Natural Phytotoxins Research Articles

Plant cell membrane as a marker for light-dependent and light-independent herbicide mechanisms of action

Plant cells possess a number of membrane bound organelles that play important roles in compartmentalizing a large number of biochemical pathways and physiological functions that have potentially harmful intermediates or by-products. The plasma membrane is particularly important as it holds the entire cellular structure whole and is at the interface between the cell and its environment. Consequently, breaches in the integrity of the lipid bilayer, often via reactive oxygen species (ROS)-induced stress membrane peroxidation, result in uncontrolled electrolyte leakage and in cell death. A simple 3-step bioassay was developed to identify compounds that cause electrolyte leakage and to differentiate light-dependent mechanisms of action from those that work in darkness. Herbicides representative of all known modes of action (as well as several natural phytotoxins) were selected to survey their effects on membrane integrity of cucumber cotyledon discs. The most active compounds were those that are known to generate ROS such as the electron diverters and uncouplers (paraquat and dinoterb) and those that either were photodynamic (cercosporin) or caused the accumulation of photodynamic products (acifluorfen-methyl and sulfentrazone). Other active compounds targeted lipids (diclofop-methyl, triclosan and pelargonic acid) or formed pores in the plasma membrane (syringomycin). Herbicides that inhibit amino acid, protein, nucleotide, cell wall or microtubule synthesis did not have any effect. Therefore, it was concluded that the plant plasma membrane is a good biomarker to help identify certain herbicide modes of action and their dependence on light for bioactivity.

Differential Exudation of Two BenzoxazinoidsOne of the Determining Factors for Seedling Allelopathy of Triticeae Species

Benzoxazinoids (Bx) are natural phytotoxins that function as chemical defense compounds in several species. The release of Bx by intact plant roots associated these compounds with root allelopathy in Triticeae species; however, the significance of exudate concentrations of Bx for plant-plant interactions is still a controversial question. A biological screening of 146 cultivars of four Triticeae species (Triticum aestivum L., Triticum durum Desf., Triticum spelta L., and Secale cereale L.) demonstrated a high cultivar dependence to suppress the root growth of Sinapis alba L. by root allelopathy in a dose-response bioassay. Only a few cultivars possessed a marked high or low allelopathic activity, whereby high-performance liquid chromatography-diode array detection analysis of root exudates revealed that these cultivars differed considerably in their ability to exude the two Bx aglucones, DIBOA [2,4-dihydroxy-2H-1,4-benzoxazin-3(4H)-one] and DIMBOA [2,4-dihydroxy-7-methoxy-2H-1,4-benzoxazin-3(4H)-one]. The total amount of DIBOA and DIMBOA exuded showed a significant correlation to the growth inhibition in bioassay with a statistically estimated contribution to the overall allelopathic effect of 48-72%. In a bioassay with pure phytotoxins, Bx concentrations consistent with the amounts quantified in the screening bioassay caused detrimental effects on S. alba and almost reproduced the statistically estimated contribution. The observed causal association between the allelopathic activity under laboratory conditions and the exudate concentrations of Bx suggests that this association might have implications for the interference of Triticeae species in natural plant communities.

