Swainsonine (SW) is a toxic alkaloid biosynthesized by the endophytic fungus Alternaria oxytropis in Oxytropis glabra. The biosynthetic pathway of SW is poorly understood. Saccharopine reductase/dehydrogenase of fungus plays an important role in this pathway. The gene knocked out mutant M1 in A. oxytropis was constructed in our previous work. In this study, the transcriptome of wild-strain OW7.8 and M1 was firstly sequenced to understand the biosynthetic pathway and molecular mechanism of SW in A. oxytropis. A total of 45,634 Unigenes were annotated. 5 genes were up-regulated and 11,213 genes were down-regulated. 41 Unigenes possibly related to the biosynthesis of SW were identified by data analyzing. The biosynthesis pathway of SW in the fungus was speculated, including two branches of P6C and P2C. Delta1-piperidine-2-carboxylate reductase, lysine 6-dehydrogenase, and saccharopine oxidase/L-pipecolate oxidase were involved in P6C. 1-piperidine-2-carboxylate/1-pyrroline-2- carboxylate reductase [NAD(P)H] and delta1-piperidine-2-carboxylate reductase were involved in P2C. Saccharopine reductase was involved in both. In addition, 1-indolizidineone was considered to be the direct precursor in the synthesis of SW, and the hydroxymethylglutaryl-CoA lyase catalyzed the synthesis of SW. Here we analyzed details of the metabolic pathway of A. oxytropis SW, which is of great significance for the follow-up research.

Locoweeds, including Oxytropis and Astragalus species, are globally recognized as plants containing swainsonine (SW), a neurotoxic alkaloid that induces neurological dysfunction and growth inhibition in livestock. SW is produced by endophytic fungi in plants; the pyrroline-5-carboxylate reductase (P5CR) gene is critical in the fungal SW biosynthetic pathway. In this study, a P5CR gene knockout mutant (ΔP5CR) was constructed from the endophytic fungus Alternaria oxytropis OW7.8 isolated from Oxytropis glabra. Compared to the wild-type strain (A. oxytropis OW7.8), the SW content in the ΔP5CR mycelia was significantly reduced, indicating that the P5CR gene plays a crucial role in promoting SW biosynthesis. Compared to the wild-type strain A. oxytropis OW7.8, the ΔP5CR mutant exhibited distinct morphological alterations in both colony and mycelial structures. The transcriptomic analysis of A. oxytropis OW7.8 and ΔP5CR revealed the downregulation of six genes associated with SW biosynthesis. Metabolomic profiling further demonstrated altered levels of six metabolites linked to SW synthesis. These findings provide foundational insights into the molecular mechanisms and metabolic pathways underlying SW biosynthesis in fungi. They hold significant value for future strategies to control SW in Oxytropis glabra and contribute positively to the protection and sustainable development of grassland ecosystems.

The swnN gene in the endophytic fungus Alternaria oxytropis OW 7.8 isolated from Oxytropis glabra was identified, and the gene knockout mutant ΔswnN was first constructed in this study. Compared with A. oxytropis OW 7.8, the ΔswnN mutant exhibited altered colony and mycelia morphology, slower growth rate, and no swainsonine (SW) in mycelia. SW was detected in the gene function complementation strain ΔswnN/swnN, indicating that the function of the swnN gene promoted SW biosynthesis. Six differentially expressed genes (DEGs) closely associated with SW synthesis were identified by transcriptomic analysis of A. oxytropis OW 7.8 and ΔswnN, with P5CR, swnR, swnK, swnH2, and swnH1 down-regulating, and sac up-regulating. The expression levels of the six genes were consistent with the transcriptomic analysis results. Five differential metabolites (DEMs) closely associated with SW synthesis were identified by metabolomic analysis, with L-glutamate, α-ketoglutaric acid, and L-proline up-regulating, and phosphatidic acid (PA) and 2-aminoadipic acid down-regulating. The SW biosynthetic pathways in A. oxytropis OW 7.8 were predicted and refined. The results lay the foundation for in-depth elucidation of molecular mechanisms and the SW synthesis pathway in fungi. They are also of importance for the prevention of locoism in livestock, the control and utilization of locoweeds, and the protection and sustainable development of grassland ecosystems.

Genomic sequencing of Alternaria oxytropis OW 7.8 isolated from Oxytropis glabra was performed, the swnK gene was cloned and knocked out firstly. The colony morphology of ΔswnK-ACP differed from that of A. oxytropis OW 7.8, appearing milky white or pale yellow, with irregular shapes and a slower growth rate. SW was not detected in ΔswnK-ACP, indicating that the swnK gene promotes SW synthesis in A. oxytropis OW 7.8. 8 differentially expressed genes (DEGs) and 7 differentially expressed metabolites (DEMs) closely associated with SW biosynthesis were identified in the transcriptome and metabolome analysis of A. oxytropis OW 7.8 and ΔswnK-ACP. The SW biosynthesis pathway in A. oxytropis OW 7.8 was predicted and refined. These results may lay foundation of in-depth analysis of the molecular mechanisms and metabolic pathway of SW synthesis in fungi, and provide reference for future control of SW in locoweed, which would benefit the development of grassland animal husbandry and the sustainable use of grassland ecosystem.

The swnH1 gene in the endophytic fungus Alternaria oxytropis OW 7.8 isolated from Oxytropis glabra was identified, and the gene knockout mutant ΔswnH1 was first constructed in this study. Compared with A. oxytropis OW 7.8, the ΔswnH1 mutant exhibited altered colony and mycelium morphology, slower growth rate, and no swainsonine (SW) in mycelia, indicating that the function of the swnH1 gene promoted SW biosynthesis. Five differential expressed genes (DEGs) closely associated with SW synthesis were identified by transcriptomic analysis of A. oxytropis OW 7.8 and ΔswnH1, with sac, swnR, swnK, swnN, and swnH2 down-regulating. Six differential metabolites (DEMs) closely associated with SW synthesis were identified by metabolomic analysis, with P450, PKS-NRPS, saccharopine, lipopolysaccharide kinase, L-PA, α-aminoadipic, and L-stachydrine down-regulated, while L-proline was up-regulated. The SW biosynthetic pathways in A. oxytropis OW 7.8 were predicted and refined. The results lay the foundation for in-depth exploration of the molecular mechanisms and metabolic pathways of SW synthesis in fungi and provide reference for future control of SW in locoweeds, which would benefit the development of animal husbandry and the sustainable use of grassland ecosystems.

The swnR gene was cloned in the endophytic fungus Alternaria oxytropis OW 7.8 isolated from Oxytropis glabra, and the gene knockout mutant ΔswnR was first constructed in this study. Compared with A. oxytropis OW 7.8, the ΔswnR exhibited distinct morphological alterations in both colony and mycelial structure, a slower growth rate, and significant reductions in swainsonine (SW) levels, indicating that the function of the swnR gene promoted SW biosynthesis. Six differentially expressed genes (DEGs) closely associated with SW synthesis were identified by transcriptomic analysis of A. oxytropis OW 7.8 and ΔswnR, with P5CR, swnR, swnK, swnH2, and swnH1 downregulating, and sac upregulating. The expression levels of the six genes were consistent with the transcriptomic analysis results. Five differential metabolites (DEMs) closely associated with SW synthesis were identified by metabolomic analysis, with L-Lys, L-Glutamic acid, Saccharopine, and L-Proline upregulating, and L-PA downregulating. The results lay the foundation for the in-depth elucidation of molecular mechanisms and SW synthesis pathways in fungi, and are also of importance for the prevention of locoism in livestock, the control and utilization of locoweeds, and the protection and sustainable development of grassland ecosystems.