Locoweed dose responses to nitrogen: Positive for biomass and primary physiology, but inconsistent for an alkaloid

Extensive Local Geographic Variation in Locoweed Toxin Produced by a Fungal Endophyte

Legumes are notorious for coevolutionary arms races where chemical defenses are employed to ward off herbivores-particularly insect seed predators. Locoweeds are legumes containing the toxic alkaloid swainsonine which can poison livestock, but its role as a deterrent for insects is unknown. Swainsonine is produced by the fungal endophyte Alternaria section Undifilum, and the chemical composition of the toxin has been well characterized. Despite this knowledge, the ecological roles and evolutionary drivers of swainsonine toxins in locoweeds remain uncertain. Here, we quantify swainsonine concentrations and herbivory levels in the hyper-diverse locoweed Astragalus lentiginosus to evaluate its role as an evolved chemical defense. We found that A. lentiginosus shows considerable variation in swainsonine concentrations according to variety, in particular showing presence/absence variation at both population and local geographic scales. Surprisingly, herbivory levels from presumed generalist insects emerging from fruits showed no correlation with swainsonine concentrations. Conversely, seed and fruit herbivory levels linked to specialist Acanthoscelides seed beetles increased with concentrations of swainsonine-suggesting a possible coevolutionary arms race. Our results highlight that variation in endophyte-produced toxin systems may not follow classical expectations for geographic variation and ecological roles of plant chemicals. We discuss the implications of these results on plant-endophytic toxin systems and coevolutionary dynamics more broadly, highlighting a considerable need for more research in these systems.

Assembly of high-quality genomes of the locoweed Oxytropis ochrocephala and its endophyte Alternaria oxytropis provides new evidence for their symbiotic relationship and swainsonine biosynthesis

Locoweeds are perennial forbs poisonous to livestock and cause extreme losses to animal husbandry. Locoweed toxicity is attributed to the symbiotic endophytes in Alternaria sect. Undifilum, which produce a mycotoxin swainsonine (SW). We performed a de novo whole genome sequencing of the most common locoweed in China, Oxytropis ochrocephala (2n = 16), and assembled a high-quality, chromosome-level reference genome. Its genome size is 958.83 Mb with 930.94 Mb (97.09 %) anchored and oriented onto 8 chromosomes, and 31,700 protein-coding genes were annotated. Phylogenetic and collinearity analysis showed it is closely related to Medicago truncatula with a pair of large interchromosomal rearrangements, and both species underwent a whole-genome duplication event. We also derived the genome of A. oxytropis at 74.48 Mb with a contig N50 of 8.87 Mb and 10,657 protein-coding genes, and refined the genes of SW biosynthesis. Multiple Alternaria species containing the swnK gene were grouped into a single clade, but in other genera, swnK's homologues are diverse. Resequencing of 41 A. oxytropis strains revealed one SNP in the SWN cluster causing changes in SW concentration. Comparing the transcriptomes of symbiotic and non-symbiotic interactions identified differentially expressed genes (DEG) linked to defense and secondary metabolism in the host. Within the endophyte DEGs were linked to cell wall degradation, fatty acids and nitrogen metabolism. Symbiosis induced the up-regulation of most of the SW biosynthetic genes. These two genomes and relevant sequencing data should provide valuable genetic resources for the study of the evolution, interaction, and SW biosynthesis in the symbiont.

Effects of Elevated CO2 on the Swainsonine Chemotypes of Astragalus lentiginosus and Astragalus mollissimus

Rapid changes in the Earth's atmosphere and climate associated with human activity can have significant impacts on agriculture including livestock production. CO₂ concentration has risen from the industrial revolution to the current time, and is expected to continue to rise. Climatic changes alter physiological processes, growth, and development in numerous plant species, potentially changing concentrations of plant secondary compounds. These physiological changes may influence plant population density, growth, fitness, and toxin concentrations and thus influence the risk of toxic plants to grazing livestock. Locoweeds, swainsonine-containing Astragalus species, are one group of plants that may be influenced by climate change. We evaluated how two different swainsonine-containing Astragalus species responded to elevated CO₂ concentrations. Measurements of biomass, crude protein, water soluble carbohydrates and swainsonine concentrations were measured in two chemotypes (positive and negative for swainsonine) of each species after growth at CO₂ levels near present day and at projected future concentrations. Biomass and water soluble carbohydrate concentrations responded positively while crude protein concentrations responded negatively to elevated CO₂ in the two species. Swainsonine concentrations were not strongly affected by elevated CO₂ in the two species. In the different chemotypes, biomass responded negatively and crude protein concentrations responded positively in the swainsonine-positive plants compared to the swainsonine-negative plants. Ultimately, changes in CO₂ and endophyte status will likely alter multiple physiological responses in toxic plants such as locoweed, but it is difficult to predict how these changes will impact plant herbivore interactions.

