Mannitol concentration is a determinant of the mycorrhizal infectivity of the Laccaria trichodermophora mycelium produced in liquid culture

Ectomycorrhizal fungi (EMF) play a crucial role in temperate forest ecosystems, facilitating enhanced nutrient uptake and increased plant resilience. Laccaria trichodermophora, an EMF commonly associated with juvenile pine species, has recently emerged as a model organism for studying fungal development and symbiotic interactions. Submerged liquid culture of this species has shown promise for producing high volumes of vegetative EMF inoculum. In this context, the carbon-to-nitrogen (C:N) ratio in culture media is a crucial factor that affects fungal metabolism, particularly the production of storage carbohydrates such as trehalose and mannitol. These storage molecules are linked to the infectivity of pathogenic fungi and symbionts. However, it remains unclear how the storage metabolic status influences the infectivity of the EMF inoculum and whether these storage molecules can serve as indicators of infectivity. Here, we show that while trehalose accumulation does not affect infectivity, intracellular mannitol concentration, modulated by the C:N ratio, positively correlates with inoculum infectivity. Our findings indicate that intracellular mannitol plays a pivotal role in enhancing the infective ability of EMF inoculum, aiding effective colonization of host roots during the early stages of symbiosis establishment. Considering the above, liquid cultures of ectomycorrhizal fungi under conditions that favor intracellular mannitol concentration produce a high-quality inoculant with higher mycorrhizal infectivity. These findings have significant implications for biotechnological production of mycorrhizal inoculants intended for reforestation and sustainable forestry.