The mycelial network organization adapts to facilitate new substrate colonization and fruiting in Agaricus bisporus

Fungal mycelial network organization determines how efficiently the substrate is degraded, resources are taken up, and translocated to developing mushrooms. Substrate breakdown and resource uptake are best performed by a network of loose hyphae. Hyphal aggregates called cords are better at transporting resources over longer distances due to having wide hyphae, lowering hydraulic resistance. Network organization, and in particular the formation of cords, remains largely unexplored under cultivation-relevant conditions. Here, we quantified adaptations of the mycelial network during vegetative growth and fructification of Agaricus bisporus grown on commercial cultivation substrate. Using imaging-based approaches, we assessed optical mycelial density (grayscale) as a proxy for substrate colonization and quantified cord abundance based on the surface area covered by cords. During homogeneous vegetative colonization, cord formation was minimal. When uncolonized substrate was placed next to a pre-colonized substrate, directional growth occurred by hyphal extension into the uncolonized substrate, acting as a resource sink, since newly encountered substrate functions as a sink because colonization of it requires import of resources. Cord formation was observed both in newly colonized regions and within the pre-existing mycelial network. During fructification, compost hyphal density continued to increase up to the second flush. Cord abundance within the compost rose during casing layer colonization and peaked during mushroom development, especially in the middle compost stratum. Cords on the casing layer, connecting the compost mycelial network and the primordia, markedly widened during exponential expansion of the mushrooms. Together, these findings suggest that cord formation is induced when transport demand increases.