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

Dhurrin: A naturally occurring phytochemical as a weapon against insect herbivores

Dhurrin, a cyanogenic glucoside, is a plant defensive chemical synthesized from aliphatic amino acids and consists of β-d-glucopyranose linked to α-hydroxy nitrile. It is catabolized by the consecutive action of hydroxynitrilase and β-glucosidase to release hydrogen cyanide on tissue disruption during herbivory. These phytoanticipins are widely distributed across various monocot and dicot plants such as Sorghum, Macadamia, Ostrya sp., and many other plant species with ornamental, pharmaceutical, medicinal, and food value. Although repellent properties of dhurrin against herbivores are often reported, less is known about its distribution, metabolism, mode of action against insects, and application for pest control. Herein, we highlight recent updates on dhurrin distribution, biosynthesis, and catabolism along with the cyanide detoxification pathway. Additionally, this article focuses on biological activities of dhurrin against various herbivores and opportunities to explore the utilization of dhurrin as a natural pest control agent and a substitute for chemically synthesized pesticides.

Cyanogenesis in Aralia spinosa (Araliaceae)

A systematic survey of Aralia spinosa (Araliaceae), covering an entire growing season and including aboveground organs at various developmental stages, revealed that only about half of all samples collected showed cyanogenesis. Cyanogenesis was detected in inflorescences and leaves but is apparently restricted to certain harvest times or developmental stages. The structurally unusual triglochinin, characterized by a hex-2-enedioic acid partial structure, was the only cyanogenic glycoside detected. This is the first description of triglochinin in this species and in the family of Araliaceae. Triglochinin is biogenetically derived from tyrosine, which is in good agreement with the few cyanogenic glycosides previously detected in members of the Araliaceae family. Triglochinin was identified, characterized, and quantified by modern chromatographic methods, and the amount of enzymatically releasable hydrocyanic acid was determined qualitatively and quantitatively. Two isomers of triglochinin were detected chromatographically at minor levels. The isomeric pattern agreed well with literature data from other triglochinin-containing plants. This was confirmed in the two species, Triglochin maritima and Thalictrum aquilegiifolium, which were comparatively studied. In the case of A. spinosa, inflorescence buds harvested in July showed the highest content of triglochinin, just under 0.2% on a dry weight basis. The detection of triglochinin adds to the knowledge of toxicological properties and the dereplication of U(H)PLC/MS² data provides a comprehensive phytochemical profile of A. spinosa.

Plant Secondary Metabolites Produced in Response to Abiotic Stresses Has Potential Application in Pharmaceutical Product Development

Plant secondary metabolites (PSMs) are vital for human health and constitute the skeletal framework of many pharmaceutical drugs. Indeed, more than 25% of the existing drugs belong to PSMs. One of the continuing challenges for drug discovery and pharmaceutical industries is gaining access to natural products, including medicinal plants. This bottleneck is heightened for endangered species prohibited for large sample collection, even if they show biological hits. While cultivating the pharmaceutically interesting plant species may be a solution, it is not always possible to grow the organism outside its natural habitat. Plants affected by abiotic stress present a potential alternative source for drug discovery. In order to overcome abiotic environmental stressors, plants may mount a defense response by producing a diversity of PSMs to avoid cells and tissue damage. Plants either synthesize new chemicals or increase the concentration (in most instances) of existing chemicals, including the prominent bioactive lead compounds morphine, camptothecin, catharanthine, epicatechin-3-gallate (EGCG), quercetin, resveratrol, and kaempferol. Most PSMs produced under various abiotic stress conditions are plant defense chemicals and are functionally anti-inflammatory and antioxidative. The major PSM groups are terpenoids, followed by alkaloids and phenolic compounds. We have searched the literature on plants affected by abiotic stress (primarily studied in the simulated growth conditions) and their PSMs (including pharmacological activities) from PubMed, Scopus, MEDLINE Ovid, Google Scholar, Databases, and journal websites. We used search keywords: "stress-affected plants," "plant secondary metabolites, "abiotic stress," "climatic influence," "pharmacological activities," "bioactive compounds," "drug discovery," and "medicinal plants" and retrieved published literature between 1973 to 2021. This review provides an overview of variation in bioactive phytochemical production in plants under various abiotic stress and their potential in the biodiscovery of therapeutic drugs. We excluded studies on the effects of biotic stress on PSMs.

