The Effect of Stereochemistry on the Biological Activity of Natural Phytotoxins,Fungicides, Insecticides and Herbicides

Phytotoxins are secondary microbial metabolites that play an essential role in the development of disease symptoms induced by fungi on host plants. Although phytotoxins can cause extensive—and in some cases devastating—damage to agricultural crops, they can also represent an important tool to develop natural herbicides when produced by fungi and plants to inhibit the growth and spread of weeds. An alternative strategy to biologically control parasitic plants is based on the use of plant and fungal metabolites, which stimulate seed germination in the absence of the host plant. Nontoxigenic fungi also produce bioactive metabolites with potential fungicide and insecticide activity, and could be applied for crop protection. All these metabolites represent important tools to develop eco‐friendly pesticides. This review deals with the relationships between the biological activity of some phytotoxins, seed germination stimulants, fungicides and insecticides, and their stereochemistry. Chirality 25:59–78, 2013. © 2012 Wiley Periodicals, Inc.     

Fungal phytotoxins: from basic studies to practical use (a review)

Recent materials are summarized, pertaining to classification of fungal phytotoxins, methods of their isolation, and assays for biological activity. Producers of phytotoxic substances have been characterized, and the chemical nature of phytotoxins has been subjected to analysis. The review gives consideration to the mechanisms of action of phytotoxins on susceptible plants and the mechanisms of plant resistance to such agents. Other matters discussed include prospects of utilizing basic knowledge of the nature and mechanisms of action of phytotoxins for developing means of plant protection against diseases and weeds and identifying or classifying fungi (chemosystematics).     

A review of fungal phytotoxins: from basic studies to practical use

Recent materials are summarized, pertaining to classification of fungal phytotoxins, methods of their isolation, and assays for biological activity. Producers of phytotoxic substances have been characterized, and the chemical nature of phytotoxins has been subjected to analysis. The review gives consideration to the mechanisms of action of phytotoxins on susceptible plants and the mechanisms of plant resistance to such agents. Other matters discussed include prospects of utilizing basic knowledge of the nature and mechanisms of action of phytotoxins for (1) developing means of plant protection against diseases and weeds and (2) identifying or classifying fungi (chemosystematics).     

ALLELOPATHIC EFFECTS OF DECAYING LITTER OF DOMINANT TREES AND THEIR ASSOCIATED SOIL IN A LOWLAND FOREST COMMUNITY

Phytotoxins released by the dominant species in a lowland forest showed highest accumulations in their respective stands. The accumulation of such phytotoxins corresponded with the amount of litter produced and its decaying rate. The release of phytotoxins and their activity depended on whether they were free or bound, and on their solubility. Therefore, the phytotoxins in soils were highly variable in amounts from one species stand to another species stand, during the same and different times of the year. The persistent accumulation of phytotoxins from all dominant tree species continues to influence the soil properties around the trees. Fourteen phytotoxins were isolated from the leaf litter and the soil. Seven persistent toxins were quantified in soils under different tree stands at various times of the year. The cycle of release of bound and free phenolics is discussed.     

Leaf Spot of Hemp Dogbane Caused by Stagonospora apocyni, and its Phytotoxins

Stagonospora apocyni causes a leaf spot disease on hemp dogbane (Apocynum cannabinum L.). The fungus produced phytotoxins citrinin, mellein, tyrosol and α–acetylorcinol in liquid culture. All toxins caused necrosis when placed on leaves of hemp dogbane, and eight other weed species. All four toxins were non–specific phytotoxins. Citrinin showed antimicrobial properties against some bacteria and fungi.     

Penicimutanin C,a New Alkaloidal Compound,Isolated from a Neomycin‐Resistant Mutant 3‐f‐31of Penicillium purpurogenum G59

Penicimutanin C, a new alkaloidal compound, was isolated from the neomycin‐resistant mutant strain 3‐f‐31 of the marine‐derived fungus, Penicillium purpurogenum G59, together with four known compounds. The structure of penicimutanin C, including the absolute configuration, was determined by spectroscopic and chemical methods. The absolute configuration of penicimutanin A was also re‐confirmed by Marfey's and chiral HPLC analyses of the hydrolyzed products. Penicimutanins C and A inhibited the proliferation of five human cancer cell lines to some extent. Penicimutanin C is the third dimer of diketopiperazine and penicimutanolone, which are only produced by mutants of P. purpurogenum G59 isolated to date, and it showed cytotoxic activity against human cancer cell lines. The neomycin‐resistant screening strategy has been previously successfully used to discover new compounds by activating silent metabolites in fungi, and the present results provide an additional example of the effectiveness of this method.     

