Mechanisms underlying beneficial plant–fungus interactions in mycorrhizal symbiosis

Abstract

Mycorrhizal fungi are a heterogeneous group of diverse fungal taxa, associated with the roots of over 90 % of all plant species. Recently, state-of-the-art molecular and genetic tools, coupled to high-throughput sequencing and advanced microscopy, have led to the genome and transcriptome analysis of several symbionts. Signalling pathways between plants and fungi have now been described and the identifi cation of several novel nutrient transporters has revealed some of the cellular processes that underlie symbiosis. Thus, the contributions of each partner in a mycorrhizal association are starting to be unravelled. This new knowledge is now available for use in agricultural practices.O wing to their fi lamentous organization, fungi exploit very diverse substrates on the basis of their nutritional strategy. Saprobes thrive in soil, water and on decaying animal and plant tissues. A smaller group of fungi, the parasitic and mutualistic symbionts, feed on living organisms 1 . Such a classifi cation cannot easily be applied to mycorrhizal fungi, a heterogeneous group of species spread over diverse fungal taxa. Although they can spend part of their life cycle as free-living organisms, mycorrhizal fungi always associate with the roots of higher plants, indeed over 90 % of plant species, including forest trees, wild grasses and many crops. Both partners benefi t from the relationship: mycorrhizal fungi improve the nutrient status of their host plants, infl uencing mineral nutrition, water absorption, growth and disease resistance, whereas in exchange, the host plant is necessary for fungal growth and reproduction 2 .Mycorrhizal fungi colonize environments such as alpine and boreal zones, tropical forests, grasslands and croplands. Th ey have a major role in nutrient cycling through the specifi c activity of their mycelium in absorbing soil nutrients and supplying them to the plant, although their role in carbon fl ux is less well defi ned 3 .Th e term mycorrhiza is derived from the Greek words for ' fungus ' and ' root ' . Mycorrhizal fungi develop an extensive hyphal network in the soil, the aptly named wood-wide web 4 , which can connect whole plant communities off ering effi cient horizontal transfer of nutrients. Mycorrhizas develop specialized areas, called symbiotic interfaces, to interact with the host plant 5 -7 . Mycorrhizal fungi can be divided into two major groups: aseptate endophytes such as Glomeromycota, or septate Asco-and Basidiomycota (see Box 1 Glossary) 2 . More commonly, mycorrhiza classifi cations refl ect anatomical aspects and identify two broad categories 2 , referred to as ectomycorrhizas (EMs) and endomycorrhizas, depending on whether the fungus colonizes the root intercellular spaces or develops inside cells ( Fig. 1 ). Endomycorrhizas are further divided into orchid, ericoid and arbuscular mycorrhizas (AMs).A descriptive approach has dominated the investigation of mycorrhizas for at least 50 years until the advent of molecular biology, and the ' omics ' era provided insight into their mechanis...

Gr and hp-1 tomato mutants unveil unprecedented interactions between arbuscular mycorrhizal symbiosis and fruit ripening

“…Both partners gain an advantage from the symbiosis; the fungus supplies the plant with mineral nutrients (mainly phosphate) increasing, plant biomass and improving its resistance to pathogens and environmental stresses including oxidative stress conditions (Garcia-Sànchez et al 2014), while the host provides organic carbon to the fungus (Bonfante and Genre 2010). Plant and fungal nutrition is thus mutually improved (Kiers et al 2011).…”

The expression of GintPT, the phosphate transporter of Rhizophagus irregularis, depends on the symbiotic status and phosphate availability

“…In order to increase Pi uptake, plants have evolved different strategies, such as the enhanced growth of lateral roots and root hairs, and/or the solubilization of soil Pi by means of organic acid and phosphatase secretion: all these processes are probably orchestrated by a systemic signalling that is triggered during Pi starvation (Doener 2008;Lambers et al 2011;Péret et al 2011) and involves specific gene expression regulators, as has been demonstrated in Arabidopsis (Pérez Torres et al 2009). Another widespread and evolutionary ancient strategy is the establishment of arbuscular mycorrhizal (AM) symbiosis which involves the majority of land plants and fungi belonging to the Glomeromycota phylum (Parniske 2008;Bonfante and Genre 2010). AM fungi ensure that their host plants have an efficient supply of mineral nutrients, particularly P and nitrogen (Allen and Shachar-Hill 2009;Tian et al 2010;.…”

Strigolactones fine-tune the root system

“…Hence, diverse regulatory signals, including plant-derived chemicals, are present in the soil to translate a wealth of information that ultimately leads to growth responses of the interacting partners. An important interaction, which proved crucial for land plant evolution, is the symbiosis with mycorrhizal fungi belonging to the Glomeromycota (Bonfante and Genre 2010). These fungi are obligate heterotrophs that supply the plant with nutrients [mainly phosphate (Pi) and nitrogen (N)] and, in return, the plant delivers photosynthetic assimilates to the fungi.…”