Translocation of water and nutrients during growth and fructification of Agaricus bisporus

Fungi need water for all stages of life. In fact, mushrooms consist of 90 % of water. Fungi degrade organic matter by secreting enzymes. These enzymes need water to break down the substrate. For instance, when the substrate is too dry, fungi are known to transport water from moist areas to arid areas by hydraulic redistribution. Once nutrients are freed from the substrate, they are taken up by transporters lining the cell membrane. Thereby, an intracellular osmotic potential is created that is greater than that of the substrate, and water follows by osmosis. Aquaporins facilitate water uptake at cold temperatures. Since fungi possess a cell wall, the cell volume will not increase by water uptake, but the cytoplasm will exert higher pressure on the cell wall, thereby building up turgor. Fungi have tightly coordinated osmotic regulatory controls via the HOG pathway. When water is getting scarce, this pathway makes sure enough osmolytes are synthesized to allow sufficient water uptake for maintaining turgor homeostasis. The fungal network is interconnected and allows water flow when small pressure differences exist. These pressure differences can be the result of growth, differential osmolyte uptake/synthesis or external osmotic conditions. Overall, the water potential of the substrate and fungal tissues determine whether water will flow, since water flows from an area of high- to a low water potential area. In this review we aim to give a comprehensive view on how fungi obtain and translocate water needed for their development. We have taken Agaricus bisporus growing on compost and casing soil as a case study, to discuss water relations during fruiting in detail. Using the current state-of-the-art we found that there is a discrepancy between the models describing water transport to mushrooms and the story that water potentials tell us.