Cyanogenesis in the Australian tropical rainforest endemic Brombya platynema (Rutaceae): chemical characterisation and polymorphism

Divergent responses of above- and below-ground chemical defence to nitrogen and phosphorus supply in waratahs (Telopea speciosissima)

Plant nutrition can affect the allocation of resources to plant chemical defences, yet little is known about how phosphorus (P) supply, and relative nitrogen (N) and P supply, affect chemical defences, especially in species with intrinsically conservative nutrient use adapted to P-impoverished soils. Waratah (Telopea speciosissima (Sm.) R.Br.), like other Proteaceae, is adapted nutrient-poor soils. It was identified as having cyanogenic glycosides (CNglycs) throughout the plant. T. speciosissima seedlings were grown for 15 weeks under two N and P concentrations. CNglycs (N-based defence) and nutrients were quantified in above- and below-ground organs; foliar carbon (C)-based phenolics and tannins were also quantified. CNglyc concentrations in roots were on average 51-fold higher than in above-ground tissues and were affected by both N and P supply, whereas foliar CNglyc concentrations only responded to N supply. Leaves had high concentrations of C-based defences, which increased under low N, and were not correlated with N-based defences. Greater root chemical defence against herbivores and pathogens may be important in a non-mycorrhizal species that relies on basal resprouting following disturbance. The differing responses of secondary chemistry in above- and below-ground organs to P and N demonstrate the importance of broadening the predominantly foliar focus of plant defence studies.

Age versus stage: does ontogeny modify the effect of phosphorus and arbuscular mycorrhizas on above- and below-ground defence in forage sorghum?

Arbuscular mycorrhizas (AM) can increase plant acquisition of P and N. No published studies have investigated the impact of P and AM on the allocation of N to the plant defence, cyanogenic glucosides. We investigated the effects of soil P and AM on cyanogenic glucoside (dhurrin) concentration in roots and shoots of two forage sorghum lines differing in cyanogenic potential (HCNp). Two harvest times allowed plants grown at high and low P to be compared at the same age and the same size, to take account of known ontogenetic changes in shoot HCNp. P responses were dependent on ontogeny and tissue type. At the same age, P-limited plants were smaller and had higher shoot HCNp but lower root HCNp. Ontogenetically controlled comparisons showed a P effect of lesser magnitude, and that there was also an increase in the allocation of N to dhurrin in shoots of P-limited plants. Colonization by AM had little effect on shoot HCNp, but increased root HCNp and the allocation of N to dhurrin in roots. Divergent responses of roots and shoots to P, AM and with ontogeny demonstrate the importance of broadening the predominantly foliar focus of plant defence studies/theory, and of ontogenetically controlled comparisons.

Cyanogenic glycosides: synthesis, physiology, and phenotypic plasticity

Cyanogenic glycosides (CNglcs) are bioactive plant products derived from amino acids. Structurally, these specialized plant compounds are characterized as α-hydroxynitriles (cyanohydrins) that are stabilized by glucosylation. In recent years, improved tools within analytical chemistry have greatly increased the number of known CNglcs by enabling the discovery of less abundant CNglcs formed by additional hydroxylation, glycosylation, and acylation reactions. Cyanogenesis--the release of toxic hydrogen cyanide from endogenous CNglcs--is an effective defense against generalist herbivores but less effective against fungal pathogens. In the course of evolution, CNglcs have acquired additional roles to improve plant plasticity, i.e., establishment, robustness, and viability in response to environmental challenges. CNglc concentration is usually higher in young plants, when nitrogen is in ready supply, or when growth is constrained by nonoptimal growth conditions. Efforts are under way to engineer CNglcs into some crops as a pest control measure, whereas in other crops efforts are directed toward their removal to improve food safety. Given that many food crops are cyanogenic, it is important to understand the molecular mechanisms regulating cyanogenesis so that the impact of future environmental challenges can be anticipated.

