Aminotransferase SsAro8 Regulates Tryptophan Metabolism Essential for Filamentous Growth of Sugarcane Smut Fungus Sporisorium scitamineum

Polyamine oxidation enzymes regulate sexual mating/filamentation and pathogenicity in Sporisorium scitamineum

Sugarcane smut fungus Sporisorium scitamineum produces polyamines putrescine (PUT), spermidine (SPD), and spermine (SPM) to regulate sexual mating/filamentous growth critical for pathogenicity. Besides de novo biosynthesis, intracellular levels of polyamines could also be modulated by oxidation. In this study, we identified two annotated polyamine oxidation enzymes (SsPAO and SsCuAO1) in S. scitamineum. Compared to the wild type (MAT-1), the ss1paoΔ and ss1cuao1Δ mutants were defective in sporidia growth, sexual mating/filamentation, and pathogenicity. The addition of a low concentration of cAMP (0.1 mM) could partially or fully restore filamentation of ss1paoΔ × ss2paoΔ or ss1cuao1Δ × ss2cuao1Δ. cAMP biosynthesis and hydrolysis genes were differentially expressed in the ss1paoΔ × ss2paoΔ or ss1cuao1Δ × ss2cuao1Δ cultures, further supporting that SsPAO- or SsCuAO1-based polyamine homeostasis regulates S. scitamineum filamentation by affecting the cAMP/PKA signalling pathway. During early infection, PUT promotes, while SPD inhibits, the accumulation of reactive oxygen species (ROS) in sugarcane, therefore modulating redox homeostasis at the smut fungus-sugarcane interface. Autophagy induction was found to be enhanced in the ss1paoΔ mutant and reduced in the ss1cuao1Δ mutant. Exogenous addition of cAMP, PUT, SPD, or SPM at low concentration promoted autophagy activity under a non-inductive condition (rich medium), suggesting a cross-talk between polyamines and cAMP signalling in regulating autophagy in S. scitamineum. Overall, our work proves that SsPAO- and SsCuAO1-mediated intracellular polyamines affect intracellular redox balance and thus play a role in growth, sexual mating/filamentation, and pathogenicity of S. scitamineum.

Quantifying chemical correlations between fruits and processed fruit products: A non-targeted analysis approach

Juices and beverages are produced by industry for long-distance distribution and shelf-stability, providing valuable nutrients. However, their nutritional value is often underestimated due to insufficient analytical methods. We have employed non-targeted analysis through a standardized analytical protocol, taking advantage of Data Independent Acquisition (DIA) technique and a novel Chromatographic Retention Behavior (CRB) data deconvolution algorithm. After analyzing 9 fruits and their products, correlations between fruits and their juices are accurately digitalized by similarities of their LC-MS fingerprints. We also specify non-targeted molecules primarily associate with nutrient loss in these analyzed juice products, including nitrogenous nutrients, flavonoids, glycosides, and vitamins. Moreover, we unveiled previously unreported fruit-characteristic metabolites, of which reconstituted-from-concentrate (RFC) juices contain over 40% of the content found in their fresh counterparts. Conclusively, our method establishes a quantitative benchmark for rational selection of RFC juices to substitute natural fruits.

An efficient inoculation method to evaluate virulence differentiation of field strains of sugarcane smut fungus

Sugarcane smut, caused by the fungal pathogen Sporisorium scitamineum, is a prominent threat to the sugarcane industry. The development of smut resistant varieties is the ultimate solution for controlling this disease, due to the lack of other efficient control methods. Artificial inoculation method is used to evaluate the virulence differentiation of pathogens. The mostly used artificial inoculation methods are soaking of the seed canes in the teliospore solution and injection of teliospores or haploid sporidia into the sugarcane sprouts. However, due to the infection nature of the pathogen that invades the sugarcane plant through meristem tissue of the sprout or shoot, the rate of successful infection is often low and fluctuated, resulting in low confidence of the assays. We recently reported a rapid and high-throughput inoculation method called plantlet soaking by using tissue culture-derived sugarcane plantlets as the test plants. Here, we compare different inoculation methods and report the characterization of parameters that may affect the sensitivity and efficiency of the plantlet soaking technique. The results showed that sugarcane plantlets were highly vulnerable to infection, even with the inoculum density at 6.0 × 105 basidial spores/ml, and this method could be applied to all varieties tested. Notably, varieties showing high smut resistance in the field exhibited high susceptibility when inoculated with the plantlet soaking method, suggesting that the plantlet soaking method is a good complement to the traditional methods for screening germplasms with internal resistance. In addition, this method could also be used to monitor the variation of cellular virulence of the smut pathogen strains in the field.

