Evolving insights to understanding mycorrhizas

Almost all land plant species form a symbiosis with mycorrhizal fungi. These soil fungi provide nutrients and other services to plants in return for plant carbohydrates. The recent application of microbial metagenomics, metatranscriptomics, and metabolomics to plants and their immediate surroundings confirms the key role of mycorrhizal fungi, rhizosphere bacteria and fungi, and suggests a world of hitherto undiscovered interactions (van der Heijden et al., this issue, pp. 1406–1423). This novel knowledge is leading to a paradigm-shifting view: plants cannot be considered as isolated individuals any more, but as metaorganisms, or holobionts (Hacquard & Schadt, this issue, pp. 1424– 1430) encompassing an active microbial community re-programming host physiology (see Pozo et al., this issue, pp. 1431–1436). This bears tremendous implications for plant ecophysiology and evolution, plant breeding, crop management and sustainable ecosystem management. Mycorrhizal associations are centerpieces in this wide cortege of plant-associated soil biota. To exploit these evolving insights, critical gaps need to be filled in our current understanding of mycorrhizal interactions. This special issue of New Phytologist addresses fundamental gaps and contains 30 new contributions on mycorrhizal science, covering topics from genomes to ecosystems. These contributions result from the 33 New Phytologist Symposium Networks of power and influence: ecology and evolution of symbioses between plants and mycorrhizal fungi (Z€ urich, 14–16 May 2014, http://www.newphytologist.org/symposiums/view/4; see also Bender et al., 2014). A series of key questions addressed in this issue explore some of the most cutting-edge approaches, and include: (1) How is the balance of mutualism maintained between plants and fungi? (2) What is the role of mycorrhizal fungi in the soil ecosystem? (3) What controls fungal community composition, and how is diversity maintained? While many of these questions originated in the earliest days of mycorrhizal science (Koide & Mosse, 2004), the development of new tools and approaches, from genomics to mathematical models to isotopes, is allowing them to be addressed in greater detail, clarity and depth than ever before.