Trading carbon between arbuscular mycorrhizal fungi and their hyphae-associated microbes

With the mycorrhizal hyphae on one side connected to plant roots and from the rhizosphere extending to the soil, arbuscular mycorrhizal (AM) fungi colonize and interconnect simultaneously two environments: specifically the roots of a host plant and the rhizosphere. Whereas the hyphae inside the roots are mainly surrounded by plant cells and in a stable environment, the hyphae extending to the soil are exposed to great variations in abiotic properties and they constantly interact with other organisms in soil such as fungi, macrofauna, microfauna, and bacteria. The total soil volume under the influence of mycorrhizal plants is referred to as the mycorrhizosphere and includes the combined effects exerted on the soil microbial communities by the roots (rhizosphere) and by the mycorrhizal hyphae (hyphosphere). Hence the mycorrhizosphere might be considered the crossroad of the root-soil habitat where complex fine-scale gradients of substrate availability, water potential, and redox state modify the root-soil environment, and consequently the composition, activity, and colonization ability of the surrounding beneficial, pathogenic, and commensal microbial communities. In this chapter we provide an overview of the available scientific knowledge on the identity and putative roles of hyphae-associated microbes in relation to the AM fungi and also to the mycorrhizal host plants. We explore the dynamics of these associations under fluctuating environmental conditions and the evolving insights to understand hyphae-associated microbes. More specifically, we analyze the potential involvement of the microbes in nutrient cycling and carbon transformation in the hyphosphere.