Species-specific and leaf-age dependent effects of ultraviolet radiation on two Brassicaceae

Ultraviolet (UV) radiation affects the chemical composition of a plant. Since young leaves are of higher value due to their increased photosynthetic activity, for these a more efficient protection and thus stronger responses to a short-term exposure to natural radiation including or excluding UV-A plus UV-B radiation ("+UV" vs. "-UV") were expected than for old leaves. Nutrients and characteristic secondary metabolites of two species of Brassicaceae were analysed after two days exposure in foil-tents with different UV filtering qualities. Contents of water, carbon, nitrogen and soluble protein were found to be affected by both UV and leaf-age in Sinapis alba L. but mainly by leaf-age in Nasturtium officinale L. Glucosinolates and myrosinases, both partners of the defence system of Brassicaceae, responded highly species-specific to UV exposure. Moreover, leaf-age mainly affected total glucosinolate concentrations in S. alba, but myrosinase activities in N. officinale. The most pronounced response to UV was found in the accumulation of flavonoids which are needed to shield the leaf interior against UV. In S. alba, relative contents of quercetin flavonols increased at the expense of kaempferols in +UV exposed leaves. In N. officinale, total flavonoid quantities were 10-fold lower in -UV exposed young leaves compared to S. alba, and flavonoid accumulation was induced by UV specifically in old leaves. Hydroxycinnamic acid concentrations were not affected in both species. In total, these herbaceous species showed a highly species-specific and age-dependent plasticity in response to short-term exposure to UV which is discussed with respect to their defence strategies.     

Natural products with calmodulin inhibitor properties

This review summarizes properties of various naturally occurring compounds with reported calmodulin (CaM)-inhibitory properties which include about 159 natural products belonging to different structural classes. Most inhibitors are alkaloid and peptide type of compounds and have been isolated from a wide variety of natural sources, including many plant species. Among the most potent natural anti-CaM substances, however, are several animal venoms and the antibiotic polymixin. The largest number of compounds described were discovered by means of enzymatic functional assays.     

Flavonoids induce germination of basidiospores of the ectomycorrhizal fungus <Emphasis Type="Italic">Suillus bovinus</Emphasis>

Under laboratory conditions, spores of ectomycorrhizal fungi usually germinate very poorly or not at all. In a previous study, we showed that spores of the ectomycorrhizal fungus Suillus bovinus germinated through the combination of activated charcoal treatment of media and co-culture with seedlings of Pinus densiflora, which suggested that some substances contained in root exudates induced the germination. Among the compounds reported from root exudates, flavonoids have been elucidated to play various and substantial roles in plant–microbe interactions; we therefore investigated the effects of flavonoids on basidiospore germination of S. bovinus by the diffusion gradient assay on water agar plates pretreated with charcoal powder. Seven out of the 11 flavonoids tested, hesperidin, morin, rutin, quercitrin, naringenin, genistein, and chrysin, had greater effects than controls, whereas flavone, biochanin A, luteolin, and quercetin showed no positive effects. The effective concentration presumably corresponded to several micromolar levels, which was equivalent to those effective for pollen development, nod gene induction, and spore germination of F. solani f. sp. pisi and AM fungi. The results suggest that flavonoids play a role as signaling molecules in symbiotic relationships between woody plants and ectomycorrhizal fungi.     

Biotechnology of flavonoids and other phenylpropanoid-derived natural products. Part I: Chemical diversity, impacts on plant biology and human health

Plant natural products derived from phenylalanine and the phenylpropanoid pathway are impressive in their chemical diversity and are the result of plant evolution, which has selected for the acquisition of large repertoires of pigments, structural and defensive compounds, all derived from a phenylpropanoid backbone via the plant-specific phenylpropanoid pathway. These compounds are important in plant growth, development and responses to environmental stresses and thus can have large impacts on agricultural productivity. While plant-based medicines containing phenylpropanoid-derived active components have long been used by humans, the benefits of specific flavonoids and other phenylpropanoid-derived compounds to human health and their potential for long-term health benefits have been only recognized more recently. In this part of the review, we discuss the diversity and biosynthetic origins of phenylpropanoids and particularly of the flavonoid and stilbenoid natural products. We then review data pertaining to the modes of action and biological properties of these compounds, referring on their effects on human health and physiology and their roles as plant defense and antimicrobial compounds. This review continues in Part II discussing the use of biotechnological tools targeting the rational reconstruction of multienzyme pathways in order to modify the production of such compounds in plants and model microbial systems for the benefit of agriculture and forestry.     

Biosynthesis of 5-deoxyflavanones in microorganisms

Flavanones are the common precursors of plant polyphenolic compounds collectively known as flavonoids. Leguminous plants have evolved a distinct class of flavanone molecules, known as 5-deoxyflavanones that play important roles in their symbiotic interactions. A four-step metabolic circuit was constructed in Escherichia coli with plant genes from heterologous origins: 4-coumarate:coenzyme A ligase from Petroselinum crispum, chalcone synthases (CHS) from Medicago sativa and Petunia x hybrida and chalcone reductase and chalcone isomerase from M. sativa. Evaluation of the different recombinant strains in shake flask experiments demonstrated that P. hybrida rather than M. sativa CHS resulted in the highest liquiritigenin production levels in glucose minimal medium, starting from precursor p-coumaric acid. Expression of the same recombinant pathway in Saccharomyces cerevisiae resulted in the accumulation of both 5-hydroxyflavanone and 5-deoxyflavanone, with the yields of the later lower than that achieved in E. coli. Other phenylpropanoid acid precursors, such as cinnamic acid and caffeic acid could also be metabolized through the recombinant pathway, yielding corresponding 5-deoxyflavanone compounds. The construction of such recombinant strains for 5-deoxyflavanone biosynthesis offers an alternative way to biochemically characterize flavonoid biosynthetic enzymes and promising production platforms for the biosynthesis of such high-value natural products.     

