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SummaryWood and litter degrading fungi are the main decomposers of lignocellulose and thus play a key role in carbon cycling in nature. Here we provide evidence for a novel lignocellulose degradation strategy employed by the litter degrading fungusAgaricus bisporus(known as the white button mushroom). Fusion of hyphae allows this fungus to synchronize the activity of its mycelium over large distances (50 cm). The synchronized activity has an 13-hour interval that increases to 20 h before becoming irregular and is associated with a 3.5-fold increase in respiration while compost temperature increases up to 2 °C. Transcriptomic analysis of this burst-like phenomenon supports a cyclic degradation of lignin, deconstruction of (hemi-) cellulose and microbial cell wall polymers, and uptake of degradation products during vegetative growth ofA. bisporus. Cycling in expression of the ligninolytic system, enzymes involved in saccharification, and nutrient uptake is proposed to provide an efficient way for degradation of substrates such as litter.

The vegetative mycelium of Agaricus bisporus supplies developing white button mushrooms with water and nutrients. However, it is not yet known which part of the mycelium contributes to the feeding of the mushrooms and how this depends on growth conditions. Here we used photon counting scintillation imaging to track translocation of the 14C-radiolabeled metabolically inert amino acid analogue α-aminoisobutyric acid (14C-AIB). Translocation to the periphery of the mycelium was observed in actively growing vegetative mycelium with a velocity of up to 6.6 mm h−1, which was 30-fold higher than the growth rate. Furthermore, 14C-AIB translocated to neighboring colonies after fusion by anastomosis depending on the relative growth rate in these colonies. When mushrooms started to develop, translocation of 14C-AIB was redirected to the fruiting bodies via mycelium and hyphal cords. More abundant mycelial cord formation and a 5-fold higher rate of translocation was observed for cultures growing directionally from inoculum located at one side of the substrate, when compared to non-directional growth (inoculum mixed throughout the substrate). The maximum translocation distance was also greater (≥50 and 22 cm, respectively). In conclusion, 14C-AIB translocation switches between vegetative growth and towards developing mushrooms, especially via cords and when source–sink relationships change.

Hyphae at the outer part of colonies of Aspergillus niger and Aspergillus oryzae are heterogeneous with respect to transcriptional and translational activity. This heterogeneity is maintained by Woronin body mediated closure of septal pores that block interhyphal mixing of cytoplasm. Indeed, heterogeneity between hyphae is abolished in ΔhexA strains that lack Woronin bodies. The subpopulation of hyphae with high transcriptional and translational activity secretes enzymes that degrade the substrate resulting in breakdown products that serve as nutrients. The role of hyphae with low transcriptional and translational activity was not yet known. Here, we show that this subpopulation is more resistant to environmental stress in A. oryzae, in particular to temperature stress, when compared to hyphae with high transcriptional and translational activity. Notably, all hyphae of the ΔhexA strain of A. oryzae were sensitive to heat stress explained by the reduced heterogeneity in this strain. Together, we show that different subpopulations of hypha secrete proteins and resist heat stress showing the complexity of a fungal mycelium.

Pores in fungal septa enable cytoplasmic streaming between hyphae and their compartments. Consequently, the mycelium can be considered unicellular. However, we show here that Woronin bodies close ~50% of the three most apical septa of growing hyphae of Aspergillus niger. The incidence of closure of the 9th and 10th septa was even ≥94%. Intercompartmental streaming of photoactivatable green fluorescent protein (PA-GFP) was not observed when the septa were closed, but open septa acted as a barrier, reducing the mobility rate of PA-GFP ~500 times. This mobility rate decreased with increasing septal age and under stress conditions, likely reflecting a regulatory mechanism affecting septal pore diameter. Modeling revealed that such regulation offers effective control of compound concentration between compartments. Modeling also showed that the incidence of septal closure in A. niger had an even stronger impact on cytoplasmic continuity. Cytoplasm of hyphal compartments was shown not to be in physical contact when separated by more than 4 septa. Together, data show that apical compartments of growing hyphae behave unicellularly, while older compartments have a multicellular organization., IMPORTANCE The hyphae of higher fungi are compartmentalized by porous septa that enable cytosolic streaming. Therefore, it is believed that the mycelium shares cytoplasm. However, it is shown here that the septa of Aspergillus niger are always closed in the oldest part of the hyphae, and therefore, these compartments are physically isolated from each other. In contrast, only part of the septa is closed in the youngest part of the hyphae. Still, compartments in this hyphal part are physically isolated when separated by more than 4 septa. Even open septa act as a barrier for cytoplasmic mixing. The mobility rate through such septa reduces with increasing septal age and under stress conditions. Modeling shows that the septal pore width is set such that its regulation offers maximal control of compound concentration levels within the compartments. Together, we show for the first time that Aspergillus hyphae switch from a unicellular to multicellular organization.

Aspergillus species are highly abundant fungi worldwide. Their conidia are among the most dominant fungal spores in the air. Conidia are formed in chains on the vesicle of the asexual reproductive structure called the conidiophore. Here, it is shown that the velvet protein VeA of Aspergillus niger maximizes the diameter of the vesicle and the spore chain length. The length and width of the conidiophore stalk and vesicle were reduced nearly twofold in a ΔveA strain. The latter implies a fourfold reduced surface area to develop chains of spores. Over and above this, the conidial chain length was approximately fivefold reduced. The calculated 20-fold reduction in formation of conidia by ΔveA fits the 8- to 17-fold decrease in counted spore numbers. Notably, morphology of the ΔveA conidiophores of A. niger was very similar to that of wild-type Aspergillus sydowii. This suggests that VeA is key in conidiophore architecture diversity in the fungal kingdom. The finding that biomass formation of the A. niger ΔveA strain was reduced twofold shows that VeA not only impacts dispersion capacity but also colonization capacity of A. niger.

Hyphae of ascomycetes are compartmentalized by septa. The central pore in these septa allows for cytoplasmic streaming. However, many of these pores are closed by Woronin bodies in Aspergillus, which prevents cytoplasmic mixing and thus maintains hyphal heterogeneity. Here, glucose uptake and transport was studied in Aspergillus niger. Glucose uptake was higher in the hyphal population with high transcriptional activity when compared to the population with low transcriptional activity. Glucose was transported from the colony center to the periphery, but not vice versa. This unidirectional flow was similar in the wild-type and the ΔhexA strain that does not form Woronin bodies. This indicated that septal plugging by Woronin bodies does not impact long distance glucose transport. Indeed, the glucose analogue 2-NBDG (2-(N-[7-nitrobenz-2-oxa-1,3-diazol-4-yl]amino)-2-deoxyglucose) translocated to neighboring hyphal compartments despite Woronin body mediated plugging of the septum that separated these compartments. Notably, 2-NBDG accumulated in septal cross walls, indicating that intercompartmental glucose transport is mediated by transporters that reside in the plasma membrane lining the septal cross-wall. The presence of such transporters would thus enable selective transport between heterogeneous compartments.