Nitrogen availability and allocation in sorghum and its wild relatives: Divergent roles for cyanogenic glucosides

Crop plants are assumed to have become more susceptible to pests as a result of selection for high growth rates during the process of domestication, consistent with resource allocation theories. We compared the investment by domesticated sorghum into cyanogenic glucosides, nitrogen-based specialised metabolites that break down to release hydrogen cyanide, with five wild relatives native to Australia. Plants were grown in pots in a greenhouse and supplied with low and high concentrations of nitrogen and monitored for 9 weeks. The concentrations of nitrate, total phenolics and silicon were also measured. Domesticated Sorghum bicolor had the highest leaf and root cyanogenic glucoside concentrations, and among the lowest nitrate and silicon concentrations under both treatments. Despite partitioning a much higher proportion of its stored nitrogen to cyanogenic glucosides than the wild species, S. bicolor's nitrogen productivity levels were among the highest. Most of the wild sorghums had higher concentrations of silicon and phenolics, which may provide an alternative defence system. Cyanogenic glucosides appear to be integral to S. bicolor's physiology, having roles in both growth and defence. Sorghum macrospermum displayed consistently low cyanogenic glucoside concentrations, high growth rates and high nitrogen productivity and represents a particularly attractive genetic resource for sorghum improvement.

Unique and highly specific cyanogenic glycoside localization in stigmatic cells and pollen in the genus Lomatia (Proteaceae)

BACKGROUND AND AIMS

Floral chemical defence strategies remain understudied despite the significance of flowers to plant fitness, and the fact that many flowers contain secondary metabolites that confer resistance to herbivores. Optimal defence and apparency theories predict that the most apparent plant parts and/or those most important to fitness should be most defended. To test whether within-flower distributions of chemical defence are consistent with these theories we used cyanogenic glycosides (CNglycs), which are constitutive defence metabolites that deter herbivores by releasing hydrogen cyanide upon hydrolysis.

METHODS

We used cyanogenic florets of the genus Lomatia to investigate at what scale there may be strategic allocation of CNglycs in flowers, what their localization reveals about function, and whether levels of floral CNglycs differ between eight congeneric species across a climatic gradient. Within-flower distributions of CNglycs during development were quantified, CNglycs were identified and their localization was visualized in cryosectioned florets using matrix-assisted laser desorption ionization mass spectrometry imaging (MALDI-MSI).

KEY RESULTS

Florets of all congeneric species studied were cyanogenic, and concentrations differed between species. Within florets there was substantial variation in CNglyc concentrations, with extremely high concentrations (up to 14.6 mg CN g-1 d. wt) in pollen and loose, specialized surface cells on the pollen presenter, among the highest concentrations reported in plant tissues. Two tyrosine-derived CNglycs, the monoglycoside dhurrin and diglycoside proteacin, were identified. MALDI-MSI revealed their varying ratios in different floral tissues; proteacin was primarily localized to anthers and ovules, and dhurrin to specialized cells on the pollen presenter. The mix of transient specialized cells and pollen of L. fraxinifolia was ~11 % dhurrin and ~1.1 % proteacin by mass.

CONCLUSIONS

Tissue-specific distributions of two CNglycs and substantial variation in their concentrations within florets suggests their allocation is under strong selection. Localized, high CNglyc concentrations in transient cells challenge the predictions of defence theories, and highlight the importance of fine-scale metabolite visualization, and the need for further investigation into the ecological and metabolic roles of CNglycs in floral tissues.

