Nitrogen and phosphate metabolism in ectomycorrhizas

The Hebeloma cylindrosporum HcPT2 Pi transporter plays a key role in ectomycorrhizal symbiosis

Through a mutualistic relationship with woody plant roots, ectomycorrhizal fungi provide growth-limiting nutrients, including inorganic phosphate (Pi), to their host. Reciprocal trades occur at the Hartig net, which is the symbiotic interface of ectomycorrhizas where the two partners are symplasmically isolated. Fungal Pi must be exported to the symbiotic interface, but the proteins facilitating this transfer are unknown. In the present study, we combined transcriptomic, microscopy, whole plant physiology, X-ray fluorescence mapping, ³² P labeling and fungal genetic approaches to unravel the role of HcPT2, a fungal Pi transporter, during the Hebeloma cylindrosporum-Pinus pinaster ectomycorrhizal association. We localized HcPT2 in the extra-radical hyphae and the Hartig net and demonstrated its determinant role for both the establishment of ectomycorrhizas and Pi allocation towards P. pinaster. We showed that the host plant induces HcPT2 expression and that the artificial overexpression of HcPT2 is sufficient to significantly enhance Pi export towards the central cylinder. Together, our results reveal that HcPT2 plays an important role in ectomycorrhizal symbiosis, affecting both Pi influx in the mycelium and efflux towards roots under the control of P. pinaster.

The first complete mitochondrial genome from the family Hygrophoraceae (Hygrophorus russula) by next-generation sequencing and phylogenetic implications

Hygrophorus russula (Schaeff.) Kauffman is an edible ectomycorrhizal fungus that is widely distributed in the world. In this study, the mitogenome of H. russula was sequenced and assembled. The mitogenome of H. russula is composed of circular DNA molecules, with a total size of 55,769 bp. Further analysis indicated that the frequent use of A and T in codons contributes to the high AT content (80.87%) in the H. russula mitogenome. Comparative analysis indicated that the length and base composition of the core protein-encoding genes, and the number of tRNA genes in the H. russula mitogenome varied from that of other Agaricales mitogenomes. Gene arrangement analysis revealed a novel gene order in the H. russula mitogenome. In addition, the expansion of the mitogenome in Agaricales was found to be closely related to the increase in the number of introns. Phylogenetic analysis of the combined mitochondrial gene set showed strong support for tree topologies, and H. russula was determined to be relatively distant from other Agaricales species. This study is the first report on the mitogenome of a member of genus Hygrophorus as well as family Hygrophoraceae, which improves our understanding of mitochondrial differentiation and evolution in the important ectomycorrhizal fungi Hygrophorus species.

HcPT1.2 participates in Pi acquisition in Hebeloma cylindrosporum external hyphae of ectomycorrhizas under high and low phosphate conditions

Ectomycorrhizal fungi improve tree phosphorus nutrition through transporters specifically localized at soil-hyphae and symbiotic interfaces. In the model symbiosis between the fungus Hebeloma cylindrosporum and the maritime pine (Pinus pinaster), several transporters possibly involved in phosphate fluxes were identified, including three H⁺:Pi transporters. Among these three, we recently unraveled the function of one of them, named HcPT2, in both pure culture and symbiotic interaction with P. pinaster. Here we investigated the transporter named HcPT1.2, by analyzing inorganic phosphate transport ability in a yeast complementation assay, assessing its expression in the fungus associated or not with the plant, and immunolocalizing the proteins in ectomycorrhizas. We also evaluated the effect of external Pi concentration on expression and localization of HcPT1.2. Our results revealed that HcPT1.2 is involved in Pi acquisition by H. cylindrosporum mycelium, irrespective of the external Pi concentrations.