Chemistry of Crop Protection: Progress and Prospects in Science and Regulation

Preface.Prologue - Reacting to Change: Past Success Stories and Future Opportunities for the Agricultural Enterprise (J. Abernathy).I: PLENARY LECTURES.Crop Protection in the New Millenium (M. Pragnell).Homologs of Amino Acids and Explorations into the Worlds of s- and y-Peptides (D. Seebach).Cost Effective Production of Agrochemicals: How to Satisfy the Needs of Farmers, Industry, and the Environment (W. Shiang & R. Swafford).Obsolete Pesticide Stocks in Developing Countries: Strategies, Policies, and Practical Steps for their Safe Disposal (W. Schimpf).A Food Processor's Efforts to Minimise Pesticide Residues in the Food Chain (H. Johr & E. Bruckner).The Risk to Eat: Natural versus Man-Made Toxins (A. Bast).II: DISEASE CONTROL: CHEMISTRY, BIOCHEMISTRY, MOLECULAR BIOLOGY.Molecular Diagnostics for Fungicide Resistance in Plant Pathogens (H. Sierotzki & U. Gisi).New Antifungal Modes of Action (M. Henry & G. Gustafson).Elucidating Pathways Controlling Induced Resistance (J. Ton & B. Mauch-Mani).Molecular Modelling of Inhibitors at Qi and Qo Sites in Cytochrome bc1 Complex (T. Link, et al.).III: WEED CONTROL: CHEMISTRY, BIOCHEMISTRY, MOLECULAR BIOLOGY.News from Old Compounds: The Mode of Action of Auxin Herbicides (K. Grossmann).Natural Phytotoxins with Potential for Development in Weed Management Strategies (S. Duke, et al.).Weed Resistance to Herbicides: Lessons Learned and Future Directions (M. Devine).IV: INSECT CONTROL: CHEMISTRY, BIOCHEMISTRY, MOLECULAR BIOLOGY. Voltage-Gated Sodium Channels as Insecticide Targets (K. Dong).Structural Studies of Insect Nuclear Receptors (I. Billas & D. Moras).Molecular Design of Neonicotenoids: Past, Present and Future (S. Kagabu).Mode of Action of Bacillus thuringiensis Toxins (M. Zhuang & S. Gill).V: FORMULATION AND APPLICATION: METHODOLOGY, PREFORMANCE, SAFETY.Foliar Interaction and Uptake of Agrochemical Formulations: Present Limits and Future Potential (J. Zabkiewicz).Standards for Sprayers and Cooperation of Test Stations for Mutual Recognition in Europe (S. Liberatori).Suspoemulsion: Key Technology for Tailor-Made Ready-Mix Formulations (M. Bratz, et al.).Controlled Release in Crop Protection: Past Experience and Future Opportunities (G. Beestman).VI: ENVIRONMENTAL FATE AND SAFETY.Advances in Validation of Environmental Exposure Model Predictions for Regulatory Purposes (P. Hendley).Environmental Fate and Impact Considerations Related to the Use of Transgenic Crops (G. Kleter & H. Kuiper).Development and Registration of Pesticides with Reduced Risk Characteristics (K. Racke).Environmental Fate Considerations for Pesticides in Tropical Ecosystems (E. Carazo).VII: RESIDUES AND CONSUMER SAFETY.European Food Agency and Consumer Risk Assessment in the EU (M. Walsh).A Comparison of Dietary Exposure and Risk Assessment Methods in US and EU (J. Tomerlin).MRLs and International Trade, a Developing Country Perspective (C. Boh).Consumer Perception of Food Risk and Consumer Trust in Europe (B. Kettlitz).Keyword Index.

Natural products as sources for new mechanisms of herbicidal action

New mechanisms of action for herbicides are highly desirable to fight evolution of resistance in weeds, to create or exploit unique market niches, and to cope with new regulatory legislation. Comparison of the known molecular target sites of synthetic herbicides and natural phytotoxins reveals that there is little redundancy. Comparatively little effort has been expended on determination of the sites of action of phytotoxins from natural sources, suggesting that intensive study of these molecules will reveal many more novel mechanisms of action. Examples of natural products that inhibit unexploited steps in the amino acid, nucleic acid, and other biosynthetic pathways are given. AAL-toxin, hydantocidin, and various plant-derived terpenoids are discussed. Strategies and potential pitfalls of using natural products as leads for new herbicide classes are summarized.

Investigating the mode of action of natural phytotoxins

The potential use of natural phytotoxins (including allelochemicals) to develop novel tools for weed management is enhanced by the elucidation of their modes of action. This approach has not been emphasized by the agrochemical industry, although the possibility of discovering new target sites may be promising, since natural products tend to have modes of action different from synthetic herbicides. The approach of testing a compound on all known herbicide molecular target sites for commercial herbicides and other potent phytotoxins is feasible. However, this would preclude the discovery of new mechanisms of action. Discovering new target sites requires more challenging holistic approaches, initiated with physiological and biochemical studies that use whole plant assays. Studying basic plant responses to a compound may yield important clues to the specific physiological processes affected by the compounds and uncover novel mechanisms of action.