Interactions between mammals and ectomycorrhizal fungi

Many ectomycorrhizal (ECM) fungi produce fruit-bodies below ground and rely on animals, especially mammals, for dispersal of spores. Mammals may therefore play an important role in the maintenance of mycorrhizal symbiosis and biodiversity of ECM fungi in many forest ecosystems. Given the pivotal role played by mycorrhizal fungi In the nutrition of their plant hosts and, possibly, in the determination of plant community structure, the ecological significance of mycophagous mammals may extend to the productivity and diversity of plant communities. Mycologists and mammalogists have been aware of the interaction between their study organisms for many years, but recent research has produced new insights Into the evolution of mammal-vectored spore dispersal among ECM fungi, the ecological importance of mycophagy to small mammals, and the effectiveness of mammals as spore-dispersal agents.     

Further secondary metabolites produced by the fungus Pyricularia grisea isolated from buffelgrass (Cenchrus ciliaris)

The fungal pathogen Pyricularia grisea has been studied to evaluate its production of phytotoxins for the biocontrol of the buffelgrass (Cenchrus ciliaris L.) weed. A first investigation allowed to isolate several new and known phytotoxic metabolites. However, the further investigation on the organic extract obtained from the fungus liquid culture showed the presence of other metabolites possibly contributing to its phytotoxicity. Thus, four known metabolites were isolated and identified by spectroscopic (nuclear magnetic resonance [NMR] and high-resolution electrospray ionization mass spectrometry [HRESIMS]) methods as dihydropyriculol ( 1 ), epi-dihydropyriculol ( 2 ), 3-methoxy-6,8-dihydroxy-3-methyl-3,4-dihydroisocoumarin ( 3 ), and (R)-mevalonolactone ( 4 ). The absolute configuration of 1 – 3 was determined for the first time by a computational analysis of their electronic circular dichroism (ECD) spectra. When the isolated compounds were bioassayed at a concentration of 5 × 10^–3 M in a buffelgrass coleoptile and radicle elongation test no toxicity was detected. On the contrary, compounds 1 and 3 showed a significant stimulating effect of radical elongation. Furthermore, the difference in growth stimulation between 1 and its epimer 2 highlights the tight relationship between absolute configuration and biological activity of these fungal metabolites.     

Conceptual problems of Ecological Continuity and its bioindicators

Very old, undisturbed forest stands may be important for biodiversity through their content of microhabitats or for the long periods available for colonisation, or for both. The term Ecological Continuity (EC) has been used to ascribe value to old forest stands. The relative importance of microhabitat and time for colonisation are usually not kept apart when EC is used as a conservation criterium. EC is broadly applied but poorly defined. Use of EC may lead to underestimation of the importance of forest dynamics and dispersal, and to overestimation of the importance of local land use history. If bioindicators of long-term habitat persistence are to be used, species with low dispersal capacity should be chosen. However, many lichens and other fungi, bryophytes and insects cited as indicators of EC, seem to have a patch-tracking lifestyle. They are 'colonists' according to life history strategy classification, and rather seem to indicate specific microhabitats. Terrestrial molluscs, some vascular forest plants, and those bryophytes and lichenized fungi classified as 'perennial stayers' in life history strategy classification, might be used to indicate long-term habitat persistence in forests, but more research is needed to evaluate such indicators.     

The biology of ophiobolins

This review article aims at summarizing the research findings on the biological aspects of ophiobolins, phytotoxins produced by the pathogenic fungi Bipolaris species, which usually infect rice, maize and sorghum. The topics covered include the organisms that produce the various ophiobolins, the structural variations of ophiobolins, the biological actions of ophiobolins in plants, animals and microorganisms, and the mode of action and the possible use of ophiobolin A as a calmodulin antagonist.     

West Siberian Latitudinal Xylomycological Scale and Its Use for Indicating Forest Conditions

