the silverleaf whitefly, host plants, competing herbivores, and phytopathogens.

The Role of Plant Rapidly Induced Responses in Asymmetric Interspecific Interactions Among Insect Herbivores