Proteins are secreted throughout the mycelium of Aspergillus niger except for the sporulating zone. A link between sporulation and repression of protein secretion was underlined by the finding that inactivation of the sporulation gene flbA results in mycelial colonies that secrete proteins throughout the colony. However, ΔflbA strain hyphae also lyse and have thinner cell walls. This pleiotropic phenotype is associated with differential expression of 36 predicted transcription factor genes, one of which, rpnR, was inactivated in this study. Sporulation, biomass, and secretome complexity were not affected in the ΔrpnR deletion strain of the fungus. In contrast, ribosomal subunit expression and protein secretion into the medium were reduced when A. niger was grown on xylose. Moreover, the ΔrpnR strain showed decreased resistance to H 2 O 2 and the proteotoxic stress-inducing agent dithiothreitol. Taking the data together, RpnR is involved in proteotoxic stress resistance and impacts protein secretion when A. niger is grown on xylose. IMPORTANCE Aspergillus niger secretes a large amount and diversity of industrially relevant enzymes into the culture medium. This makes the fungus a widely used industrial cell factory. For instance, carbohydrate-active enzymes of A. niger are used in biofuel production from lignocellulosic feedstock. These enzymes represent a major cost factor in this process. Higher production yields could substantially reduce these costs and therefore contribute to a more sustainable economy and less dependence on fossil fuels. Enzyme secretion is inhibited in A. niger by asexual reproduction. The sporulation protein FlbA is involved in this process by impacting the expression of 36 predicted transcription factor genes. Here, we show that one of these predicted transcriptional regulators, RpnR, regulates protein secretion and proteotoxic stress resistance. The gene is thus an interesting target to improve enzyme production in A. niger.

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<p>Methane (CH<sub>4</sub>) is a potent greenhouse gas contributing to climate change, with a global warming potential of 24x of CO<sub>2</sub> on a 100-year time-frame. More importantly, the atmospheric methane concentration has been rising rapidly in the last decade. Soils are as yet the only known biological sink for atmospheric methane, but the methane uptake capacity of agricultural soils is substantially reduced when compared to native soils. This may be due to a reduction of soil organic matter and soil aggregate stability as a result of agricultural management practices. In this study, we improved a laser-diffraction analysis-based modelling of soil aggregate stability. Using data from an extensive field study, we show new relationships between soil aggregate stability, atmospheric methane uptake, and soil organic matter. The use of organic amendments like compost increases soil organic matter content, which improves soil aggregate stability, and in this study, we show that an improved soil aggregate stability enhances atmospheric soil methane uptake. These results provide new insights on the use of organic amendments like compost on agricultural soils as an atmospheric methane mitigation strategy.</p>