Transgenic mimicry of pathogen attack stimulates growth and secondary metabolite accumulation

Plant secondary metabolites, including pharmaceuticals, flavorings and aromas, are often produced in response to stress. We used chemical inducers of the pathogen defense response (jasmonic acid, salicylate, killed fungi, oligosaccharides and the fungal elicitor protein, cryptogein) to increase metabolite and biomass production in transformed root cultures of the medicinal plant, Withania somnifera, and the weed, Convolvulus sepium. In an effort to genetically mimic the observed effects of cryptogein, we employed Agrobacterium rhizogenes to insert a synthetic gene encoding cryptogein into the roots of C. sepium, W. somnifera and Tylophora tanakae. This genetic transformation was associated with stimulation in both secondary metabolite production and growth in the first two species, and in growth in the third. In whole plants of Convolvulus arvensis and Arabidopsis thaliana, transformation with the cryptogein gene led, respectively, to increases in the calystegines and certain flavonoids. A similar transgenic mimicry of pathogen attack was previously employed to stimulate resistance to the pathogen and abiotic stress. In the present study of biochemical phenotype, we show that transgenic mimicry is correlated with increased secondary metabolite production in transformed root cultures and whole plants. We propose that natural transformation with genes encoding the production of microbial elicitors could influence interactions between plants and other organisms.     

Perturbation of the lipid phase of a membrane is not involved in the modulation of MRP1 transport activity by flavonoids

The expression of transmembrane transporter multidrug resistance-associated protein 1 (MRP1) confers the multidrug-resistant phenotype (MDR) on cancer cells. Since the activity of the other MDR transporter, P-glycoprotein, is sensitive to membrane perturbation, we aimed to check whether the changes in lipid bilayer properties induced by flavones (apigenin, acacetin) and flavonols (morin, myricetin) were related to their MRP1 inhibitory activity. All the flavonoids inhibited the efflux of MRP1 fluorescent substrate from human erythrocytes and breast cancer cells. Morin was also found to stimulate the ATPase activity of erythrocyte ghosts. All flavonoids intercalated into phosphatidylcholine bilayers as judged by differential scanning calorimetry and fluorescence spectroscopy with the use of two carbocyanine dyes. The model of an intramembrane localization for flavones and flavonols was proposed. No clear relationship was found between the membrane-perturbing activity of flavonoids and their potency to inhibit MRP1. We concluded that mechanisms other than perturbation of the lipid phase of membranes were responsible for inhibition of MRP1 by the flavonoids.     

Repair of UVB-Damaged Skin by the Antioxidant Sulphated Flavone Glycoside Thalassiolin B Isolated from the Marine Plant Thalassia testudinum Banks ex König

Daily topical application of the aqueous ethanolic extract of the marine sea grass, Thalassia testudinum, on mice skin exposed to UVB radiation resulted in a dose-dependent recovery of the skin macroscopic alterations over a 6-day period. Maximal effect (90%) occurred at a dose of 240 μg/cm², with no additional effects at higher doses. Bioassay-guided fractionation of the plant extract resulted in the isolation of thalassiolin B (1). Topical application of 1 (240 μg/cm²) markedly reduces skin UVB-induced damage. In addition, thalassiolin B scavenged 2,2-diphenyl-2-picrylhydrazyl radical with an EC₅₀ = 100 μg/ml. These results suggest that thalassiolin B is responsible for the skin-regenerating effects of the crude extract of T. testudinum. Erik L. Regalado and María Rodríguez have contributed equally to this work and should be considered as first authors.     

In vitro analysis of iron chelating activity of flavonoids

Flavonoids have been demonstrated to possess miscellaneous health benefits which are, at least partly, associated with iron chelation. In this in vitro study, 26 flavonoids from different subclasses were analyzed for their iron chelating activity and stability of the formed complexes in four patho/physiologically relevant pH conditions (4.5, 5.5, 6.8, and 7.5) and compared with clinically used iron chelator deferoxamine. The study demonstrated that the most effective iron binding site of flavonoids represents 6,7-dihydroxy structure. This site is incorporated in baicalein structure which formed, similarly to deferoxamine, the complexes with iron in the stoichiometry 1:1 and was not inferior in all tested pH to deferoxamine. The 3-hydroxy-4-keto conformation together with 2,3-double bond and the catecholic B ring were associated with a substantial iron chelation although the latter did not play an essential role at more acidic conditions. In agreement, quercetin and myricetin possessing all three structural requirements were similarly active to baicalein or deferoxamine at the neutral conditions, but were clearly less active in lower pH. The 5-hydroxy-4-keto site was less efficient and the complexes of iron in this site were not stable at the acidic conditions. Isolated keto, hydroxyl, methoxyl groups or an ortho methoxy-hydroxy groups were not associated with iron chelation at all.