Inhibition of Cyanogenesis by tannins

Role of Synthetic Plant Extracts on the Production of Silver-Derived Nanoparticles

The main antioxidants present in plant extracts-quercetin, β-carotene, gallic acid, ascorbic acid, hydroxybenzoic acid, caffeic acid, catechin and scopoletin-are able to synthesize silver nanoparticles when reacting with a Ag NO3 solution. The UV-visible absorption spectrum recorded with most of the antioxidants shows the characteristic surface plasmon resonance band of silver nanoparticles. Nanoparticles synthesised with ascorbic, hydroxybenzoic, caffeic, and gallic acids and scopoletin are spherical. Nanoparticles synthesised with quercetin are grouped together to form micellar structures. Nanoparticles synthesised by β-carotene, were triangular and polyhedral forms with truncated corners. Pentagonal nanoparticles were synthesized with catechin. We used Fourier-transform infrared spectroscopy to check that the biomolecules coat the synthesised silver nanoparticles. X-ray powder diffractograms showed the presence of silver, AgO, Ag2O, Ag3O4 and Ag2O3. Rod-like structures were obtained with quercetin and gallic acid and cookie-like structures in the nanoparticles obtained with scopoletin, as a consequence of their reactivity with cyanide. This analysis explained the role played by the various agents responsible for the bio-reduction triggered by nanoparticle synthesis in their shape, size and activity. This will facilitate targeted synthesis and the application of biotechnological techniques to optimise the green synthesis of nanoparticles.

Cyanogenesis in animal-plant interactions

When there is variation between individuals of the same plant species for a putative defensive chemical it is possible to prove beyond reasonable doubt that the chemical does indeed have a defence function for the plant. The polymorphism for cyanogenesis in Lotus corniculatus and Trifolium repens provides ample evidence that cyanogenesis has an important role in plant defence. This herbivore-plant interaction is, however, far from simple: the degree of selectivity varies between individuals of the same animal species and is influenced by differences in hunger and the type of food previously consumed. The primary effect of cyanogenesis is at the feeding and not at the digesting stage, although it seems that it is the carbonyl compounds released on cyanogenesis which have the deterrent effect and not hydrogen cyanide itself. Although cyanogenesis is not a particularly effective method of defence, it is a conservative system--the compounds involved are recycled and the plant has to be physically damaged before cyanogenesis occurs. Because there is no pre-damage volatile compound involved, cyanogenesis cannot be used by insects or other animals as a means of finding the plants.

The role of plant secondary metabolites in mammalian herbivory: ecological perspectives

Plant secondary metabolites (PSM) have many ecological functions, but have long been considered as defences against pathogens or herbivores (vertebrate or invertebrate), reducing the likelihood and extent of attack. However, mammalian herbivores ingest many foods containing PSM and use both behavioural methods and physiological strategies to limit their negative effects. Most physiological counter-adaptations are inducible in response to ingested PSM, providing efficient protection against toxic effects. Possible positive effects of PSM include antioxidant and anthelminthic properties and complex formation between protein and condensed tannins that protects dietary protein from degradation by the symbiotic microflora of foregut fermenters, increasing its utilisation by the animal. This protein effect is probably only beneficial to animals under a narrow range of nutrient-rich conditions found mainly in agricultural systems. There are many examples of PSM causing food avoidance or reducing food intake, but there is as yet relatively little evidence for positive selection of them by herbivores. Although the feedback mechanisms relating the post-ingestive consequences of PSM to subsequent foraging behaviour are beginning to be understood, knowledge of the integration of behavioural and physiological strategies for regulating the effects of PSM is relatively poor. The opportunities for learned avoidance of PSM may be restricted in animals with complex diets that cannot associate a particular feedback signal with a given food type. A greater emphasis on the study of subclinical effects of PSM rather than acute effects, on pharmaco-kinetic studies in relation to behavioural studies and on the use of realistic experimental models is advocated.

Cyanogenic glucosides in grapevine: polymorphism, identification and developmental patterns

Twelve grapevine (Vitis vinifera L.) cultivars were surveyed for 'cyanide potential' (i.e. the total cyanide measured in beta-glucosidase-treated crude, boiled tissue extract) in mature leaves. Two related cultivars (Carignan and Ruby Cabernet) had mean cyanide potential (equivalent to 110 mgHCNkg-1fr.wt) ca. 25-fold greater than that of the other 10 cultivars, and so the trait is polymorphic in the species. In boiled leaf extracts of Carignan and Ruby Cabernet, free cyanide constituted a negligible fraction of the total cyanide potential because beta-glucosidase treatment was required to liberate the major cyanide fraction - which is therefore bound in glucosylated cyanogenic compound(s) (or cyanogenic glucosides). In addition, cyanide was liberated from ground leaf tissue of Ruby Cabernet but not Sultana (a cultivar with low cyanide potential). Hence, the high cyanide potential in Ruby Cabernet leaves is coupled with endogenous beta-glucosidase(s) activity and this cultivar may be considered 'cyanogenic'. A method was developed to detect and identify cyanogenic glucosides using liquid chromatography combined with tandem mass spectrometry (LC-MS/MS). Two putative cyanogenic glucosides were found in extracts from leaves of Carignan and Ruby Cabernet and were identified as the epimers prunasin and sambunigrin. Cyanide potential measured at three times over the growing season in young and mature leaves, petioles, tendrils, flowers, berries, seeds and roots of Ruby Cabernet was substantially higher in the leaves compared with all other tissues. This characterisation of cyanogenic glucoside accumulation in grapevine provides a basis for gauging the involvement of the trait in interactions of the species with its pests and pathogens.