Exogenous indole acetic acid alleviates Cd toxicity in tea (Camellia sinensis)

Cadmium (Cd), a toxic heavy metal, restrains the growth and development of plants and threatens global food safety. Many studies on the alleviation of heavy metal toxicity by exogenous phytohormones have emerged, but reports on tea (Camellia sinensis) are still scarce. In this study, the effects of indole acetic acid (IAA) (2 μM and 10 μM) on Cd uptake and on the physiological and biochemical characteristics of the 'Xiangfeicui' tea cultivar were investigated for the first time. The order of Cd accumulation in tea seedlings was root > stem > mature leaf > tender leaf. Under Cd stress (30 mg kg^-1), photosynthetic pigment levels, antioxidant enzyme activity, root vigor, root IAA content, and the levels of most metabolites (including caffeine, soluble sugar, total amino acids, some amino acid components, and most catechins) were significantly reduced, while levels of malondialdehyde, proline, epicatechin, and some amino acids increased. We therefore propose that by reducing Cd accumulation, exogenous IAA can lessen the adverse effects of Cd on the physiology and biochemistry of tea seedlings, promoting the growth of healthier tea plants.

Cyanogenesis in Macadamia and Direct Analysis of Hydrogen Cyanide in Macadamia Flowers, Leaves, Husks, and Nuts Using Selected Ion Flow Tube-Mass Spectrometry

Macadamia has increasing commercial importance in the food, cosmetics, and pharmaceutical industries. However, the toxic compound hydrogen cyanide (HCN) released from the hydrolysis of cyanogenic compounds in Macadamia causes a safety risk. In this study, optimum conditions for the maximum release of HCN from Macadamia were evaluated. Direct headspace analysis of HCN above Macadamia plant parts (flower, leaves, nuts, and husks) was carried out using selected ion flow tube-mass spectrometry (SIFT-MS). The cyanogenic glycoside dhurrin and total cyanide in the extracts were analyzed using HPLC-MS and UV-vis spectrophotometer, respectively. HCN released in the headspace was at a maximum when Macadamia samples were treated with pH 7 buffer solution and heated at 50 °C for 60 min. Correspondingly, treatment of Macadamia samples under these conditions resulted in 93%-100% removal of dhurrin and 81%-91% removal of total cyanide in the sample extracts. Hydrolysis of cyanogenic glucosides followed a first-order reaction with respect to HCN production where cyanogenesis is principally induced by pH changes initiating enzymatic hydrolysis rather than thermally induced reactions. The effective processing of different Macadamia plant parts is important and beneficial for the safe production and utilization of Macadamia-based products.

Does foliage metal accumulation influence plant-insect interactions? A field study of two sympatric tree metallophytes

Gossia (Myrtaceae) is a highly restricted tree genus most speciose in New Caledonia and eastern Australia. The latter group accumulates above-normal foliar manganese (Mn) concentrations, with some individuals exhibiting the rare Mn-hyperaccumulative trait. Whether foliar metals contribute to chemical defence has been addressed via numerous feeding experiments and very few field studies. This investigation exploited specifically different insect activities on the foliage of sympatric Gossia grayi (N.Snow & Guymer) and Gossia shepherdii (F.Muell.) N.Snow & Guymer, endemic to north-eastern Australia, to test for direct and indirect effects of foliar Mn enrichment on plant-insect interactions. Leaf organic and inorganic chemistries, specific weight, surface damage, gall infestation and occupancy were quantified. Discovery that both species are Mn hyperaccumulators augments the world listing by 5-7%. Highly elevated gall-Mn concentrations coupled with negligible gall parasitisation suggested chemical fortification and adaptation by the host insect - a Cecidomyiidae fly. Linear mixed modelling (LMM) showed differences in leaf Mn, phenolics, toughness and surface damage across tree species and leaf age. There was no direct relationship between leaf Mn and insect impact. However, LMM did resolve indirect effects, i.e. between insect impact and certain foliar elements, consistent with nutritional dynamics in a physiologically novel plant system where Mn is vastly overaccumulated.

