Phytochemistry Reviews (v.10, #2)

A novel fungal genus is described that produces extremely bioactive volatile organic compounds (VOC’s). The initial fungal isolate was discovered as an endophyte in Cinnamomum zeylanicum in a botanical garden in Honduras. This endophytic fungus was named Muscodor albus because of its odor and its white color. This fungus produces a mixture of VOC’s that are lethal to a wide variety of plant and human pathogenic fungi and bacteria. It also is effective against nematodes and certain insects. The mixture of VOC’s has been analyzed using GC/MS and consists primarily of various alcohols, acids, esters, ketones, and lipids. Final verification of the identity of the VOC’s was carried out by using artificial mixtures of the putatively identified compounds and showing that the artificial mixture possessed the identical retention times and mass spectral qualities as those of the fungal derived substances. Artificial mixtures of the available VOC’s mimicked some but not all of the biological effects of the fungal VOC’s when tested against a wide range of fungal and bacterial pathogens. Other species and isolates of this genus have been found in various tropical forests in Australia, Bolivia, Ecuador, and Thailand. The most recent discovery is Muscodor crispans whose VOCs are active against many plant and human pathogens. Potential applications for “mycofumigation” by members of the Muscodor genus are currently being investigated and include uses for treating plant diseases, buildings, soils, agricultural produce and many more. This report will describe how the fungus was discovered, identified, and found potentially useful to agriculture, medicine and industry.
Keywords: Endophytes; Mycofumigation; Volatile organic compounds; Volatile antibiotics; DNA; Biological control; GC/MS

Artemisinin production in Artemisia annua: studies in planta and results of a novel delivery method for treating malaria and other neglected diseases by Pamela J. Weathers; Patrick R. Arsenault; Patrick S. Covello; Anthony McMickle; Keat H. Teoh; Darwin W. Reed (173-183).
Artemisia annua L. produces the sesquiterpene lactone, artemisinin, a potent antimalarial drug that is also effective in treating other parasitic diseases, some viral infections and various neoplasms. Artemisinin is also an allelopathic herbicide that can inhibit the growth of other plants. Unfortunately, the compound is in short supply and thus, studies on its production in the plant are of interest as are low cost methods for drug delivery. Here we review our recent studies on artemisinin production in A. annua during development of the plant as it moves from the vegetative to reproductive stage (flower budding and full flower formation), in response to sugars, and in concert with the production of the ROS, hydrogen peroxide. We also provide new data from animal experiments that measured the potential of using the dried plant directly as a therapeutic. Together these results provide a synopsis of a more global view of regulation of artemisinin biosynthesis in A. annua than previously available. We further suggest an alternative low cost method of drug delivery to treat malaria and other neglected tropical diseases.
Keywords: Artemisinin pharmacokinetics; ROS; DMSO; Artemisia annua development; Trichomes

Nature is a rich source of compounds exhibiting biological activity against weeds, plant diseases, insects and mites. Many of these natural products have complex structures, insufficient biological activity and low persistence under field conditions. Thus the share of natural products being used as active ingredients per se in today’s crop protection market is relatively small. In some cases the natural products have been further modified to provide semi-synthetic derivatives with improved biological properties. More importantly, natural products served as lead structures inspiring chemists to prepare new synthetic analogues with often improved biological activity, simplified structures, increased safety towards humans and the environment and an optimized persistence. This article is not an extensive review of natural products in crop protection, but it discusses some examples illustrating the use of natural products per se, their use as starting materials to prepare semi-synthetic products, and their use as lead structures to prepare new synthetic products which may in the end bear no resemblance to the initial lead.
Keywords: Acaricides; Fungicides; Herbicides; Insecticides; Lead structures

In spite of intensive research on plant natural products and insect-plant chemical interactions over the past three decades, only two new types of botanical insecticides have been commercialized with any success in the past 15 years, those based on neem seed extracts (azadirachtin), and those based on plant essential oils. Certain plant essential oils, obtained through steam distillation and rich in mono- and sesquiterpenes and related phenols, are widely used in the flavouring and fragrance industries and in aromatherapy. Some aromatic plants have traditionally been used for stored product protection, but the potential for development of pesticides from plant essential oils for use in a wide range of pest management applications has only recently been realized. Many plant essential oils and their major terpenoid constituents are neurotoxic to insects and mites and behaviourally active at sublethal concentrations. Most plant essential oils are complex mixtures. In our laboratory we have demonstrated that individual constituents of oils rarely account for a major share of the respective oil’s toxicity. Further, our results suggest synergy among constituents, including among those that appear non-toxic in isolation. Repellent effects may be particularly useful in applications against public health and domestic pests, but may be useful in specific agricultural applications as well. In all of these applications, there is a premium on human and animal safety that takes priority over absolute efficacy. In agriculture, the main market niche for essential oil-based pesticides is in organic food production, at least in developed countries, where there are fewer competing pest management products. There is also scope for mixing these oils with conventional insecticides and for enhancing their efficacy with natural synergists. Some examples of field efficacy against agricultural pests are discussed.
Keywords: Plant essential oils; Natural pesticides; Insect repellents; Pest management