Cyanogenic allosides and glucosides from Passiflora edulis and Carica papaya

Leaf and stem material of Passiflora edulis (Passifloraceae) contains the new cyanogenic glycosides (2R)-beta-D-allopyranosyloxy-2-phenylacetonitrile (1a) and (2S)-beta-D-allopyranosyloxy-2-phenylacetonitrile (1b), along with smaller amounts of (2R)-prunasin (2a), sambunigrin (2b), and the alloside of benzyl alcohol (4); the major cyanogens of the fruits are (2R)-prunasin (2a) and (2S)-sambunigrin (2b). The major cyanogenic glycoside of Carica papaya (Caricaceae) is 2a; only small amounts of 2b also are present. We were not able to confirm the presence of a cyclopentenoid cyanogenic glycoside, tetraphyllin B, in Carica papaya leaf and stem materials. In detailed 1H NMR studies of 1a/b and 2a/b, differences in higher order effects in glucosides and allosides proved to be valuable for assignment of structures in this series. The diagnostic chemical shifts of cyanogenic methine and anomeric protons in 1a/b are sensitive to anisotropic environmental effects. The assignment of C-2 stereochemistry of 1a/b was made in analogy to previous assignments in the glucoside series and was supported by GLC analysis of the TMS ethers.

Differences in host use efficiency of larvae of a generalist moth, Operophtera brumata on three chemically divergent Salix species

The food selection, growth, and fecundity of insect herbivores are largely dictated by the chemical composition and nutritive values of plant foliage. We studied the host-use efficiency of larvae of the generalist moth, Operophtera brumata (Lepidoptera: Geometridae) on three chemically divergent but nutritively similar willows (Salix spp.). The 4th instars were able to use the salicylate-free leaves of S. phylicifolia efficiently. Growth was slightly reduced on S. pentandra, which contained a moderate level of acetylated salicylates. The high concentration of salicylates found in the leaves of S. myrsinifolia seemed to provide efficient protection against non-specialized O. brumata. We also studied assimilation of nutrients and degradation of salicylates and other secondary compounds in the digestive tract of O. brumata larvae. Neither the assimilation of nitrogen nor of carbon were affected by secondary chemicals of ingested food. Salicylates were shown to be degraded to salicin and catechol, while further degradation of salicin to saligenin was rather slow. In an artificial diet experiment, we showed that two degradation products of salicylates, catechol and saligenin markedly reduced the growth of the larvae. Neither salicin nor chlorogenic acid affected larval growth. We conclude that salicylates reduced the growth of the generalist winter moth mainly by feeding deterrence caused by 6-hydroxy-2-cyclohexenone and catechol. Compared to the deleterious effects of salicylates the effects of other secondary compounds were minor.

Chemical defense production in Lotus corniculatus L. II. Trade-offs among growth, reproduction and defense

Ecological trade-offs between growth, reproduction and both condensed tannins and cyanogenic glycosides were examined in Lotus corniculatus by correlating shoot (leaves and stem) size and reproductive output with chemical concentrations. We found that cyanide concentration was not related to shoot size, but that condensed tannin concentrations were positively correlated with shoot size; larger plants contained higher tannin concentrations. Both tannin and cyanide concentrations were depressed when plants produced fruits. Defense costs change as plants mature and begin to reproduce. These trade-offs indicate that cost of defense chemical production cannot be predicted merely on the basis of molecular size, composition or concentration.

Chemical defense production in Lotus corniculatus L. I. The effects of nitrogen source on growth, reproduction and defense

The carbon to nitrogen balance theory was examined for a legume, Lotus corniculatus L., which allocates carbon to nitrogen fixation. N-fixation can influence the ratio of carbon to nitrogen in legumes by providing nitrogen in nutrient-poor habitats, and by consuming carbon for support of symbiotic N-fixation. L. corniculatus clones (genotypes) were grown under two levels of nitrogen fertilization: a treatment which suppressed nodulation with fertilization and a treatment which received no additional fertilization. These plants relied solely on symbiotic N-fixation. Plants which supported symbionts had lower biomass and lower tannin concentrations than fertilized plants; this appears to be a result of the large carbon demand on N-fixation. Plants supporting symbionts often had relatively lower protein concentrations than fertilized plants. Cyanide concentration was influenced by plant genotype but not by nitrogen source. Although symbiotic N-fixing plants were smaller, they had three times the reproductive output of fertilized plants.

Relationship between contents of leucoanthocyanidin and dhurrin in sorghum leaves

Flag leaves of 'Colman' forage sorghum (Sorghum bicolor) contain at least 25 times as much leucoanthocyanidin (LAC) and approximately half as much of the cyanogenic glucoside, dhurrin, as do flag leaves of 'White Collier' forage sorghum. Assays of flag leaves from 119 F2 plants and 11 F5 lines from crosses between these two cultivars revealed a statistically significant negative association between levels of LAC and dhurrin. Both LAC and dhurrin are aromatic compounds, and the negative association between the two may be the result of competition for intermediates or products of the aromatic biosynthetic pathway. This rationale appears to be quite different from that for the negative association reported for levels of tannin and cyanide in Lotus corniculatus. Although the negative relationship between LAC and dhurrin in sorghum was statistically significant, the association was not consistent enough to suggest that either trait could be used reliably in selecting or breeding to modify the other trait.