Tyrosine biosynthesis, metabolism, and catabolism in plants

L-Tyrosine (Tyr) is an aromatic amino acid (AAA) required for protein synthesis in all organisms, but synthesized de novo only in plants and microorganisms. In plants, Tyr also serves as a precursor of numerous specialized metabolites that have diverse physiological roles as electron carriers, antioxidants, attractants, and defense compounds. Some of these Tyr-derived plant natural products are also used in human medicine and nutrition (e.g. morphine and vitamin E). While the Tyr biosynthesis and catabolic pathways have been extensively studied in microbes and animals, respectively, those of plants have received much less attention until recently. Accumulating evidence suggest that the Tyr biosynthetic pathways differ between microbes and plants and even within the plant kingdom, likely to support the production of lineage-specific plant specialized metabolites derived from Tyr. The interspecies variations of plant Tyr pathway enzymes can now be used to enhance the production of Tyr and Tyr-derived compounds in plants and other synthetic biology platforms.

Genome and transcriptome sequencing characterises the gene space of Macadamia integrifolia (Proteaceae)

BACKGROUND

The large Gondwanan plant family Proteaceae is an early-diverging eudicot lineage renowned for its morphological, taxonomic and ecological diversity. Macadamia is the most economically important Proteaceae crop and represents an ancient rainforest-restricted lineage. The family is a focus for studies of adaptive radiation due to remarkable species diversification in Mediterranean-climate biodiversity hotspots, and numerous evolutionary transitions between biomes. Despite a long history of research, comparative analyses in the Proteaceae and macadamia breeding programs are restricted by a paucity of genetic information. To address this, we sequenced the genome and transcriptome of the widely grown Macadamia integrifolia cultivar 741.

RESULTS

Over 95 gigabases of DNA and RNA-seq sequence data were de novo assembled and annotated. The draft assembly has a total length of 518 Mb and spans approximately 79% of the estimated genome size. Following annotation, 35,337 protein-coding genes were predicted of which over 90% were expressed in at least one of the leaf, shoot or flower tissues examined. Gene family comparisons with five other eudicot species revealed 13,689 clusters containing macadamia genes and 1005 macadamia-specific clusters, and provides evidence for linage-specific expansion of gene families involved in pathogen recognition, plant defense and monoterpene synthesis. Cyanogenesis is an important defense strategy in the Proteaceae, and a detailed analysis of macadamia gene homologues potentially involved in cyanogenic glycoside biosynthesis revealed several highly expressed candidate genes.

CONCLUSIONS

The gene space of macadamia provides a foundation for comparative genomics, gene discovery and the acceleration of molecular-assisted breeding. This study presents the first available genomic resources for the large basal eudicot family Proteaceae, access to most macadamia genes and opportunities to uncover the genetic basis of traits of importance for adaptation and crop improvement.

Lotus japonicus flowers are defended by a cyanogenic β-glucosidase with highly restricted expression to essential reproductive organs

Flowers and leaves of Lotus japonicus contain α-, β-, and γ-hydroxynitrile glucoside (HNG) defense compounds, which are bioactivated by β-glucosidase enzymes (BGDs). The α-HNGs are referred to as cyanogenic glucosides because their hydrolysis upon tissue disruption leads to release of toxic hydrogen cyanide gas, which can deter herbivore feeding. BGD2 and BGD4 are HNG metabolizing BGD enzymes expressed in leaves. Only BGD2 is able to hydrolyse the α-HNGs. Loss of function mutants of BGD2 are acyanogenic in leaves but fully retain cyanogenesis in flowers pointing to the existence of an alternative cyanogenic BGD in flowers. This enzyme, named BGD3, is identified and characterized in this study. Whereas all floral tissues contain α-HNGs, only those tissues in which BGD3 is expressed, the keel and the enclosed reproductive organs, are cyanogenic. Biochemical analysis, active site architecture molecular modelling, and the observation that L. japonicus accessions lacking cyanogenic flowers contain a non-functional BGD3 gene, all support the key role of BGD3 in floral cyanogenesis. The nectar of L. japonicus flowers was also found to contain HNGs and additionally their diglycosides. The observed specialisation in HNG based defence in L. japonicus flowers is discussed in the context of balancing the attraction of pollinators with the protection of reproductive structures against herbivores.

