Phosphorus/nitrogen sensing and signaling in diverse root-fungus symbioses

Establishing mutualistic relationships between plants and fungi is crucial for overcoming nutrient deficiencies in plants. This review highlights the intricate nutrient sensing and uptake mechanisms used by plants in response to phosphate and nitrogen starvation, as well as their interactions with plant immunity. The coordination of transport systems in both host plants and fungal partners ensures efficient nutrient uptake and assimilation, contributing to the long-term maintenance of these mutualistic associations. It is also essential to understand the distinct responses of fungal partners to external nutrient levels and forms, as they significantly impact the outcomes of symbiotic interactions. Our review also highlights the importance of evolutionarily younger and newly discovered root-fungus associations, such as endophytic associations, which offer potential benefits for improving plant nutrition. Mechanistic insights into the complex dynamics of phosphorus and nitrogen sensing within diverse root-fungus associations can facilitate the identification of molecular targets for engineering symbiotic systems and developing plant phenotypes with enhanced nutrient use efficiency. Ultimately, this knowledge can inform tailored fertilizer management practices to optimize plant nutrition.

Direct nitrogen, phosphorus and carbon exchanges between Mucoromycotina 'fine root endophyte' fungi and a flowering plant in novel monoxenic cultures

Most plants form mycorrhizal associations with mutualistic soil fungi. Through these partnerships, resources are exchanged including photosynthetically fixed carbon for fungal-acquired nutrients. Recently, it was shown that the diversity of associated fungi is greater than previously assumed, extending to Mucoromycotina fungi. These Mucoromycotina 'fine root endophytes' (MFRE) are widespread and generally co-colonise plant roots together with Glomeromycotina 'coarse' arbuscular mycorrhizal fungi (AMF). Until now, this co-occurrence has hindered the determination of the direct function of MFRE symbiosis. To overcome this major barrier, we developed new techniques for fungal isolation and culture and established the first monoxenic in vitro cultures of MFRE colonising a flowering plant, clover. Using radio- and stable-isotope tracers in these in vitro systems, we measured the transfer of 33 P, 15 N and 14 C between MFRE hyphae and the host plant. Our results provide the first unequivocal evidence that MFRE fungi are nutritional mutualists with a flowering plant by showing that clover gained both 15 N and 33 P tracers directly from fungus in exchange for plant-fixed C in the absence of other micro-organisms. Our findings and methods pave the way for a new era in mycorrhizal research, firmly establishing MFRE as both mycorrhizal and functionally important in terrestrial ecosystems.

Agricultural land-use favours Mucoromycotinian, but not Glomeromycotinian, arbuscular mycorrhizal fungi across ten biomes

Globally, agricultural land-use negatively affects soil biota that contribute to ecosystem functions such as nutrient cycling, yet arbuscular mycorrhizal fungi (AMF) are promoted as essential components of agroecosystems. Arbuscular mycorrhizal fungi include Glomeromycotinian AMF (G-AMF) and the arbuscule-producing fine root endophytes, recently re-classified into the Endogonales order within Mucoromycotina. The correct classification of Mucoromycotinian AMF (M-AMF) and the availability of new molecular tools can guide research to better the understanding of their diversity and ecology. To investigate the impact on G-AMF and M-AMF of agricultural land-use at a continental scale, we sampled DNA from paired farm and native sites across 10 Australian biomes. Glomeromycotinian AMF were present in both native and farm sites in all biomes. Putative M-AMF were favoured by farm sites, rare or absent in native sites, and almost entirely absent in tropical biomes. Temperature, rainfall, and soil pH were strong drivers of richness and community composition of both groups, and plant richness was an important mediator. Both fungal groups occupy different, but overlapping, ecological niches, with M-AMF thriving in temperate agricultural landscapes. Our findings invite exploration of the origin and spread of M-AMF and continued efforts to resolve the phylogeny of this newly reclassified group of AMF.