Intracellular polyamines regulate redox homeostasis with cAMP-PKA signalling during sexual mating/filamentation and pathogenicity of Sporisorium scitamineum

Sugarcane smut caused by Sporisorium scitamineum seriously impairs sugarcane production and quality. Sexual mating/filamentation is a critical step of S. scitamineum pathogenesis, yet the regulatory mechanisms are not fully understood. In this study, we identified the SsAGA, SsODC, and SsSAMDC genes, which are involved in polyamine biosynthesis in S. scitamineum. Deletion of SsODC led to complete loss of filamentous growth after sexual mating, and deletion of SsAGA or SsSAMDC caused reduced filamentation. Double deletion of SsODC and SsSAMDC resulted in auxotrophy for putrescine (PUT) and spermidine (SPD) when grown on minimal medium (MM), indicating that these two genes encode enzymes that are critical for PUT and SPD biosynthesis. We further showed that low PUT concentrations promoted S. scitamineum filamentation, while high PUT concentrations suppressed filamentation. Disrupted fungal polyamine biosynthesis also resulted in a loss of pathogenicity and reduced fungal biomass within infected plants at the early infection stage. SPD formed a gradient from the diseased part to nonsymptom parts of the cane stem, suggesting that SPD is probably favourable for fungal virulence. Mutants of the cAMP-PKA (SsGPA3-SsUAC1-SsADR1) signalling pathway displayed up-regulation of the SsODC gene and elevated intracellular levels of PUT. SsODC directly interacted with SsGPA3, and sporidia of the ss1uac1ΔodcΔ mutant displayed abundant pseudohyphae. Furthermore, we found that elevated PUT levels caused accumulation of intracellular reactive oxygen species (ROS), probably by suppressing transcription of ROS-scavenging enzymes, while SPD played the opposite role. Overall, our work proves that polyamines play important roles in the pathogenic development of sugarcane smut fungus, probably by collaboratively regulating intracellular redox homeostasis with the cAMP-PKA signalling pathway.

Cytochrome c-peroxidase modulates ROS homeostasis to regulate the sexual mating of Sporisorium scitamineum

IMPORTANCE

Reactive oxygen species play an important role in pathogen-plant interactions. In fungi, cytochrome c-peroxidase maintains intracellular ROS homeostasis by utilizing H2O2 as an electron acceptor to oxidize ferrocytochrome c, thereby contributing to disease pathogenesis. In this study, our investigation reveals that the cytochrome c-peroxidase encoding gene, SsCCP1, not only plays a key role in resisting H2O2 toxicity but is also essential for the mating/filamentation and pathogenicity of S. scitamineum. We further uncover that SsCcp1 mediates the expression of SsPrf1 by maintaining intracellular ROS homeostasis to regulate S. scitamineum mating/filamentation. Our findings provide novel insights into how cytochrome c-peroxidase regulates sexual reproduction in phytopathogenic fungi, presenting a theoretical foundation for designing new disease control strategies.

Manipulation of Auxin Signaling by Smut Fungi during Plant Colonization

A common feature of many plant-colonizing organisms is the exploitation of plant signaling and developmental pathways to successfully establish and proliferate in their hosts. Auxins are central plant growth hormones, and their signaling is heavily interlinked with plant development and immunity responses. Smuts, as one of the largest groups in basidiomycetes, are biotrophic specialists that successfully manipulate their host plants and cause fascinating phenotypes in so far largely enigmatic ways. This review gives an overview of the growing understanding of how and why smut fungi target the central and conserved auxin growth signaling pathways in plants.