A quantitative analysis of the aphyllophoroid macromycetes of Betula complex on 207 sites of the territory of the West Siberian Plain in the range from forest tundra to forest steppe allows us to determine the width of the zone and its patterns for the biota of xylotrophic fungi. It is shown that the width and structure of the zonal xylomycocomplexes is determined, first and foremost, by climatic factors, which are characterized by an expressed hydrothermal gradient. They influence fungi directly also by the formation of the forest stand corresponding to climate. The sequence of width and zone xylomycocomplex of the region is a scale for the indication of forest vegetation conditions and parameters of the forest stand of a particular timber stand. Possibilities of indication are expanded with the differentiation of the xylomycocomplex and, respectively, its width and zone scale on functional fractions of fungi (stem, peripheral, and wound types). The scale is used for assessing the condition of woods in the Numto Natural Park (northern taiga). For the characteristic of each site, coefficients of correlation of numerical structure of this mycocenos and the zone xylomycocomplex in the range from the forest tundra to the forest steppe were obtained and calculated for functional groups. The configuration of the growth spectral curves obtained in this way and their mutual position indicates the peculiarities of the hydrothermal regime of a plot; the influence of mechanical damages on tree stands; and their physiological conditions, completeness, quality, and age. It is shown that, according to the results of mycological indication, forest conditions of the Numto National Park forest plots damaged by fires and clearcuttings correspond to forest conditions of the zones to the south of the park. Regularities of the West Siberian latitude and zone xylomycological scale are characterized by a certain universality for the forest biome of Siberia, which is confirmed by similar regularities obtained during a study of elevation and zone distribution of the xylomycocomplex of Transbaikalia.     

Detection of extracellular protease activity in different species and genera of ectomycorrhizal fungi

In northern forest ecosystems, most soil nitrogen (N) is in organic form and forest trees are largely dependent on ectomycorrhizal (ECM) fungi and their degradative abilities for N uptake. The ability of ECM fungi to acquire N from organic substrates should, therefore, be a widespread trait given its ecological importance. However, little is known about the degradative abilities of most ECM fungi as they remain untested due to problems of isolation or extremely slow growth in pure culture. In this paper, we present data on extracellular protease activity of 32 species of ECM fungi, most of which have not previously been cultured. Milk powder plates and zymograms were compared for detecting protease activity in these intractable species. In total, 29/32 of the species produced extracellular protease activity, but detection was method dependent. Growth on milk powder plates detected protease activity in 28 of 32 species, while zymograms only detected proteases in Amanita muscaria, Russula chloroides, Lactarius deterrimus and Lactarius quieticolor. The study supports the hypothesis that protease excretion is a widespread physiological trait in ECM fungi and that this ability is of considerable significance for nitrogen uptake in forest ecosystems.     

Polyphenols as fungal phytotoxins, seed germination stimulants and phytoalexins

This review deals with the sources and chemical and biological characterization of phytotoxic polyphenols produced essentially by pathogenic fungi of forest and crop plants and of weeds. Their potential use as natural herbicides and fungicides is discussed. The use of some polyphenols which could be applied as an alternative method to control parasitic weeds, the so called “suicidal germination”, will be covered. The sources and the isolation and identification of polyphenols produced by some crop plants in consequence of the attack of pathogenic fungi as plant defence compounds (phytoalexins), are also described.     

Diversity and evolutionary origins of fungi associated with seeds of a neotropical pioneer tree: a case study for analysing fungal environmental samples

Fungi associated with seeds of tropical trees pervasively affect seed survival and germination, and thus are an important, but understudied, component of forest ecology. Here, we examine the diversity and evolutionary origins of fungi isolated from seeds of an important pioneer tree (Cecropia insignis, Cecropiaceae) following burial in soil for five months in a tropical moist forest in Panama. Our approach, which relied on molecular sequence data because most isolates did not sporulate in culture, provides an opportunity to evaluate several methods currently used to analyse environmental samples of fungi. First, intra- and interspecific divergence were estimated for the nu-rITS and 5.8S gene for four genera of Ascomycota that are commonly recovered from seeds. Using these values we estimated species boundaries for 527 isolates, showing that seed-associated fungi are highly diverse, horizontally transmitted, and genotypically congruent with some foliar endophytes from the same site. We then examined methods for inferring the taxonomic placement and phylogenetic relationships of these fungi, evaluating the effects of manual versus automated alignment, model selection, and inference methods, as well as the quality of BLAST-based identification using GenBank. We found that common methods such as neighbor-joining and Bayesian inference differ in their sensitivity to alignment methods; analyses of particular fungal genera differ in their sensitivity to alignments; and numerous and sometimes intricate disparities exist between BLAST-based versus phylogeny-based identification methods. Lastly, we used our most robust methods to infer phylogenetic relationships of seed-associated fungi in four focal genera, and reconstructed ancestral states to generate preliminary hypotheses regarding the evolutionary origins of this guild. Our results illustrate the dynamic evolutionary relationships among endophytic fungi, pathogens, and seed-associated fungi, and the apparent evolutionary distinctiveness of saprotrophs. Our study also elucidates the diversity, taxonomy, and ecology of an important group of plant-associated fungi and highlights some of the advantages and challenges inherent in the use of ITS data for environmental sampling of fungi.     