Fungicide treatment and clipping of Oxytropis sericea does not disrupt swainsonine concentrations

Swainsonine, an indolizidine alkaloid, is an α-mannosidase and mannosidase II inhibitor that causes lysosomal storage disease and alters glycoprotein processing. Swainsonine is found in a number of plant species worldwide, and is produced by associated endophytic fungi. Prolonged consumption of swainsonine-containing plants by livestock causes a condition characterized by weight loss, depression, altered behavior, decreased libido, infertility, and death. In contrast, Astragalus and Oxytropis that do not contain swainsonine may present a valuable food source for grazing livestock in regions where palatable forage is scarce. This study tested the hypothesis that swainsonine concentrations may be reduced by fungicide treatment or by clipping, thus reducing plant toxicity. Additionally we hypothesized that clipping plants may provide a mechanism for horizontal transmission of the endophyte. To this end, four different fungicides were applied to render the endophyte non-viable, and plant vegetative tissues were periodically clipped. Treatment of Oxytropis sericea with any of four different fungicides did not alter swainsonine concentrations in plants at any of three harvest times. Additionally, we found that individual or multiple clippings had no effect on swainsonine concentrations; plants that contained swainsonine maintained concentrations, and plants low or absent in swainsonine also remained as such at each harvest. These results suggest that there is no evidence of horizontal transmission of the endophyte among individual plants due to clipping.

Simple Indolizidine and Quinolizidine Alkaloids

This review of simple indolizidine and quinolizidine alkaloids (i.e., those in which the parent bicyclic systems are in general not embedded in polycyclic arrays) is an update of the previous coverage in Volume 55 of this series (2001). The present survey covers the literature from mid-1999 to the end of 2013; and in addition to aspects of the isolation, characterization, and biological activity of the alkaloids, much emphasis is placed on their total synthesis. A brief introduction to the topic is followed by an overview of relevant alkaloids from fungal and microbial sources, among them slaframine, cyclizidine, Steptomyces metabolites, and the pantocins. The important iminosugar alkaloids lentiginosine, steviamine, swainsonine, castanospermine, and related hydroxyindolizidines are dealt with in the subsequent section. The fourth and fifth sections cover metabolites from terrestrial plants. Pertinent plant alkaloids bearing alkyl, functionalized alkyl or alkenyl substituents include dendroprimine, anibamine, simple alkaloids belonging to the genera Prosopis, Elaeocarpus, Lycopodium, and Poranthera, and bicyclic alkaloids of the lupin family. Plant alkaloids bearing aryl or heteroaryl substituents include ipalbidine and analogs, secophenanthroindolizidine and secophenanthroquinolizidine alkaloids (among them septicine, julandine, and analogs), ficuseptine, lasubines, and other simple quinolizidines of the Lythraceae, the simple furyl-substituted Nuphar alkaloids, and a mixed quinolizidine-quinazoline alkaloid. The penultimate section of the review deals with the sizable group of simple indolizidine and quinolizidine alkaloids isolated from, or detected in, ants, mites, and terrestrial amphibians, and includes an overview of the "dietary hypothesis" for the origin of the amphibian metabolites. The final section surveys relevant alkaloids from marine sources, and includes clathryimines and analogs, stellettamides, the clavepictines and pictamine, and bis(quinolizidine) alkaloids.

Swainsonine-containing plants and their relationship to endophytic fungi

Swainsonine, an indolizidine alkaloid with significant physiological activity, is an α-mannosidase and mannosidase II inhibitor that alters glycoprotein processing and causes lysosomal storage disease. Swainsonine is present in a number of plant species worldwide and causes severe toxicosis in livestock grazing these plants. Consumption of these plants by grazing animals leads to a chronic wasting disease characterized by weight loss, depression, altered behavior, decreased libido, infertility, and death. This review focuses on the three plant families and the associated taxa that contain swainsonine; the fungi that produce swainsonine, specifically the fungal endophytes associated with swainsonine-containing taxa; studies investigating the plant, endophyte, and swainsonine relationship; the influence of environmental factors on swainsonine concentrations in planta; and areas of future research.

Influence of endophyte genotype on swainsonine concentrations in Oxytropis sericea

Locoism is a toxic syndrome of livestock caused by the ingestion of a subset of legumes belonging to the Astragalus and Oxytropis genera known as "locoweeds". Locoweeds contain the toxic indolizidine alkaloid swainsonine, which is produced by the endophytic fungi Undifilum species. Previously we reported that swainsonine concentrations differ between populations of Oxytropis sericea. We hypothesized that the genotype of the plant, endophyte, or an interaction of the two may be responsible for the differences in swainsonine concentration between populations of O. sericea. To test this hypothesis, plants derived from seeds collected at each location were grown in a common garden, Undifilum oxytropis isolates from each location were cultured and grown in a common environment, and a plant genotype by endophyte cross inoculation was performed. Here we show that the genotype of the endophyte is responsible for the differences in swainsonine concentrations observed in the two populations of O. sericea.