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.

The rare cyanogen proteacin, and dhurrin, from foliage of Polyscias australiana, a tropical Araliaceae

The tyrosine-derived cyanogenic di-glucoside proteacin and related mono-glucoside dhurrin were identified as the cyanogens in foliage of the tropical tree species Polyscias australiana, present in the approximate ratio 9:1. To date cyanogenic glycosides have not been characterised from the Araliaceae or the Apiales. Concentrations of cyanogenic glycosides varied significantly between plant parts and with leaf age, with the highest concentrations in young emerging leaves (mean 2217.1 μg CN g(-1) dry wt), petioles (rachis; 1487.1 μg CN g(-1) dry wt) and floral buds (265.8 μg CN g(-1) dry wt). Between 2% and 10% of nitrogen in emerging leaves and petioles was present as cyanogenic glycosides. With the exception of floral buds, all tissues apparently lack a specific cyanogenic β-glucosidase to catalyse the first step in the breakdown of these cyanogenic glycosides. Only with the addition of emulsin, an exogenous non-specific β-glucosidase from almonds, were high concentrations of cyanogenic glycosides detected, as much as 20-fold greater than the low to negligible cyanogenic glycoside concentrations determined in the absence of exogenous enzyme. High concentrations of cyanogens in young tissues may confer protection, but may also be a nitrogen source during leaf expansion.

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.

A combined biochemical screen and TILLING approach identifies mutations in Sorghum bicolor L. Moench resulting in acyanogenic forage production

Cyanogenic glucosides are present in several crop plants and can pose a significant problem for human and animal consumption, because of their ability to release toxic hydrogen cyanide. Sorghum bicolor L. contains the cyanogenic glucoside dhurrin. A qualitative biochemical screen of the M2 population derived from EMS treatment of sorghum seeds, followed by the reverse genetic technique of Targeted Induced Local Lesions in Genomes (TILLING), was employed to identify mutants with altered hydrogen cyanide potential (HCNp). Characterization of these plants identified mutations affecting the function or expression of dhurrin biosynthesis enzymes, and the ability of plants to catabolise dhurrin. The main focus in this study is on acyanogenic or low cyanide releasing lines that contain mutations in CYP79A1, the cytochrome P450 enzyme catalysing the first committed step in dhurrin synthesis. Molecular modelling supports the measured effects on CYP79A1 activity in the mutant lines. Plants harbouring a P414L mutation in CYP79A1 are acyanogenic when homozygous for this mutation and are phenotypically normal, except for slightly slower growth at early seedling stage. Detailed biochemical analyses demonstrate that the enzyme is present in wild-type amounts but is catalytically inactive. Additional mutants capable of producing dhurrin at normal levels in young seedlings but with negligible leaf dhurrin levels in mature plants were also identified. No mutations were detected in the coding sequence of dhurrin biosynthetic genes in this second group of mutants, which are as tall or taller, and leafier than nonmutated lines. These sorghum mutants with reduced or negligible dhurrin content may be ideally suited for forage production.

