Postharvest sclerotinia rot control in carrot by the natural product hinokitiol and the potential mechanisms involved

Efficacy and toxic action of the natural product natamycin against Sclerotinia sclerotiorum

BACKGROUND

Sclerotinia stem rot caused by Sclerotinia sclerotiorum seriously endangers oilseed rape production worldwide, and the occurrence of fungicide-resistant mutants of S. sclerotiorum leads to control decline. Thus, it is critical to explore new green substitutes with different action mechanisms and high antifungal activity. Herein, the activity and the action mechanism of natamycin against S. sclerotiorum were evaluated.

RESULTS

Natamycin showed potent inhibition on the mycelial growth of S. sclerotiorum, and half-maximal effective concentration (EC50 ) values against 103 S. sclerotiorum strains ranged from 0.53 to 4.04 μg/mL (mean 1.44 μg/mL). Natamycin also exhibited high efficacy against both carbendazim- and dimethachlone-resistant strains of S. sclerotiorum on detached oilseed rape leaves. No cross-resistance was detected between natamycin and carbendazim. Natamycin markedly disrupted hyphal form, sclerotia formation, integrity of the cell membrane, and reduced the content of oxalic acid and ergosterol, whereas it increased the reactive oxygen species (ROS) and malondialdehyde content. Interestingly, exogenous addition of ergosterol could reduce the inhibition of natamycin against S. sclerotiorum. Importantly, natamycin significantly inhibited expression of the Cyp51 gene, which is contrary to results for the triazole fungicide flusilazole, indicating a different action mechanism from triazole fungicides.

CONCLUSION

Natamycin is a promising effective candidate for the resistance management of S. sclerotiorum. © 2023 Society of Chemical Industry.

The schizotrophic lifestyle of Sclerotinia sclerotiorum

Sclerotinia sclerotiorum is a cosmopolitan and typical necrotrophic phytopathogenic fungus that infects hundreds of plant species. Because no cultivars highly resistant to S. sclerotiorum are available, managing Sclerotinia disease caused by S. sclerotiorum is still challenging. However, recent studies have demonstrated that S. sclerotiorum has a beneficial effect and can live mutualistically as an endophyte in graminaceous plants, protecting the plants against major fungal diseases. An in-depth understanding of the schizotrophic lifestyle of S. sclerotiorum during interactions with plants under different environmental conditions will provide new strategies for controlling fungal disease. In this review, we summarize the pathogenesis mechanisms of S. sclerotiorum during its attack of host plants as a destructive pathogen and discuss its lifestyle as a beneficial endophytic fungus.

Natural terpene-based derivatives to control postharvest sclerotinia rot and the involved potential mechanism

In an effort to develop novel postharvest preservatives of satisfactory environmental compatibility from natural monoterpene, a series of terpene-based derivatives containing oxime ester were designed and prepared. In this research, the inhibitory effect of target compounds against S. sclerotiorum were evaluated though in vitro and in vivo tests. It was investigated that most compounds exhibited promising antifungal activity, especially compound 4k with EC50 value of 3.02 μg/mL, which was significantly superior to commercial fungicide trifloxystrobin. Notably, compound 4k improved the physicochemical quality of carrot including weight loss, contents of titratable acidity, ascorbic acid, carotenoid, malondialdehyde, and enzymes activities, ensuring post-harvest preservation. Simultaneously, the mycelial morphology, ultrastructure, cell wall permeability, and defense/respiration-related enzymes of S. sclerotiorum were destructed. The preliminary toxicity evaluation of target compounds indicated that the prepared target compounds possessed safety and low toxicity. Additionally, the essential chemical features for activity and interaction mode between molecule and cytochrome bc1 complex were inquired by computer-aid technology. The study provided meaningful insight into formulation of natural terpene-based fresh-keeping agent to resist postharvest decay infected by S. sclerotiorum.