Phytotoxins as tools in studying plant disease resistance

Many plant pathogenic fungi and bacteria produce phytotoxins which are a vital part of their arsenal in causing plant disease. Some host-specific toxins have proved useful in enhancing our understanding of the biochemical mechanisms underlying plant disease resistance.     

Non-target impacts of forest defoliator management options: Decision for no spraying may have worse impacts on non-target Lepidoptera than Bacillus thuringiensis insecticides

Management programs for major forest defoliators such as gypsy moths or forest tent caterpillars, and crop pests such as the European corn borer have shifted from broad-spectrum insecticides to more environmentally benign microbial pesticides such as Bacillus thuringiensis (foliage sprays and transgenic toxin expression in plant tissues). Phytochemically resistant host plants and natural enemies have been used as alternative pest management strategies (including generalist tachinid flies such as Compsilura, viruses, microsporidians, and fungi), but all of these have some non-target impacts, as described from literature review. A sequence of lab and field studies were conducted to determine non-target impacts on native Lepidoptera in North America. The conclusions reached are that a decision not to spray Bt pesticides (i.e. to allow defoliation and natural pest outbreaks to run their course) could be as bad or worse for non-target Lepidoptera as the microbial insecticides would be. The important concept that must be maintained is that all pest management programs have some risk of negative non-target impacts, but it is the magnitude and relative importance that will remain the most critical issue for environmental impacts and pest management.     

Effects of environment,habitat configuration and forest continuity on the distribution of forest plant species

In present day European landscapes many forest plant species are restricted to isolated remnants of a formerly more or less continuous forest cover. The two major objectives of this study were (1) to determine the relative importance of habitat quality (mainly in terms of soil parameters), habitat configuration (patch size and isolation) and habitat continuity for the distribution of herbaceous forest plant species in a highly fragmented landscape and (2) to examine if groups of species with different habitat requirements are affected differently. Deciduous forest patches in northwestern Germany were surveyed for the presence of a large set of forest species. For each patch, habitat quality, configuration and continuity were determined. Data were analysed by Redundancy Analysis with variation partitioning for effects on total species composition and multivariate logistic regression for effects on individual species, for two different data sets (base‐rich and base‐poor forest patches). Overall, we found strong effects of habitat quality (particularly of soil pH, water content and topographic heterogeneity in the base‐rich forest patches; and of calcium content and disturbance in the base‐poor patches), but only relatively weak effects of habitat configuration and habitat continuity. However, a number of species were positively affected by patch area and negatively affected by patch isolation. Furthermore, the relative importance of habitat configuration tended to be higher for species predominantly growing in closed forests compared to species occurring both in the forest and in the open landscape.     

Flow cytometry as a method for assaying the biological activity of phytotoxins

A flow cytometric method has been developed for the qualitative and quantitative evaluation of the biological activities of phytotoxins from plant pathogenic fungi. The method utilized fresh wheat (Triticum aestivum L.) leaf protoplast preparations treated with purified phytotoxins, triticone A-B and triticone D. Subsequently, protoplasts were exposed to fluorescein diacetate, and analyzed by flow cytometry. Information acquired included fluorescence owing to esterase activity on fluorescein diacetate, and chlorophyll autofluorescence. Results indicate that triticone A-B has a rapid dose-dependent toxic effect on wheat protoplasts but triticone D has no toxic effect. This method can also yield information on the mechanism of action of phytotoxins that are relatively unstable or available only in small quantities.     

Diterpenes and tetranorditerpenes from Acritopappus species

The investigation of four Acritopappus species afforded twenty-three new diterpenes of the labdane or kolavane type and three tetranorditerpenes as well as two new benzofuran derivatives and a hydroxybicyclogermacrene. The structures were elucidated by spectroscopic methods and by some chemical transformations. Though the absolute configuration were not established in all cases, most probably all diterpenes were ent-labdanes or kolavanes (clerodanes). The chemotaxonomic importance of this investigation is discussed briefly.     

Biotransformation of terpenes

The main application of terpenes as fragrances and flavors depends on the absolute configuration of the compounds because enantiomers present different organoleptic properties. Biotransformations allow the production of regio- and stereoselective compounds under mild conditions. These products may be labeled as "natural". Commercially useful chemical building-blocks and pharmaceutical stereo isomers can also be produced by bioconversion of terpenes. Enzymes and extracts from bacteria, cyanobacteria, yeasts, microalgae, fungi, plants, and animal cells have been used for the production and/or bioconversion of terpenes. In addition, whole cell catalysis has also been used. A variety of media and reactors have been assessed for these biotransformations and have produced encouraging results, as discussed in this review.