An overview of NMR-based metabolomics to identify secondary plant compounds involved in host plant resistance by Kirsten A. Leiss; Young H. Choi; Robert Verpoorte; Peter G. L. Klinkhamer (205-216).
Secondary metabolites provide a potential source for the generation of host plant resistance and development of biopesticides. This is especially important in view of the rapid and vast spread of agricultural and horticultural pests worldwide. Multiple pests control tactics in the framework of an integrated pest management (IPM) programme are necessary. One important strategy of IPM is the use of chemical host plant resistance. Up to now the study of chemical host plant resistance has, for technical reasons, been restricted to the identification of single compounds applying specific chemical analyses adapted to the compound in question. In biological processes however, usually more than one compound is involved. Metabolomics allows the simultaneous detection of a wide range of compounds, providing an immediate image of the metabolome of a plant. One of the most universally used metabolomic approaches comprises nuclear magnetic resonance spectroscopy (NMR). It has been NMR which has been applied as a proof of principle to show that metabolomics can constitute a major advancement in the study of host plant resistance. Here we give an overview on the application of NMR to identify candidate compounds for host plant resistance. We focus on host plant resistance to western flower thrips (Frankliniella occidentalis) which has been used as a model for different plant species.
Keywords: Eco-metabolomic approach; Host plant resistance; NMR metabolomics; Secondary plant metabolites; Thrips (Frankliniella occidentalis)

Ligand-gated chloride channels mediate a variety of functions in excitable membranes of nerve and muscle in insects, and have a long history as targets for neurotoxic insecticides. Recent findings from our laboratory confirm that the natural product silphinenes and their semi-synthetic analogs share a mode of action with the established ligand-gated chloride channel antagonist, picrotoxinin. The silphinenes are non-selective, being roughly equipotent on insect and mammalian receptors, but also possess lethal and neurotoxic effects on a dieldrin-resistant strain of Drosophila melanogaster. These findings suggest that silphinenes act on insect GABA receptors in a way that is different from picrotoxinin, and it is possible that resistant insect populations in the field could be controlled with insecticidal compounds derived from the silphinenes. Voltage-gated chloride channels and anion transporters provide additional classes of validated targets for insecticidal/nematicidal action. Anion transporter blockers are toxic to insects via an action on the gut, and RNAi studies implicate voltage-gated chloride channels in nematode muscle as another possible target. There was no cross resistance to DIDS in a dieldrin-resistant strain of Drosophila melanogaster, and no evidence for neurotoxicity. The potent paralytic actions of anion transporter blockers against nematodes, and stomach poisoning activity against lepidopteran larvae suggests they are worthy of further investigation as commercial insecticidal/nematicidal agents.
Keywords: Silphinene; GABA receptor; Fipronil; Fipronil analog; CLC; rdl ; Insecticide resistance; GluRCl; Glutamate receptor; Ligand-gated chloride channel; Voltage-sensitive chloride channel; Anion transporter

Plant essential oils as potential control agents of varroatosis by María Laura Umpiérrez; Estela Santos; Andrés González; Carmen Rossini (227-244).
Beekeeping has always been vulnerable to various sanitary drawbacks. The mite Varroa destructor (Mesostigmata: Varroidae), an obligated ectoparasite of honeybees, has been in recent times one of the major problems leading not only to economical losses but also to ecological problems related to the role of honeybees as the most important pollinators on Earth. Varroatosis has been treated by methods that include special practices of beekeeping, physical removal, and the use of synthetic acaricides. None of these methods have, however, rendered complete protection, and there is clear evidence for the evolution of resistance to conventional acaricides. Consequently, the need for alternative control means has prompted the onset of several prospecting programs on botanicals with anti-varroa potential. Among these, essential oils appear as good candidates for safer control. In this review, we provide a concise compilation of the information generated in the last years on essential oils with anti-varroa activity. Future developments in this area demand standardization of bioassay protocols (either in the laboratory for primary screening or in hives), a detailed chemical characterization of the essential oils tested, and comprehensive studies on application methods.
Keywords: Botanical pesticides; Honeybees; Plant extracts; Varroa destructor

Triterpene-based plant defenses by Azucena González-Coloma; Carmen López-Balboa; Omar Santana; Matías Reina; Braulio M. Fraga (245-260).
Pentacyclic triterpenes are abundant in the plant kingdom and have a wide array of pharmacological activities. They also have insect antifeedant effects and therefore apparently play a role in plant defense. In this paper, we describe the insecticidal activity of pentacyclic triterpenes of plant origin from different chemical classes on several insect pests (Spodoptera littoralis, Leptinotarsa decemlineata and Myzus persicae), their phytotoxic properties and their selective cytotoxic effects on insect-derived Sf9 and mammalian CHO cells. We also discuss the role they play in plant defense based on these activities.
Keywords: Triterpenes; Bioactivity; Antifeedant; Phytotoxic; Cytotoxic

Metabolites from freshwater aquatic microalgae and fungi as potential natural pesticides by Beatriz Hernández-Carlos; M. Marcela Gamboa-Angulo (261-286).
Microorganisms are recognized worldwide as the major source of secondary metabolites with mega diverse structures and promissory biological activities. However, as yet many of them remain little or under-explored like the microbiota from freshwater aquatic ecosystems. In the present review, we undertook a recompilation of metabolites reported with pesticidal properties from microalgae (cyanobacteria and green algae) and fungi, specifically from freshwater aquatic habitats.
Keywords: Bioactive metabolites; Cyanobacteria; Freshwater ecosystems; Fungi; Microalgae