Search for: All records

The fungal plant pathogen Slafractonia leguminicola produces two mycotoxins that affect animals: slaframine, which causes slobbers, and swainsonine, which causes locoism. Slafractonia leguminicola contains the swainsonine-associated orthologous gene clusters, “SWN”, which include a multifunctional swnK gene (NRPS-PKS hybrid), swnH1 and swnH2 (nonheme iron dioxygenase genes), swnN and swnR (reductase genes), and swnT (transmembrane transporter). In addition to these genes, two paralogs of swnK, swnK1 (paralog1) and swnk2 (paralog2), are found in S. leguminicola. cDNAs from total mRNA were isolated from the S. leguminicola mycelia grown in the culture plates as well as from leaves inoculated with the fungal mycelia at different time points, and expression pattern of the SWN genes were analyzed using RT-qPCR. The concentrations of swainsonine and slaframine production from this fungus at different time points were also examined using liquid chromatography–mass spectrometry. The timing of gene expression was similar in cultured fungus and inoculated leaves and agreed with our proposed biosynthetic pathway. Substantially more swainsonine was produced than slaframine during time course studies. 

Slafractonia leguminicola infects red clover and other legumes, causing black patch disease. This pathogenic fungus also produces two mycotoxins, slaframine and swainsonine, that are toxic to livestock grazing on clover hay or pasture infested with S. leguminicola. Swainsonine toxicosis causes locoism, while slaframine causes slobbers syndrome. The mechanism of toxin secretion by S. leguminicola is poorly understood. The aim of this research was to investigate the role of a putative transmembrane transporter, SwnT, in mycotoxin transport. The swnT gene was silenced by RNA interference using the silencing vector Psilent1, which included inverted repeat transgenes of swnT. This resulted in a significant reduction of swnT transcript levels compared with the controls. Silencing caused a decline in the active efflux of toxins from the mycelia to the media, as shown by LC–MS analysis. Transformants in which swnT was silenced showed higher concentrations of both toxins in the mycelia compared with the concentrations in the media. These transformants exhibited a visibly distinct phenotype with much thicker and shorter mycelia than in the wild type. These transformants were also unable to infect detached clover leaves, unlike the controls, suggesting that SwnT function may play an important role in pathogenesis in addition to mycotoxin transport. This research demonstrates the importance of this transporter to the secretion of mycotoxins for this phytopathogenic fungus. 

Endosymbionts play important roles in the life cycles of many macro-organisms. The indolizidine alkaloid swainsonine is produced by heritable fungi that occurs in diverse plant families, such as locoweeds (Fabaceae) and morning glories (Convolvulaceae) plus two species of Malvaceae. Swainsonine is known for its toxic effects on livestock following the ingestion of locoweeds and the potential for pharmaceutical applications. We sampled and tested herbarium seed samples ( n = 983) from 244 morning glory species for the presence of swainsonine and built a phylogeny based on available internal transcribed spacer (ITS) sequences of the sampled species. We show that swainsonine occurs only in a single morning glory clade and host species are established on multiple continents. Our results further indicate that this symbiosis developed ∼5 mya and that swainsonine-positive species have larger seeds than their uninfected conspecifics. 

Abstract Endophytes are microbes that live, for at least a portion of their life history, within plant tissues. Endophyte assemblages are often composed of a few abundant taxa and many infrequently observed, low-biomass taxa that are, in a word, rare. The ways in which most endophytes affect host phenotype are unknown; however, certain dominant endophytes can influence plants in ecologically meaningful ways—including by affecting growth and immune system functioning. In contrast, the effects of rare endophytes on their hosts have been unexplored, including how rare endophytes might interact with abundant endophytes to shape plant phenotype. Here, we manipulate both the suite of rare foliar endophytes (including both fungi and bacteria) and Alternaria fulva–a vertically transmitted and usually abundant fungus–within the fabaceous forb Astragalus lentiginosus. We report that rare, low-biomass endophytes affected host size and foliar %N, but only when the heritable fungal endophyte (A. fulva) was not present. A. fulva also reduced plant size and %N, but these deleterious effects on the host could be offset by a negative association we observed between this heritable fungus and a foliar pathogen. These results demonstrate how interactions among endophytic taxa determine the net effects on host plants and suggest that the myriad rare endophytes within plant leaves may be more than a collection of uninfluential, commensal organisms, but instead have meaningful ecological roles.