Trends in Plant Science
OpinionPhosphorus Transport in Mycorrhiza: How Far Are We?
Section snippets
Phosphate Efflux in Mycorrhizal Roots: The Missing Step
The roots of most land plants fulfill an important part of their need for phosphorus (P) through the association with soil fungi, called mycorrhizal symbiosis 1., 2.. Fungal species belonging to the Mucoromycota, subphylum Glomeromycotina, Ascomycota, and Basidiomycota phyla form different types of mycorrhizas [3]. Arbuscular mycorrhizal (AM) symbiosis (see Glossary), characterized by the formation of fungal arbuscules and/or vesicles within plant cortical cells, is formed by Glomeromycotina
High-Affinity Transporters Mediate Inorganic Phosphate Efflux in Ectomycorrhizas
The simplest hypothesis to explain the fungal P delivery at the symbiotic interface is the release of free orthophosphate ions (Pi) through phosphate transporters (PT). Fungal P transport has been widely studied in baker’s yeast (Saccharomyces cerevisiae), in which two high-affinity Pi symporters have been characterized, one coupled with H+ (Pho84) [15] and one with Na+ (Pho89) [16]. Interestingly, alignment of proteins retrieved from publicly available mycorrhizal species using the sequences
Efflux of Inorganic Phosphate through H+:Pi Symporters Is Tightly Regulated by Undescribed Mechanisms
Determining whether an efflux of Pi is possible through fungal H+:Pi transporters from a thermodynamic point of view is a key question that was addressed recently in AM symbiosis 27., 28.. Indeed, computational modeling was used to simulate in silico the conditions required for Pi exchange through fungal and plant H+:Pi transporters located at the symbiotic interface of AM roots. Briefly, the authors modelized a network based on the activity of fungal and plant membrane ATPases that release
The Intriguing Case of High-Affinity Pi:Na+ Symporters
Based on recent data reporting the role of HcPT2 in ECM symbiosis [26], it is tempting to hypothesize that H+:Pi, and particularly PT2-like transporters, could be the main proteins unloading Pi into the symbiotic interface. Here, we show that all studied mycorrhizal fungi, except Sebacinales, harbor at least one ortholog of H+:Pi PT1-like transporter, whereas most Ascomycota (truffles and parented species) and Glomeromycotina species do not have a PT2-like member (Figure 1). This indicates that
Low-Affinity Transporters Could Also Mediate Inorganic Phosphate Efflux in Mycorrhiza
So far, the most studied mechanisms for Pi efflux in mycorrhizal symbiosis have focused on H+:Pi transporters. In yeast, Pi can also be transported through three low-affinity transporters, described as Na+/dicarboxylate/sulfate/Pi transporter in databases (Pho87, Pho90, Pho91)[41]. When Pho84 and Pho89 are either not expressed in yeast or degraded at high Pi, Pho87 and Pho90 are located at the plasma membrane and ensure Pi acquisition 41., 42., 43.. Pho91 is localized at the vacuolar membrane
Organic Phosphate, an Alternative Phosphorus Source in Mycorrhizas?
We assumed for decades that the form of P released to the symbiotic interface is Pi. However, most mycorrhizal species also harbor at least one ortholog of the yeast organic P transporter, ScGit1p (Figure 1). In yeast, this protein is upregulated at low P and is able to import several phospho-diesters, which are by order of preference: glycerophosphoinositol (GroPIns) ≫ glycerophosphoserine ≫ glycerol-3-phosphate ≫ glycerophosphoethanolamine ≫ glycerophosphocholine [49]. A release of GroPIns
Concluding Remarks and Future Perspectives
Although the improvement of plant P nutrition by mycorrhizal fungi was described for the first time more than 50 years ago 51., 52., critical steps in the transport of P from the soil to colonized roots are still missing. Specifically, only a handful of candidate proteins possibly involved in the release of P from fungal cells into the symbiotic interface have been reported so far in both AM and ECM fungi [53]. With our present survey of fungal P transporters putatively involved in this crucial
Acknowledgments
We would like to thank the anonymous reviewers as well as the editor for their helpful suggestions that helped us improve themanuscript. K.G. acknowledges support of the North Carolina Agriculture Research Service (NCARS) and the North Carolina Soybean Producers Association (2019-1656).
Glossary
- Arbuscular mycorrhiza
- symbiotic association between roots of most land plants and fungi belonging to the Mucoromycota phylum, Glomeromycotina subphylum.
- Arbuscule
- highly ramified fungal structure developing within plant cortical cells in arbuscular mycorrhizal symbiosis.
- Ectomycorrhiza
- symbiotic association between roots from trees and shrubs and fungi belonging to the Ascomycota and Basidiomycota phyla.
- Ericoid mycorrhiza
- symbiotic association between roots from Ericaceae plants and mycorrhizal fungi.
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