Discovery of Terpene-Derived Quaternary Ring Compounds Containing an Oxime Moiety as Potential Fungicides

Terpene-derived quaternary ring compounds with an oxime moiety were designed and prepared to create fungicides from natural products. A preliminary assessment of their antifungal activity against seven common pathogenic fungi was conducted, and the median effective concentration (EC50) values against Rhizoctonia solani were obtained. The effects of compound 6a19 (3-bromothiophene-containing), which had an outstanding EC50 value (1.62 μg/mL), on the morphology, ultrastructure, reactive oxygen species production, mitochondrial membrane potential, nuclear morphology, and defense-related and respiration-related enzyme activities of mycelia were evaluated. The test compound was speculated to obstruct the bio-oxidative process, inhibiting mycelial growth. Compound 6a19 exhibited a satisfactory in vivo control effect on leaf sheath-infected rice plants. After treating rice plants with 50, 100, and 200 μg/mL 6a19, the protective and therapeutic efficacy values were 48.3 and 70.3%, 58.6 and 75.7%, and 69.0 and 81.1%, respectively. Moreover, a linear quantitative structure-activity relationship (R2 = 0.932, F = 61.3, and S2 = 0.020) was established using density functional theory calculations. Four chemical descriptors that were crucial to the antifungal activity were analyzed: the number of occupied electronic levels of atoms, the minimum atomic orbital electronic population, maximum net atom charge for a H atom, and minimum net atomic charge. In overall consideration of experimental results, it was speculated that the target compounds satisfactorily inhibited R. solani by interfering with biological oxidation pathways, which provided an insight into the future intensive and systematic action mechanism. This research is promising for the invention of novel fungicides from natural terpenes with multiple potential targets and satisfactory ecological compatibility.

Structural characteristics and diversity of the rhizosphere bacterial communities of wild Fritillaria przewalskii Maxim. in the northeastern Tibetan Plateau

Introduction

Fritillaria przewalskii Maxim. is a Chinese endemic species with high medicinal value distributed in the northeastern part of the Tibetan Plateau. F. przewalskii root-associated rhizosphere bacterial communities shaped by soil properties may maintain the stability of soil structure and regulate F. przewalskii growth, but the rhizosphere bacterial community structure of wild F. przewalskii from natural populations is not clear.

Methods

In the current study, soil samples from 12 sites within the natural range of wild F. przewalskii were collected to investigate the compositions of bacterial communities via high-throughput sequencing of 16S rRNA genes and multivariate statistical analysis combined with soil properties and plant phenotypic characteristics.

Results

Bacterial communities varied between rhizosphere and bulk soil, and also between sites. Co-occurrence networks were more complex in rhizosphere soil (1,169 edges) than in bulk soil (676 edges). There were differences in bacterial communities between regions, including diversity and composition. Proteobacteria (26.47-37.61%), Bacteroidetes (10.53-25.22%), and Acidobacteria (10.45-23.54%) were the dominant bacteria, and all are associated with nutrient cycling. In multivariate statistical analysis, both soil properties and plant phenotypic characteristics were significantly associated with the bacterial community (p < 0.05). Soil physicochemical properties accounted for most community differences, and pH was a key factor (p < 0.01). Interestingly, when the rhizosphere soil environment remained alkaline, the C and N contents were lowest, as was the biomass of the medicinal part bulb. This might relate to the specific distribution of genera, such as Pseudonocardia, Ohtaekwangia, Flavobacterium (relative abundance >0.01), which all have significantly correlated with the biomass of F. przewalskii (p < 0.05).

Discussion

F. przewalskii is evidently averse to alkaline soil with high potassium contents, but this requires future verification. The results of the present study may provide theoretical guidance and new insights for the cultivation and domestication of F. przewalskii.

Hinokitiol inhibits Aspergillus fumigatus by interfering with the cell membrane and cell wall

Hinokitiol (β-thujaplicin) is an important component of the essential oil extracted from Chamaecyparis obtuse, which prevents the decay and decomposition of temple and shrine buildings in Japan. Hinokiol has been shown to have a detrimental effect on various fungi such as Candida albicans and saprophytic fungi. However how hinokitiol works against Aspergillus fumigatus (A. fumigatus) has not been claimed. This study aims to investigate the adverse effects of hinokitiol on the disruption of the cell wall and cell membrane of A. fumigatus and to explore possible potential mechanisms or pathways. According to our results, hinokitiol negatively altered mycelium morphology, growth density, and cell plasma composition content. When incubated with human corneal epithelial cells (HCECs), hinokitiol saw a safe effect with concentrations below 12 μg/ml. Hinokitiol was shown to increase the cell membrane's permeability by decreasing the cell membrane's ergosterol content. The integrity of the cell wall was disrupted, as well as a significant increase in chitin degradation and chitinase activity. As determined by RNA-seq results, subsequent analysis, and qRT-PCR, altered transcript levels of cell walls and cell membranes-related genes (such as eglC) illustrated how hinokitiol affected the genetic profile of A. fumigatus. With this study, we recommend hinokitiol as an effective anti-A. fumigatus agent by reducing the amounts of key components in the cell wall and membrane by preventing production and accelerating breakdown.