Two AMP-Binding Domain Proteins from Rhizophagus irregularis Involved in Import of Exogenous Fatty Acids

Cross-kingdom nutrient exchange in the plant-arbuscular mycorrhizal fungus-bacterium continuum

The association between plants and arbuscular mycorrhizal fungi (AMF) affects plant performance and ecosystem functioning. Recent studies have identified AMF-associated bacteria as cooperative partners that participate in AMF-plant symbiosis: specific endobacteria live inside AMF, and hyphospheric bacteria colonize the soil that surrounds the extraradical hyphae. In this Review, we describe the concept of a plant-AMF-bacterium continuum, summarize current advances and provide perspectives on soil microbiology. First, we review the top-down carbon flow and the bottom-up mineral flow (especially phosphorus and nitrogen) in this continuum, as well as how AMF-bacteria interactions influence the biogeochemical cycling of nutrients (for example, carbon, phosphorus and nitrogen). Second, we discuss how AMF interact with hyphospheric bacteria or endobacteria to regulate nutrient exchange between plants and AMF, and the possible molecular mechanisms that underpin this continuum. Finally, we explore future prospects for studies on the hyphosphere to facilitate the utilization of AMF and hyphospheric bacteria in sustainable agriculture.

The arbuscular mycorrhizal fungus Rhizophagus irregularis uses the copper exporting ATPase RiCRD1 as a major strategy for copper detoxification

Arbuscular mycorrhizal (AM) fungi establish a mutualistic symbiosis with most land plants. AM fungi regulate plant copper (Cu) acquisition both in Cu deficient and polluted soils. Here, we report characterization of RiCRD1, a Rhizophagus irregularis gene putatively encoding a Cu transporting ATPase. Based on its sequence analysis, RiCRD1 was identified as a plasma membrane Cu + efflux protein of the P1B1-ATPase subfamily. As revealed by heterologous complementation assays in yeast, RiCRD1 encodes a functional protein capable of conferring increased tolerance against Cu. In the extraradical mycelium, RiCRD1 expression was highly up-regulated in response to high concentrations of Cu in the medium. Comparison of the expression patterns of different players of metal tolerance in R. irregularis under high Cu levels suggests that this fungus could mainly use a metal efflux based-strategy to cope with Cu toxicity. RiCRD1 was also expressed in the intraradical fungal structures and, more specifically, in the arbuscules, which suggests a role for RiCRD1 in Cu release from the fungus to the symbiotic interface. Overall, our results show that RiCRD1 encodes a protein which could have a pivotal dual role in Cu homeostasis in R. irregularis, playing a role in Cu detoxification in the extraradical mycelium and in Cu transfer to the apoplast of the symbiotic interface in the arbuscules.

Lessons from arbuscular mycorrhizal fungal genomes

Arbuscular mycorrhizal fungi (AMF) have accompanied the majority of land plants since their evolution in the Devonian period with a symbiotic alliance centered on nutrient exchanges. The exploration of AMF genomes is providing clues to explain major questions about their biology, evolution, and ecology. The dynamics of nuclei across the fungal life cycle, the abundance of transposable elements, and the epigenome landscape are emerging as sources of intraspecific variability, which can be especially important in organisms with no or rare sexual reproduction such as AMF. These features have been hypothesized to support AMF adaptability to a wide host range and to environmental changes. New insights on plant-fungus communication and on the iconic function of phosphate transport were also recently obtained that overall contribute to a better understanding of this ancient and fascinating symbiosis.

Integrating plant and fungal quantitative genetics to improve the ecological and agricultural applications of mycorrhizal symbioses

Finding and targeting genes that quantitatively contribute to agricultural and ecological processes progresses food production and conservation efforts. Typically, quantitative genetic approaches link variants in a single organism's genome with a trait of interest. Recently, genome-to-genome mapping has found genome variants interacting between species to produce the result of a multiorganism (including multikingdom) interaction. These were plant and bacterial pathogen genome interactions; plant-fungal coquantitative genetics have not yet been applied. Plant-mycorrhizae symbioses exist across most biomes, for a majority of land plants, including crop plants, and manipulate many traits from single organisms to ecosystems for which knowing the genetic basis would be useful. The availability of Rhizophagus irregularis mycorrhizal isolates, with genomic information, makes dual-genome methods with beneficial mutualists accessible and imminent.

Stronger mutualistic interactions with arbuscular mycorrhizal fungi help Asteraceae invaders outcompete the phylogenetically related natives

Mutualistic interactions with arbuscular mycorrhizal fungi (AMF) greatly affect the outcome of plant-plant competition, especially for invasive plants competing against native plants. We examined the effects of AMF on the competition between invasive Asteraceae plants and the phylogenetically related native plants. We compared the performance of seven invasive Asteraceae plants from different genera with that of their phylogenetically related native counterparts in response to AMF in monocultures and mixed cultures. We investigated how interactions with AMF impact the competition between Asteraceae relatives. Total biomass increased with AMF colonization in both invasive and native plants. Arbuscular mycorrhizal fungi improved the competitiveness of invasive plants, but decreased that of native plants. Competition increased the shoot nitrogen, phosphorus and root myristic acid concentrations and relative expression of fatty acid transporter genes (RiFAT1 and RiFAT2) in AMF-colonized invasive plants, but decreased those in AMF-colonized native plants. Structural equation models indicated that the presence of AMF increased the uptake of phosphorus, but not nitrogen, by invasive plants, which probably provided more myristic acids to symbiotic AMF in return. These results suggest that invasive Asteraceae plants have greater mutualistic interactions with AMF than their phylogenetically related native counterparts, potentially contributing to invasion success.

Functions of Lipids in Development and Reproduction of Arbuscular Mycorrhizal Fungi

Arbuscular mycorrhizal fungi (AMF) form mutualistic associations with most land plants. The symbiosis is based on the exchange of nutrients: AMF receive photosynthetically fixed carbon from the plants and deliver mineral nutrients in return. Lipids are important players in the symbiosis. They act as components of the plant-derived membrane surrounding arbuscules, as carbon sources transferred from plants to AMF, as a major form of carbon storage in AMF and as triggers of developmental responses in AMF. In this review, we describe the role of lipids in arbuscular mycorrhizal symbiosis and AMF development.