7‐hydroxytropolone is the main metabolite responsible for the fungal antagonism of Pseudomonas donghuensis strain SVBP6

Abstract

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.

Pseudomonas species isolated via high-throughput screening significantly protect cotton plants against verticillium wilt

“…In addition, our P. donghuensis 22G5 showed a higher amount of 7-HT production (the OD 330 of 7-HT produced by 22G5 reached 4 to 5 when the OD 600 of the culture reached 2) (Fig. 4 d) compared with that reported for P. donghuensis SVBP6 (Muzio et al 2020 ) (the OD 330 of 7-HT produced by SVBP6 reached 1 when the OD 600 of the culture reached 2). Moreover, our data from the in planta assay first showed the effectiveness of P. donghuensis against V. dahliae , and this further supported its applications in the development of BCAs for VW manipulation in cotton farming.…”

“…3 b). We further compared P. protegens XY2F4 and P. donghuensis 22G5 genomes respectively with more published strains in the same species, including two well-documented P. protegens strains (Paulsen et al 2005 ; Shaukat and Siddiqui 2003 ) and three published P. donghuensis strains to date (Chen et al 2018 ; Muzio et al 2020 ; Ossowicki et al 2017 ). Results indicated that the P. protegens XY2F4 genome had 835 specific genes compared with P. protegens CHA0 and Pf-5 (Fig.…”

“…3 e), which is well documented in its type strain P. protegens CHA0 and Pf-5 . P. donghuensis 22G5 has a gene cluster for biosynthesis of specific siderophore metabolite 7-HT, a metabolite common to P. donghuensis species which was previously reported to have fungal antagonism activity (Chen et al 2018 ; Muzio et al 2020 ; Ossowicki et al 2017 ). We inferred that 7-HT with broad spectrum might account for their inhibitory action against V. dahliae .…”

“…7-HT was first reported as a siderophore in P. donghuensis HYS in 2016 (Jiang et al 2016 ), and the gene cluster responsible for its production has been well established (Chen et al 2018 ; Krzyżanowska et al 2016 ). Recently, 7-HT was found to be the main metabolite responsible for the fungal antagonism of P. donghuensis SVBP6 by testing its antagonism activities against Macrophomina phaseolina , Fusarium graminearum , Fusarium semitectum and Bacillus subtilis (Muzio et al 2020 ). The antagonism results were obtained from the growth inhibition of phytopathogenic fungi when they were co-cultured/co-inoculated with P. donghuensis SVBP6 or its supernatant.…”

Biocontrol Traits Correlate With Resistance to Predation by Protists in Soil Pseudomonads

“…NL81, which was in contrast to previous studies reporting adverse effects of pyoluteorin against protists (Winding et al, 2004;Jousset et al, 2006). An additional candidate antibiotic compound, 7-hydroxytropolone, was recently reported for P. donghuensis SVBP6 to be at the origin of the broad-spectrum in vitro antifungal activity displayed by this bacterium (Muzio et al, 2020). Tropolone and products containing tropolonoid motifs display antimicrobial activities attributed to their metal-chelating and redox properties (Guo et al, 2019) and derivatives have shown some anti-protozoan activities (Ren et al, 2003).…”