Phylogenetic Comparison of Swainsonine Biosynthetic Gene Clusters among Fungi

Analysis of the Mycotoxin Levels and Expression Pattern of SWN Genes at Different Time Points in the Fungus Slafractonia leguminicola

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.

Transcriptomic Screening of Alternaria oxytropis Isolated from Locoweed Plants for Genes Involved in Mycotoxin Swaisonine Production

Locoweed is a collective name for a variety of plants, such as Oxytropis and Astragalus L. When these plants are infected by some fungi or endophytes, they will produce an alkaloid (swainsonine) that is harmful to livestock. Chronic toxicity characterized by neurological disorders occurs in livestock overfed on locoweed, and swainsonine (SW) is considered a major toxic component. The mechanism of the SW synthesis of endophytic fungi from locoweed remains unknown. In order to further discover the possible synthetic pathway of SW, in this study, a mycotoxin (SW) producer, Alternaria oxytropis isolate, UA003, isolated from Locoweed plants, and its mutant were subjected to transcriptomic analyses to ascertain the genes involved in the synthesis of this toxin. Mutant strain A. oxytropis E02 was obtained by ethyl methanesulfonate (EMS) mutagenesis treatment, and the strains were sequenced with different culture times for transcriptomic analysis and screening of differentially expressed genes. The results show a highly significant (p < 0.01) increase in SW yield in the A. oxytropis E02 strain obtained by EMS mutagenesis treatment compared to A. oxytropis UA003. A total of 637 differentially expressed genes were screened by transcriptome sequencing analysis, including 11 genes potentially associated with SW biosynthesis. These genes were screened using GO and KEGG data annotation and analysis. Among the differential genes, evm.TU.Contig4.409, evm.TU.Contig19.10, and evm.TU.Contig50.48 were associated with L-lysine biosynthesis, the L-pipecolic acid pathway, and the α-aminoadipic acid synthesis pathway. This study provides new insights to elucidate the mechanism of SW synthesis of endophytic fungi in locoweed and provides data support for further exploration of A. oxytropis genomics studies.

Construction of Yeast One-Hybrid Library of Alternaria oxytropis and Screening of Transcription Factors Regulating swnK Gene Expression

The indolizidine alkaloid-swainsonine (SW) is the main toxic component of locoweeds and the main cause of locoweed poisoning in grazing animals. The endophytic fungi, Alternaria Section Undifilum spp., are responsible for the biosynthesis of SW in locoweeds. The swnK gene is a multifunctional complex enzyme encoding gene in fungal SW biosynthesis, and its encoding product plays a key role in the multistep catalytic synthesis of SW by fungi using pipecolic acid as a precursor. However, the transcriptional regulation mechanism of the swnK gene is still unclear. To identify the transcriptional regulators involved in the swnK gene in endophytic fungi of locoweeds, we first analyzed the upstream non-coding region of the swnK gene in the A. oxytropis UA003 strain and predicted its high transcriptional activity region combined with dual-luciferase reporter assay. Then, a yeast one-hybrid library of A. oxytropis UA003 strain was constructed, and the transcriptional regulatory factors that may bind to the high-transcriptional activity region of the upstream non-coding region of the swnK gene were screened by this system. The results showed that the high transcriptional activity region was located at -656 bp and -392 bp of the upstream regulatory region of the swnK gene. A total of nine candidate transcriptional regulator molecules, including a C2H2 type transcription factor, seven annotated proteins, and an unannotated protein, were screened out through the Y1H system, which were bound to the upstream high transcriptional activity region of the swnK gene. This study provides new insight into the transcriptional regulation of the swnK gene and lays the foundation for further exploration of the regulatory mechanisms of SW biosynthesis in fungal endophytic locoweeds.

Silencing of the Transmembrane Transporter (swnT) Gene of the Fungus Slafractonia leguminicola Results in a Reduction of Mycotoxin Transport

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.

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.