Resource partitioning between extraradical and intraradical AM fungal mycelium

Abstract

In nature the mycelia of arbuscular mycorrhizal (AM) fungi usually consist of one extraradical mycelium connecting several intraradical mycelia, which can be distributed between several plant species. The extraradical and intraradical mycelia differ clearly from each other in morphology and physiology, and they are characterised by specific gene expression profiles. In this chapter the interplay between AM fungal extraradical and intraradical mycelium will be discussed by addressing the physiological aspects of nutrient partitioning. The focus will be on the transport of carbon and phosphate, and the enzymatic activity related to the fungal metabolism. It will be shown that AM fungi distribute their resources between the extraradical and intraradical mycelium. The estimate of AM fungal biomass in roots and in soil can be obtained with AM fungal fatty acids. Combining this technique to stable isotope labeling enables to estimate flow rates and retention of fungal carbon in the intraradical and extraradical mycelium. The most important mechanisms for bidirectional transport in the symbiosis seem to be the translocation of triacylglycerols in lipid bodies and the translocation of phosphate in tubular vacuoles. The fungal phosphatase activity is regulated in the fungal mycelium and this activity probably mainly reflects the internal phosphate transitions in the mycelium, which are related to the overall phosphate translocation and metabolism. It is suggested to compare fungi from different taxonomical groups and to investigate the partitioning in fungal mycelium that connects multiple host plants at the same time.