Elucidating the role of monoacetylphlorogulcinol in the pathogenicity of Pseudomonas 'gingeri' against Agaricus bisporus

BACKGROUND

Agaricus bisporus is a globally important edible fungus. The occurrence of ginger blotch caused by Pseudomonas 'gingeri' during A. bisporus growth and post-harvest stages results in significant economic losses. The biotoxin monoacetylphloroglucinol (MAPG) produced by P. 'gingeri' is responsible for inducing ginger blotch on A. bisporus. However, the understanding of the toxic mechanisms of MAPG on A. bisporus remains limited, which hinders the precise control of ginger blotch disease in A. bisporus and the breeding of disease-resistant varieties.

RESULTS

Integrating transcriptomic, metabolomic, and physiological data revealed that MAPG led to an increase in intracellular superoxide anion (O2 -) levels and lipid peroxidation in A. bisporus. MAPG changed the cellular membrane composition of A. bisporus, causing to damage membrane permeability. MAPG inhibited the expression of genes associated with the 19s subunit of the proteasome, thereby impeding cellular waste degradation in A. bisporus. Unlike melanin, MAPG stimulated the synthesis of flavonoids in A. bisporus, which might explain the manifestation of ginger-colored symptoms rather than browning. Meanwhile, the glutathione metabolism pathway in A. bisporus played a pivotal role in counteracting the cytotoxic effects of MAPG. Additionally, enhanced catalase activity and up-regulation of defense-related genes, including cytochrome P450s, Major Facilitator Superfamily (MFS), and ABC transporters, were observed.

CONCLUSION

This study provides comprehensive insights into MAPG toxicity in A. bisporus and uncovers the detoxification strategies of A. bisporus against MAPG. The findings offer valuable evidence for precise control and breeding of resistant varieties against ginger blotch in A. bisporus. © 2024 Society of Chemical Industry.

Ginger blotches on Agaricus bisporus due to monoacetylphloroglucinol production by the pathogen Pseudomonas 'gingeri'

BACKGROUND

Agaricus bisporus is the most widely cultivated and consumed mushroom worldwide. Pseudomonas 'gingeri' is the only pathogenic causative agent of ginger blotch in A. bisporus. Current research on mushroom pathogenic biotoxins is limited to P. tolaasii, which causes brown blotch, while understanding of P. 'gingeri' is lacking, therefore identifying the toxins produced by P. 'gingeri' and evaluating their toxicity on A. bisporus is essential for understanding its pathogenic mechanisms.

RESULTS

A pathogenic bacterium isolated from fruiting bodies of A. bisporus with ginger blotch was identified as P. 'gingeri', and its main toxin identified as 2', 4', 6'-trihydroxyacetophenone monohydrate, also known as monoacetylphloroglucinol (MAPG). Its first known extraction from a mushroom pathogen is reported here. MAPG at 250 μg/mL significantly inhibited the host's mycelial growth, increased branching, caused the structure to become dense and resulted in folds appearing on the surface. An MAPG concentration of 750 μg/mL MAPG led to mycelial death. P. 'gingeri' had high MAPG production in medium containing 0.1 mol/L of either glucose or mannitol (4.30 and 1.85 μg/mL, respectively), and mycelia were inhibited by 69.6% and 41.1%, respectively. The MAPG content was significantly lower in other carbon source media.

CONCLUSION

This work provides a detailed description of the structure and virulence of the P. 'gingeri' biotoxin, which has implications for understanding its pathogenic mechanism and for exploring precise control strategies for A. bisporus ginger blotch disease, such as the development of MAPG inhibitory factors. © 2023 Society of Chemical Industry.

Comparative Studies on the Disease Prevalence and Population Dynamics of Ginger Blotch and Brown Blotch Pathogens of Button Mushrooms

Bacterial blotch is one of the most economically important diseases of button 'mushroom. Knowledge of mechanisms of disease expression, inoculum thresholds, and disease management is limited to the most well-known pathogen, Pseudomonas tolaasii. Recent outbreaks in Europe have been attributed to 'P. gingeri' and P. salomonii for ginger and brown blotch, respectively. Information about their identity, infection dynamics, and pathogenicity is largely lacking. The disease pressure in an experimental mushroom cultivation facility was evaluated for 'P. gingeri' and P. salomonii over varying inoculation densities, casing soil types, environmental humidity, and cultivation cycles. The pathogen population structures in the casing soils were simultaneously tracked across the cropping cycle using highly specific and sensitive TaqMan-quantitative PCR assays. 'P. gingeri' caused disease outbreaks at lower inoculum thresholds (104 CFU/g) in the soil than P. salomonii (105 CFU/g). Ginger blotch generically declined in later harvest cycles, although brown blotch did not. Casing soils were differentially suppressive to blotch diseases, based on their composition and supplementation. Endemic pathogen populations increased across the cultivation cycle although the inoculated pathogen populations were consistent between the first and second flush. In conclusion, 'P. gingeri' and P. salomonii have unique infection and population dynamics that vary over soil types. Their endemic populations are also differently abundant in peat-based casing soils. This knowledge is essential for interpreting diagnostic results from screening mushroom farms and designing localized disease control strategies.