SummaryAsian soybean rust (Phakopsora pachyrhizi) causes a devastating disease in soybean (Glycine max). We tested the hypothesis that the fungus generates high turgor pressure in its hyaline appressoria to mechanically pierce epidermal cells.Turgor pressure was determined by a microscopic technique, called transmitted light double-beam interference Mach-Zehnder microscopy (MZM), which was developed in the 1960s as a forefront of live cell imaging. We revitalized some original microscopes and equipped them for modern image capturing. MZM data were corroborated by cytorrhysis experiments.Incipient cytorrhysis determined the turgor pressure in appressoria of P. pachyrhizi to be equivalent to 5.13 MPa. MZM data revealed that osmotically active sugar alcohols only accounted for 75% of this value. Despite having a lower turgor pressure, hyaline rust appressoria were able to penetrate non-biodegradable polytetrafluoroethylene (PTFE) membranes more efficiently than do melanized appressoria of the anthracnose fungus Colletotrichum graminicola or the rice blast fungus Magnaporthe oryzae.Our findings challenge the hypotheses that force-based penetration is a specific hallmark of fungi differentiating melanized appressoria and that this turgor-driven process is solely caused by metabolic degradation products. The appressorial turgor pressure may explain the capability of P. pachyrhizi to forcefully invade a wide range of different plants and may pave the way to novel plant protection approaches.

Purpose Many decades of research have shown that humic compounds as part of the soil organic matter (SOM) are essential for the stability and ecosystem services of soils. James P. Martin and Konrad Haider based on several pioneers in humus research improved the basis for the current knowledge of key processes in the soil environment, in particular structure and formation of humus triggered mainly by soil fungi. Other major contributions are briefly described but not in the focus of this article, such as their innovative tracer experiments with isotopelabelled xenobiotic chemicals and natural litter to study their fate. Both scientists inspired generations of younger researchers to advance their approaches and laid the cornerstone of the current understanding of soil organic matter formation. Results and discussion This article values the key experiments of Martin and Haider in this field and the related follow-up research finally resulting in the current view on formation mechanisms of SOM and non-extractable residues (NER) of xenobiotics. The improved understanding of these processes considering necromass with tissue residues of plants, microbes and animals challenges the traditional views of humic matter as macromolecular organic matrix, which according to the research of the last years represents only a variable part of the total organic matter besides associates of low molecular weight molecules. We discuss views on soil organic matter and humic substances that are nowadays considered not to differ in molecular diversity. We will start by demonstrating the understanding of humus characteristics and humus formation three decades ago closely related to the findings of Martin and Haider. Methodological approaches to characterize relevant structural and mechanistic pictures of SOM such as the priming effect, clay mineral catalysed reactions and the various mechanisms by which natural and xenobiotic chemicals are protected in soil are briefly illustrated by examples. Fungal activities in producing secondary metabolites like polyketides and their probably minor contributions to SOM formation are presented. Conclusions and perspectives Open research questions stimulated by these two soil scientists are sketched which are nowadays possible to address by new sophisticated highresolution techniques.

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Extracellular, oxidative soil enzymes like monophenol oxidases and peroxidases play an important role in transformation of xenobiotics and the formation of organic matter in soil. Additionally, these enzymes may be involved in the formation of non-extractable residues (NERs) of xenobiotics during humification processes. To examine this correlation, the fate of the fungicide (14)C metalaxyl in soil samples from Ultuna (Sweden) was studied. Using different soil sterilization techniques, it was possible to differentiate between free, immobilized, and abiotic ("pseudoenzyme"-like) oxidative activities. A correlation between the formation of metalaxyl NER and soil organic matter content, biotic activities, as well as extracellular phenoloxidase and peroxidase activities in the bulk soil and its particle size fractions was determined. Extracellular soil-bound enzymes were involved in NER formation (up to 8% of applied radioactivity after 92 days) of the fungicide independently from the presence of living microbes and different distributions of the NER in the soil humic subfractions.

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