The Mechanism of Transcription Factor Swi6 in Regulating Growth and Pathogenicity of Ceratocystis fimbriata: Insights from Non-Targeted Metabolomics

Ceratocystis fimbriata (C. fimbriata) is a notorious pathogenic fungus that causes sweet potato black rot disease. The APSES transcription factor Swi6 in fungi is located downstream of the cell wall integrity (CWI)-mitogen-activated protein kinase (MAPK) signaling pathway and has been identified to be involved in cell wall integrity and virulence in several filamentous pathogenic fungi. However, the specific mechanisms by which Swi6 regulates the growth and pathogenicity of plant pathogenic fungi remain elusive. In this study, the SWI6 deletion mutants and complemented strains of C. fimbriata were generated. Deletion of Swi6 in C. fimbriata resulted in aberrant growth patterns. Pathogenicity assays on sweet potato storage roots revealed a significant decrease in virulence in the mutant. Non-targeted metabolomic analysis using LC-MS identified a total of 692 potential differentially accumulated metabolites (PDAMs) in the ∆Cfswi6 mutant compared to the wild type, and the results of KEGG enrichment analysis demonstrated significant enrichment of PDAMs within various metabolic pathways, including amino acid metabolism, lipid metabolism, nucleotide metabolism, GPI-anchored protein synthesis, and ABC transporter metabolism. These metabolic pathways were believed to play a crucial role in mediating the growth and pathogenicity of C. fimbriata through the regulation of CWI. Firstly, the deletion of the SWI6 gene led to abnormal amino acid and lipid metabolism, potentially exacerbating energy storage imbalance. Secondly, significant enrichment of metabolites related to GPI-anchored protein biosynthesis implied compromised cell wall integrity. Lastly, disruption of ABC transport protein metabolism may hinder intracellular transmembrane transport. Importantly, this study represents the first investigation into the potential regulatory mechanisms of SWI6 in plant filamentous pathogenic fungi from a metabolic perspective. The findings provide novel insights into the role of SWI6 in the growth and virulence of C. fimbriata, highlighting its potential as a target for controlling this pathogen.

Biosynthetic Pathways and Functions of Indole-3-Acetic Acid in Microorganisms

Indole-3-acetic acid (IAA) belongs to the family of auxin indole derivatives. IAA regulates almost all aspects of plant growth and development, and is one of the most important plant hormones. In microorganisms too, IAA plays an important role in growth, development, and even plant interaction. Therefore, mechanism studies on the biosynthesis and functions of IAA in microorganisms can promote the production and utilization of IAA in agriculture. This mini-review mainly summarizes the biosynthesis pathways that have been reported in microorganisms, including the indole-3-acetamide pathway, indole-3-pyruvate pathway, tryptamine pathway, indole-3-acetonitrile pathway, tryptophan side chain oxidase pathway, and non-tryptophan dependent pathway. Some pathways interact with each other through common key genes to constitute a network of IAA biosynthesis. In addition, functional studies of IAA in microorganisms, divided into three categories, have also been summarized: the effects on microorganisms, the virulence on plants, and the beneficial impacts on plants.

Recent Advancements in Pathogenic Mechanisms, Applications and Strategies for Entomopathogenic Fungi in Mosquito Biocontrol

Fungal diseases are widespread among insects and play a crucial role in naturally regulating insect populations. Mosquitoes, known as vectors for numerous infectious diseases, pose a significant threat to human health. Entomopathogenic fungi (EPF) have emerged as highly promising alternative agents to chemical mosquitocides for controlling mosquitoes at all stages of their life cycle due to their unique infection pathway through direct contact with the insect's cuticle. In recent years, significant advancements have been made in understanding the infection pathways and pathogenic mechanisms of EPF against mosquitoes. Various strategies involving the use of EPF alone or combinations with other approaches have been employed to target mosquitoes at various developmental stages. Moreover, the application of genetic technologies in fungi has opened up new avenues for enhancing the mosquitocidal efficacy of EPF. This review presents a comprehensive summary of recent advancements in our understanding the pathogenic mechanisms of EPF, their applications in mosquito management, and the combination of EPF with other approaches and employment of transgenic technologies. The biosafety concerns associated with their use and the corresponding approaches are also discussed. The recent progress suggests that EPF have the potential to serve as a future biorational tool for controlling mosquito vectors.