How do arbuscular mycorrhizal fungi handle phosphate? New insight into fine‐tuning of phosphate metabolism

Abstract

Contents Summary1116I.Introduction1116II.Foraging for phosphate1117III.Fine‐tuning of phosphate homeostasis1117IV.The frontiers: phosphate translocation and export1119V.Conclusions and outlook1120Acknowledgements1120References1120 Summary Arbuscular mycorrhizal fungi form symbiotic associations with most land plants and deliver mineral nutrients, in particular phosphate, to the host. Therefore, understanding the mechanisms of phosphate acquisition and delivery in the fungi is critical for full appreciation of the mutualism in this association. Here, we provide updates on physical, chemical, and biological strategies of the fungi for phosphate acquisition, including interactions with phosphate‐solubilizing bacteria, and those on the regulatory mechanisms of phosphate homeostasis based on resurveys of published genome sequences and a transcriptome with reference to the latest findings in a model fungus. For the mechanisms underlying phosphate translocation and export to the host, which are major research frontiers in this field, not only recent advances but also testable hypotheses are proposed. Lastly, we briefly discuss applicability of the latest tools to gene silencing in the fungi, which will be breakthrough techniques for comprehensive understanding of the molecular basis of fungal phosphate metabolism.

Partner communication and role of nutrients in the arbuscular mycorrhizal symbiosis

“…Once absorbed by the ERM, Pi is quickly converted inside vacuoles into polyphosphate (polyP) chains, linear polymers comprising up to hundreds of Pi molecules (Solaiman et al, 1999;Ezawa et al, 2003). It has been hypothesized that AMF synthesize polyP through the VTC complex (Tani et al, 2009;Ezawa & Saito, 2018), as described in yeast (Hothorn et al, 2009). PolyP is then translocated to the IRM via cytoplasmic streaming and/or along a motile tubular vacuolar network (Olsson et al, 2002;Uetake et al, 2002;Hijikata et al, 2010).…”

Structure-specific regulation of nutrient absorption, metabolism and transfer in arbuscular mycorrhizal fungi

“…Polyphosphate is probably degraded to monophosphate by polyphosphatase and subsequently released from AMF 45 . Although the mechanism by which AMF release Pi is largely unknown, orthologs of Suppressor of Yeast Gpa1 (SYG1), which share domains with plant and animal Pi exporters, are strong candidates for the Pi exporter 46, 47 . In the case of nitrogen, AMF absorb NO 3 − and NH + , and transform them into arginine in the ERM.…”

Cross‐scale integration of mycorrhizal function

“…One avenue to growth promotion is almost certainly via phosphate acquisition and transfer to the plant. Recent advances in phosphate metabolism, reviewed by Ezawa & Saito, ; in this issue of New Phytologist , pp. 1116–1121), report the identification of five secreted acid phosphatases expressed in the extracellular hyphae of Rhizophagus clarus .…”

“…1116–1121), report the identification of five secreted acid phosphatases expressed in the extracellular hyphae of Rhizophagus clarus . This is interesting because they may enable liberation of phosphate from organic sources, for which there is some biochemical evidence, but also some earlier debate (reviewed in Ezawa & Saito, ). Variation in genes such as these could potentially underlie variation in plant growth promotion among symbionts.…”