New insights into HcPTR2A and HcPTR2B, two high-affinity peptide transporters from the ectomycorrhizal model fungus Hebeloma cylindrosporum

Mycorrhizal symbiosis and the nitrogen nutrition of forest trees

Terrestrial plants form primarily mutualistic symbiosis with mycorrhizal fungi based on a compatible exchange of solutes between plant and fungal partners. A key attribute of this symbiosis is the acquisition of soil nutrients by the fungus for the benefit of the plant in exchange for a carbon supply to the fungus. The interaction can range from mutualistic to parasitic depending on environmental and physiological contexts. This review considers current knowledge of the functionality of ectomycorrhizal (EM) symbiosis in the mobilisation and acquisition of soil nitrogen (N) in northern hemisphere forest ecosystems, highlighting the functional diversity of the fungi and the variation of symbiotic benefits, including the dynamics of N transfer to the plant. It provides an overview of recent advances in understanding 'mycorrhizal decomposition' for N release from organic or mineral-organic forms. Additionally, it emphasises the taxon-specific traits of EM fungi in soil N uptake. While the effects of EM communities on tree N are likely consistent across different communities regardless of species composition, the sink activities of various fungal taxa for tree carbon and N resources drive the dynamic continuum of mutualistic interactions. We posit that ectomycorrhizas contribute in a species-specific but complementary manner to benefit tree N nutrition. Therefore, alterations in diversity may impact fungal-plant resource exchange and, ultimately, the role of ectomycorrhizas in tree N nutrition. Understanding the dynamics of EM functions along the mutualism-parasitism continuum in forest ecosystems is essential for the effective management of ecosystem restoration and resilience amidst climate change. KEY POINTS: • Mycorrhizal symbiosis spans a continuum from invested to appropriated benefits. • Ectomycorrhizal fungal communities exhibit a high functional diversity. • Tree nitrogen nutrition benefits from the diversity of ectomycorrhizal fungi.

Nitrogen Acquisition and Transport in the Ectomycorrhizal Symbiosis-Insights from the Interaction between an Oak Tree and Pisolithus tinctorius

In temperate forests, the roots of various tree species are colonized by ectomycorrhizal fungi, which have a key role in the nitrogen nutrition of their hosts. However, not much is known about the molecular mechanisms related to nitrogen metabolism in ectomycorrhizal plants. This study aimed to evaluate the nitrogen metabolic response of oak plants when inoculated with the ectomycorrhizal fungus Pisolithus tinctorius. The expression of candidate genes encoding proteins involved in nitrogen uptake and assimilation was investigated in ectomycorrhizal roots. We found that three oak ammonium transporters were over-expressed in root tissues after inoculation, while the expression of amino acid transporters was not modified, suggesting that inorganic nitrogen is the main form of nitrogen transferred by the symbiotic fungus into the roots of the host plant. Analysis by heterologous complementation of a yeast mutant defective in ammonium uptake and GFP subcellular protein localization clearly confirmed that two of these genes encode functional ammonium transporters. Structural similarities between the proteins encoded by these ectomycorrhizal upregulated ammonium transporters, and a well-characterized ammonium transporter from E. coli, suggest a similar transport mechanism, involving deprotonation of NH4+, followed by diffusion of uncharged NH3 into the cytosol. This view is supported by the lack of induction of NH4+ detoxifying mechanisms, such as the GS/GOGAT pathway, in the oak mycorrhizal roots.