The limit to the distribution of a rainforest marsupial folivore is consistent with the thermal intolerance hypothesis

Models of impacts of climate change on species are generally based on correlations between current distributions and climatic variables, rather than a detailed understanding of the mechanisms that actually limit distribution. Many of the vertebrates endemic to rainforests of northeastern Australia are restricted to upland forests and considered to be threatened by climate change. However, for most of these species, the factors controlling their distributions are unknown. We examined the role of thermal intolerance as a possible mechanism limiting the distribution of Pseudochirops archeri (green ringtail possum), a specialist arboreal folivore restricted to rainforests above an altitude of 300 m in Australia's Wet Tropics. We measured short-term metabolic responses to a range of ambient temperatures, and found that P. archeri stores heat when ambient temperatures exceed 30°C, reducing water requirements for evaporative cooling. Due to the rate at which body temperature increases with ambient temperatures >30°C, this strategy is not effective over periods longer than 5 h. We hypothesise that the distribution of P. archeri is limited by interactions between (i) the duration and severity of extreme ambient temperatures (over 30°C), (ii) the scarcity of free water in the rainforest canopy in the dry season, and (iii) constraints on water intake from foliage imposed by plant secondary metabolites and fibre. We predict that dehydration becomes limiting for P. archeri where extreme ambient temperatures (>30°C) persist for more than 5 h per day over 4-6 days or more. Consistent with our hypothesis, the abundance of P. archeri in the field is correlated with the occurrence of extreme temperatures, declining markedly at sites where the average maximum temperature of the warmest week of the year is above 30°C. Assuming the mechanism of limitation is based on extreme temperatures, we expect impacts of climate change on P. archeri to occur in discrete, rapid events rather than as a slow contraction in range.

In vitro anti-influenza screening of several Euphorbiaceae species: structure of a bioactive Cyanoglucoside from Codiaeum variegatum

A bio-guided screening against influenza A virus (FLUAV) was carried out with seven Euphorbiaceae species. The results showed that chromatographic fractions from Phyllantus niruri, Euphorbia pulcherrima and Codiaeum variegatum had relevant anti-FLUAV activity, although only chromatographical subfractions from C. variegatum kept the activity. From this plant, the active compound against FLUAV was isolated. Its structure was assigned as 2-(3,4,5)-trihydroxy-6-hydroxymethyltetrahydropyran-2-yloxymethyl)acrylonitrile (1) on the basis of NMR, mass spectrometry and X-ray diffraction analysis. The compound displayed virucidal activity without impairment of haemagglutination properties of the used virus strain. This is the first report indicating antiviral activity of a cyanoglucoside.

Resource availability and the abundance of an N-based defense in Australian tropical rain forests

Plant defense theories predict that relatively resource-rich environments (those with more fertile soil) will support a greater abundance of plants with nitrogen-based chemical defense, but this has yet to be adequately tested. We tested this prediction by measuring the diversity and contribution to total biomass of cyanogenic plants (those that release hydrogen cyanide from endogenous cyanide-containing compounds) in the Australian tropical rain forest. We examined 401 species in thirty 200-m2 plots, six at each of five sites, for cyanogenesis. In upland/highland rain forest, two pairs of sites similar in rainfall and altitude, but differing in soil nutrients, were selected, as well as one site in lowland rain forest. Sites differed markedly in species composition and foliar N was positively related to soil fertility. Holding altitude constant, we did not detect significant differences in the proportion of cyanogenic species with soil fertility, nor did we consistently detect significant increases in the contribution of cyanogenic species to total biomass on higher nutrient sites. Thus we found no clear evidence that soil fertility affects the distribution and prevalence of species investing in a constitutive N-based defense at the community level.

Frequency and distribution of cyanogenic glycosides in Eucalyptus L'Hérit

In this study approximately 420 of the described species of Eucalyptus were examined for cyanogenesis. Our work has identified an additional 18 cyanogenic species, 12 from living tissues and a further six from herbarium samples. This brings the total of known cyanogenic species to 23, representing approximately 4% of the genus. The taxonomic distribution of the species within the genus is restricted to the subgenus Symphyomyrtus, with only two exceptions. Within Symphyomyrtus, the species are in three closely related sections. The cyanogenic glycoside was found to be predominantly prunasin (1) in the 11 species where this was examined. We conclude that cyanogenesis is plesiomorphic in Symphyomyrtus (i.e. a common basal trait) but has probably arisen independently in the other two subgenera, consistent with recent phylogenetic treatments of the genus. The results of this study have important implications for the selection of trees for plantations to support wildlife, and to preserve genetic diversity.