Reports on the distribution of aromatic cyanogenic glycosides in Australian tropical rainforest tree species of the Lauraceae and Sapindaceae

The aromatic cyanogenic glycosides taxiphyllin [(R)-4-hydroxymandelonitrile β-D-glucoside] and prunasin [(R)-mandelonitrile β-D-glucoside] were identified as the main cyanogenic compounds in tissues of Australian endemic tropical rainforest tree taxa in the Lauraceae and Sapindaceae families, respectively. The tyrosine-derived taxiphyllin was the main cyanogenic glycoside in foliage of Beilschmiedia collina. This is the first reported cyanogenic compound from the Lauraceae. In addition, substantial quantitative variation in the capacity for cyanogenesis was detected in leaves from 40 individuals, with taxiphyllin concentrations ranging from 23 to 1263 μg CN g(-1) dry wt. No acyanogenic individuals were detected. Concentrations of taxiphyllin were, on average, 2.2-fold greater in young leaves than in old leaves. Prunasin was the dominant cyanogenic compound in tissues of Mischocarpus grandissimus (leaves) and Mischocarpus exangulatus (leaves and seed capsule). Better known for cyanolipids in seed oils, this is the first time a phenylalanine-derived cyanogenic glycoside has been reported in the Sapindaceae. The concentrations of prunasin varied widely, over an order of magnitude, among individuals and different tissue types in these species, with the higher concentrations found in seed capsules and young leaves.

Frequency of cyanogenesis in tropical rainforests of far north Queensland, Australia

BACKGROUND AND AIMS

Plant cyanogenesis is the release of toxic cyanide from endogenous cyanide-containing compounds, typically cyanogenic glycosides. Despite a large body of phytochemical, taxonomic and ecological work on cyanogenic species, little is known of their frequency in natural plant communities. This study aimed to investigate the frequency of cyanogenesis in Australian tropical rainforests. Secondary aims were to quantify the cyanogenic glycoside content of tissues, to investigate intra-plant and intra-population variation in cyanogenic glycoside concentration and to appraise the potential chemotaxonomic significance of any findings in relation to the distribution of cyanogenesis in related taxa.

METHODS

All species in six 200 m(2) plots at each of five sites across lowland, upland and highland tropical rainforest were screened for cyanogenesis using Feigl-Anger indicator papers. The concentrations of cyanogenic glycosides were accurately determined for all cyanogenic individuals.

KEY RESULTS

Over 400 species from 87 plant families were screened. Overall, 18 species (4.5 %) were cyanogenic, accounting for 7.3 % of total stem basal area. Cyanogenesis has not previously been reported for 17 of the 18 species, 13 of which are endemic to Australia. Several species belong to plant families or orders in which cyanogenesis has been little reported, if at all (e.g. Elaeocarpaceae, Myrsinaceae, Araliaceae and Lamiaceae). A number of species contained concentrations of cyanogenic glycosides among the highest ever reported for mature leaves-up to 5.2 mg CN g(-1) d. wt, for example, in leaves of Elaeocarpus sericopetalus. There was significant variation in cyanogenic glycoside concentration within individuals; young leaves and reproductive tissues typically had higher cyanogen content. In addition, there was substantial variation in cyanogenic glycoside content within populations of single species.

CONCLUSIONS

This study expands the limited knowledge of the frequency of cyanogenesis in natural plant communities, includes novel reports of cyanogenesis among a range of taxa and characterizes patterns in intra-plant and intra-population variation of cyanogensis.

Novel aspects of cyanogenesis in Eucalyptus camphora subsp. humeana

Cyanogenesis is the release of cyanide from certain organisms upon tissue disruption. Tissue disruption, such as that caused by folivory, brings cyanogenic glycosides into contact with catabolic enzymes and toxic HCN is subsequently released. The process provides a measure of defence against generalist herbivores. Within the genus Eucalyptus, several species have been identified as cyanogenic and all of these store cyanide exclusively in the form of the cyanogenic glycoside prunasin. Here we report for the first time cyanogenesis in Eucalyptus camphora subsp. humeana L.A.S. Johnson & K.D. Hill. We found that foliage contains at least five different cyanogenic glycosides, three of which were purified and identified (prunasin, sambunigrin and amygdalin). Two natural populations of E. camphora trees were screened for cyanogenesis, and quantitative polymorphism was measured at both sites. Trees varied in their capacity for cyanogenesis from 0.014 to 0.543 mg CN g^-1 DW in one population and from 0.011 to 0.371 mg CN g^-1 DW in the other. A progeny trial, testing both cyanogenesis and carbon-based defence (namely total phenolics and condensed tannins), was performed with seed sourced from two cyanogenic, open-pollinated maternal trees. Interestingly, the seedlings exhibited markedly lower levels of cyanogenesis and condensed tannins than the adult population, with some individuals completely lacking one or both of the chemical defences. Total phenolic concentrations, however, were significantly higher in the seedlings than in the parental population from which the seed was sourced. Eucalyptus camphora is relatively unique among cyanogenic trees having multiple foliar cyanogenic glycosides and an apparently marked ontogenetic regulation of cyanogenic capacity.