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Summary: Pseudomonas donghuensis strain SVBP6, an isolate from an agricultural plot in Argentina, displays a broad‐spectrum and diffusible antifungal activity, which requires a functional gacS gene but could not be ascribed yet to known secondary metabolites typical of Pseudomonas biocontrol species. Here, we report that Tn5 mutagenesis allowed the identification of a gene cluster involved in both the fungal antagonism and the production of a soluble tropolonoid compound. The ethyl acetate extract from culture supernatant showed a dose‐dependent inhibitory effect against the phytopathogenic fungus Macrophomina phaseolina. The main compound present in the organic extract was identified by spectroscopic and X‐ray analyses as 7‐hydroxytropolone (7HT). Its structure and tautomerism was confirmed by preparing the two key derivatives 2,3‐dimethoxy‐ and 2,7‐dimethoxy‐tropone. 7HT, but not 2,3‐ or 2,7‐dimethoxy‐tropone, mimicked the fungal inhibitory activity of the ethyl acetate extract from culture supernatant. The activity of 7HT, as well as its production, was barely affected by the presence of up to 50 μM added iron (Fe+2). To summarize, P. donghuensis SVBP6 produces 7HT under the positive control of the Gac‐Rsm cascade and is the main active metabolite responsible for the broad‐spectrum inhibition of different phytopathogenic fungi. [ABSTRACT FROM AUTHOR]

An O -specific polysaccharide (OPS) was isolated from the lipopolysaccharide (LPS) of Pseudomonas donghuensis SVBP6, a bacterium with a broad-spectrum antifungal activity in vitro , particularly that against Macrophomina phaseolina. This latter is one of the most virulent and dangerous pathogens of plants, including soybean which is an economically important crop in Argentina today. The OPS was studied by sugar analysis and spectroscopy (1D and 2D 1H and 13C NMR) showing the following trisaccharide repeating unit: →6)-ɑ-D-Man p NAc-(1 → 3)-β- l -Rha p -(1 → 4)-β-D-Glc p -(1→. The crude LPS, the purified LPS and the O -chain were assayed for their antifungal activity against M. phaseolina at 25, 50, 100, and 200 μg plug−1. The results showed that the crude LPS best inhibition was at 200 μg plug−1, able to inhibit the fungus growth by about 45%, while purified LPS and the corresponding OPS, in the same condition, reduced fungus growth by 65%, and 75%, respectively. Furthermore, the purified LPS and OPS significantly reduced the growth of M. phaseolina already at 100 μg plug−1 compared to the crude LPS. The structure of the O -chain is unique among the bacterial LPS and this is the first time that both the antifungal activity of a bacterial LPS and its corresponding O -chain were described. • Unique O -chain (OPS) structure of Pseudomonas donghuensis (Pd) lipopolysaccharide (LPS) • First report on the antifungal activityof Pd LPS/OPS against Macrophomina phaseolina • The OPS showed a repeating unit constituted by α-ManNAc, β-Rha and β-Glc [ABSTRACT FROM AUTHOR]

A bioactive disubstituted nonenolide, named truncatenolide, was produced by Colletotrichum truncatum , which was collected from infected tissues of soybean showing anthracnose symptoms in Argentina. This is a devastating disease that drastically reduces the yield of soybean production in the world. The fungus also produced a new trisubstituted oct-2-en-4-one, named truncatenone, and the well-known tyrosol and N- acetyltyramine. Truncatenolide and truncatenone were characterized by spectroscopic (essentially one-dimensional (1D) and two-dimensional (2D) 1 H and 13 C NMR and HR ESIMS) and chemical methods as (5 E, 7 R, 10 R )-7-hydroxy-10-methyl-3,4,7,8,9,10-hexahydro-2 H -oxecin-2-one and ( Z )-6-hydroxy-3,5-dimethyloct-2-en-4-one, respectively. The geometry of the double bond of truncatenolide was assigned by the value of olefinic proton coupling constant and that of truncatenone by the correlation observed in the corresponding NOESY spectrum. The relative configuration of each stereogenic center was assigned with the help of 13 C chemical shift and 1 H- 1 H scalar coupling DFT calculations, while the absolute configuration assignment of truncatenolide was performed by electronic circular dichroism (ECD). When tested on soybean seeds, truncatenolide showed the strongest phytotoxic activity. Tyrosol and N -acetyltyramine also showed phytotoxicity to a lesser extent, while truncatenone weakly stimulated the growth of the seed root in comparison to the control. When assayed against Macrophomina phaseolina and Cercospora nicotianae , other severe pathogens of soybean, truncatenolide showed significant activity against M. phaseolina and total inhibition of C. nicotianae . Thus, some other fungal nonenolides and their derivatives were assayed for their antifungal activity against both fungi in comparison with truncatenolide. Pinolidoxin showed to a less extent antifungal activity against both fungi, while modiolide A selectively and totally inhibited only the growth of C. nicotianae . The SAR results and the potential of truncatenolide, modiolide A, and pinolidoxin as biofungicides were also discussed.

Two new penta- and tetrasubstituted cyclopentenones, named phaseocyclopentenones A and B ( 1 and 2 ), together with guignardone A ( 3 ), were isolated from Macrophomina phaseolina cultures. The phytopathogenic fungus was isolated from infected soybean tissues showing charcoal rot symptoms in Argentina. Charcoal rot is a devastating disease considering that soybean is one of the main legumes cultivated in the world. Phaseocyclopentenones A and B were characterized by 1D and 2D 1 H and 13 C NMR spectroscopic and HRESIMS spectrometric data and chemical methods as 4-benzoyl-3,4,5-trihydroxy-2-phenylcyclopent-2-enone and 3,5-dihydroxy-2,4-diphenylcyclopent-2-enone, respectively. The relative configuration of phaseocyclopentenones A and B was assigned by 1 H and NOESY NMR methods, while their absolute configurations were assigned by electronic circular dichroism methods. When assayed on a nonhost plant ( Solanum lycopersicum L.) by the leaf puncture assay, phaseocyclopentenones A and B and guignardone A showed phytotoxic activity, while only 1 and 2 were toxic when tested on cuttings of the same plant. No phytotoxicity or antifungal activity was detected for the three compounds on the host plant soybean ( Glycine max L.) and against some of its fungal pathogens, namely, Cercospora nicotianae and Colletotrichum truncatum , also isolated from infected soybean plants in Argentina.