South American leaf blight of the rubber tree (Hevea spp.): new steps in plant domestication using physiological features and molecular markers

BACKGROUND

Rubber trees (Hevea spp.) are perennial crops of Amazonian origin that have been spread over the whole tropical belt to guarantee worldwide production of natural rubber. This crop plant has found its place in many national economies of producing countries, and although its domestication by selection of suitable genotypes was very slow, it contributes a lot to the welfare of small farmers worldwide. Its development is limited by severe diseases. In South America, the main fungal disease of rubber trees is the South American leaf blight (SALB) caused by the ascomycete Microcyclus ulei. This fungus inhibits natural rubber production on a commercial scale in South and Central America.

SCOPE

The disease is still restricted to its continent of origin, but its potential to be distributed around the world rises with every transcontinental airline connection that directly links tropical regions. The need to develop control measures against the disease is an urgent task and must be carried out on an international scale. All control efforts so far taken since 1910 have ended in a miserable failure. Even the use of modern systemic fungicides and use of greatly improved application techniques have failed to prevent large losses and dieback of trees. The results of research dealing with both the disease and the pathosystem over more than 50 years are summarized and placed into perspective.

FUTURE PROSPECTS

A detailed knowledge of this host-pathogen combination requires understanding of the dynamics of Hevea leaf development, the biochemical potential for cyanide liberation, and molecular data for several types of resistance factors. Resolution of the Hevea-SALB problem may serve as a model for future host-pathogen studies of perennial plants requiring a holistic approach.

Constitutive polymorphic cyanogenesis in the Australian rainforest tree, Ryparosa kurrangii (Achariaceae)

Cyanogenesis, the liberation of volatile hydrogen cyanide from endogenous cyanide-containing compounds, is a proven plant defence mechanism and the particular cyanogens involved have taxonomic utility. The cyclopentenoncyanhydrin glycoside gynocardin was the only cyanogen isolated from foliar tissue of the rare Australian rainforest tree, Ryparosa kurrangii (Achariaceae). Mechanical damage simulating foliar herbivory did not induce a significant increase in the expression of cyanogenesis over a 24h period, indicating cyanogenic herbivore defence in R. kurrangii is constitutive. The cyanogenic potential of mature leaves was quantitatively polymorphic between trees in a natural population, ranging from 0.54 to 4.77 mg CN g(-1) dry wt leaf tissue.

Variation in defence strategies in two species of the genus Beilschmiedia under differing soil nutrient and rainfall conditions

The relationships between various leaf functional traits that are important in plant growth (e.g., specific leaf area) have been investigated in recent studies; however, research in this context on plants that are highly protected by chemical defences, particularly resource-demanding nitrogen-based defence, is lacking. We collected leaves from cyanogenic (N-defended) Beilschmiedia collina B. Hyland and acyanogenic (C-defended) Beilschmiedia tooram (F. M. Bailey) B. Hyland at high- and low-soil nutrient sites in two consecutive years that varied significantly in rainfall. We then measured the relationships between chemical defence and morphological and functional leaf traits under the different environmental conditions. We found that the two species differed significantly in their resource allocation to defence as well as leaf morphology and function. The N defended species had a higher leaf nitrogen concentration, whereas the C-defended species had higher amounts of C-based chemical defences (i.e., total phenolics and condensed tannins). The C-defended species also tended to have higher force to fracture and increased leaf toughness. In B. collina, cyanogenic glycoside concentration was higher with higher rainfall, but not with higher soil nutrients. Total phenolic concentration was higher at the high soil nutrient site in B. tooram, but lower in B. collina; however, with higher rainfall an increase was found in B. tooram, while phenolics decreased in B. collina. Condensed tannin concentration decreased in both species with rainfall and nutrient availability. We conclude that chemical defence is correlated with leaf functional traits and that variation in environmental resources affects this correlation.