Selective transport between heterogeneous hyphal compartments via the plasma membrane lining septal walls of Aspergillus 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.

Heterogenic expression of genes encoding secreted proteins at the periphery of Aspergillus niger colonies.

Colonization of a substrate by fungi starts with the invasion of exploring hyphae. These hyphae secrete enzymes that degrade the organic material into small molecules that can be taken up by the fungus to serve as nutrients. We previously showed that only part of the exploring hyphae of Aspergillus niger highly express the glucoamylase gene glaA. This was an unexpected finding since all exploring hyphae are exposed to the same environmental conditions. Using GFP as a reporter, we here demonstrate that the acid amylase gene aamA, the α-glucuronidase gene aguA, and the feruloyl esterase gene faeA of A. niger are also subject to heterogenic expression within the exploring mycelium. Coexpression studies using GFP and dTomato as reporters showed that hyphae that highly express one of these genes also highly express the other genes encoding secreted proteins. Moreover, these hyphae also highly express the amylolytic regulatory gene amyR, and the glyceraldehyde-3-phosphate dehydrogenase gene gpdA. In situ hybridization demonstrated that the high expressers are characterized by a high 18S rRNA content. Taken together, it is concluded that two subpopulations of hyphae can be distinguished within the exploring mycelium of A. niger. The experimental data indicate that these subpopulations differ in their transcriptional and translational activity.

Heterogeneity of Aspergillus niger microcolonies in liquid shaken cultures.

The fungus Aspergillus niger forms (sub)millimeter microcolonies within a liquid shaken culture. Here, we show that such microcolonies are heterogeneous with respect to size and gene expression. Microcolonies of strains expressing green fluorescent protein (GFP) from the promoter of the glucoamlyase gene glaA or the ferulic acid esterase gene faeA were sorted on the basis of diameter and fluorescence using the Complex Object Parametric Analyzer and Sorter (COPAS) technology. Statistical analysis revealed that the liquid shaken culture consisted of two populations of microcolonies that differ by 90 µm in diameter. The population of small microcolonies of strains expressing GFP from the glaA or faeA promoter comprised 39% and 25% of the culture, respectively. Two populations of microcolonies could also be distinguished when the expression of GFP in these strains was analyzed. The population expressing a low level of GFP consisted of 68% and 44% of the culture, respectively. We also show that mRNA accumulation is heterogeneous within microcolonies of A. niger. Central and peripheral parts of the mycelium were isolated with laser microdissection and pressure catapulting (LMPC), and RNA from these samples was used for quantitative PCR analysis. This analysis showed that the RNA content per hypha was about 45 times higher at the periphery than in the center of the microcolony. Our data imply that the protein production of A. niger can be improved in industrial fermentations by reducing the heterogeneity within the culture.

Spatial and developmental differentiation of mannitol dehydrogenase and mannitol-1-phosphate dehydrogenase in Aspergillus niger.

The presence of a mannitol cycle in fungi has been subject to discussion for many years. Recent studies have found no evidence for the presence of this cycle and its putative role in regenerating NADPH. However, all enzymes of the cycle could be measured in cultures of Aspergillus niger. In this study we have analyzed the localization of two enzymes from the pathway, mannitol dehydrogenase and mannitol-1-phosphate dehydrogenase, and the expression of their encoding genes in nonsporulating and sporulating cultures of A. niger. Northern analysis demonstrated that mpdA was expressed in both sporulating and nonsporulating mycelia, while expression of mtdA was expressed only in sporulating mycelium. More detailed studies using green fluorescent protein and dTomato fused to the promoters of mtdA and mpdA, respectively, demonstrated that expression of mpdA occurs in vegetative hyphae while mtdA expression occurs in conidiospores. Activity assays for MtdA and MpdA confirmed the expression data, indicating that streaming of these proteins is not likely to occur. These results confirm the absence of the putative mannitol cycle in A. niger as two of the enzymes of the cycle are not present in the same part of A. niger colonies. The results also demonstrate the existence of spore-specific genes and enzymes in A. niger.

Phleomycin increases transformation efficiency and promotes single integrations in Schizophyllum commune.

Phleomycin is mutagenic by introducing double-strand breaks in DNA. The ble gene of Streptoalloteychus hindustanus, which confers resistance to this substance, is widely used as a selection marker for transformation. Schizophyllum commune grows on 25 microg of phleomycin ml(-1) after introduction of a resistance cassette based on the ble gene. However, we here report that growth of resistant colonies on this concentration of phleomycin resulted in aberrant colony morphologies. Apparently, phleomycin was mutagenic despite acquired resistance. Therefore, a new selection system was developed based on resistance to the antibiotic nourseothricin. However, the transformation efficiency was tenfold lower than that obtained with phleomycin as a selection agent. This low transformation efficiency could be rescued by addition of a nonselective concentration of phleomycin during protoplast regeneration. This was accompanied by a higher incidence of single-copy integrations and with an increase of expression of key genes involved in double-strand break repair. Taken together, we conclude that the effect of a nonselective concentration of phleomycin strongly resembles the effect of restriction enzyme-mediated integration (REMI) but, unlike REMI, it does not depend on the presence of a target restriction site.

Hyphal differentiation in the exploring mycelium of Aspergillus niger.

Mycelial fungi play a central role in element cycling in nature by degrading dead organic material such as wood. Fungal colonization of a substrate starts with the invasion of exploring hyphae. These hyphae secrete enzymes that convert the organic material into small molecules that can be taken up by the fungus to serve as nutrients. Using green fluorescent protein (GFP) as a reporter, we show for the first time that exploring hyphae of Aspergillus niger differentiate with respect to enzyme secretion; some strongly express the glucoamylase gene glaA, while others hardly express it at all. When a cytoplasmic GFP was used, 27% of the exploring hyphae of a 5-day-old colony belonged to the low expressing hyphae. By fusing GFP to glucoamylase and by introducing an ER retention signal, this number increased to 50%. This difference is due to cytoplasmic streaming of the reporter in the former case, as was shown by using a photo-activatable GFP. Our findings indicate that a fungal mycelium is highly differentiated, especially when taking into account that hyphae in the exploration zone were exposed to the same nutritional conditions.