Agricultural management and pesticide use reduce the phosphorus uptake capability of beneficial plant symbionts

Phosphorus (P) acquisition is key for global food production. Arbuscular mycorrhizal fungi (AMF) help plants acquire P and are considered key for the design of sustainable agroecosystems. However, how the functioning of AMF varies across agricultural soils and responds to management practices is still unknown. Here, we collected soils from 150 cereal fields and 60 non-cropped grassland sites across Europe, and in a greenhouse experiment, we tested the ability of AMF in these soils to forage for radioisotope-labelled 33P from a hyphal compartment. Hyphal-mediated P uptake was 64% higher in non-cropped grassland compared to cropland soils. Soil pH and organic carbon best explained the hyphal-mediated P uptake in the grasslands, while the use of fungicide in croplands reduced P uptake in the croplands by 43%. Our results suggest that land-use intensity and fungicide use are major deterrents to the natural capacity of AMF to contribute to sustainable crop production.

ParA and ParB coordinate chromosome segregation with cell elongation and division during Streptomyces sporulation

In unicellular bacteria, the ParA and ParB proteins segregate chromosomes and coordinate this process with cell division and chromosome replication. During sporulation of mycelial Streptomyces , ParA and ParB uniformly distribute multiple chromosomes along the filamentous sporogenic hyphal compartment, which then differentiates into a chain of unigenomic spores. However, chromosome segregation must be coordinated with cell elongation and multiple divisions. Here, we addressed the question of whether ParA and ParB are involved in the synchronization of cell-cycle processes during sporulation in Streptomyces . To answer this question, we used time-lapse microscopy, which allows the monitoring of growth and division of single sporogenic hyphae. We showed that sporogenic hyphae stop extending at the time of ParA accumulation and Z-ring formation. We demonstrated that both ParA and ParB affect the rate of hyphal extension. Additionally, we showed that ParA promotes the formation of massive nucleoprotein complexes by ParB. We also showed that FtsZ ring assembly is affected by the ParB protein and/or unsegregated DNA. Our results indicate the existence of a checkpoint between the extension and septation of sporogenic hyphae that involves the ParA and ParB proteins.

Nitrogen transfer and assimilation between the arbuscular mycorrhizal fungus Glomus intraradices Schenck & Smith and Ri T-DNA roots of Daucus carota L. in an in vitro compartmented system

Nitrogen metabolism was examined in monoxenic cultures of carrot roots (Daucus carota L.) colonized with the arbuscular mycorrhizal (AM) fungus Glomus intraradices Schenck & Smith. Glutamine synthetase and glutamate dehydrogenase activities were significantly increased in mycorrhizal roots for which only the extraradical mycelium had exclusive access to NH4NO3 in a distinct hyphal compartment inaccessible to the roots. This was in comparison with the water controls but was similar to the enzyme activities of non-arbuscular-mycorrhizal (non-AM) roots that had direct access to NH4NO3. In addition, glutamate dehydrogenase activity was significantly enhanced in AM roots compared with non-AM roots. Carrot roots took up 15NH4+ more efficiently than 15NO3–, and the extraradical hyphae transfered 15NH4+ to host roots from the hyphal compartment but did not transfer 15NO3–. The extraradical mycelium was shown, for the first time, to have a different glutamine synthetase monomer than roots. Our overall results highlight the active role of AM fungi in nitrogen uptake, transfer, and assimilation in their symbiotic root association.Key words: arbuscular mycorrhizal fungus, Ri T-DNA carrot roots, in vitro root-organ culture, nitrogen metabolism.

Water and nutrient translocation by hyphae of Glomus mosseae

To test the ability of vesicular–arbuscular mycorrhizal (VAM) hyphae to take up water, phosphorus, nitrogen, and potassium, mycorrhizal and nonmycorrhizal couchgrass (Agropyron repens) or white clover (Trifolium repens) plants were grown in pots with separated compartments for roots and hyphae growth. Soil solution transfer between compartments was blocked by a 2-mm air gap. Total shoot contents of phosphate and nitrogen, but not of potassium, were higher in mycorrhizal plants with access to the hyphal compartment. Hyphal uptake from the outer compartment accounted for 49% of the total phosphate and 35% of the total nitrogen taken up by mycorrhizal plants. This was associated with depletion of extractable phosphate, [Formula: see text]-nitrogen, and also [Formula: see text]-nitrogen in the soil of the hyphal compartments. In contrast, no difference in water loss from the hyphal compartments was measured by tensiometers under well-watered and water-stress conditions whether hyphae were present or not. Severance of the external hyphae did not affect water loss from the outer compartments. The results show the ability of VAM hyphae to transport considerable quantities of phosphate and nitrogen to the plant from soil zones several centimetres from the root. However, there was no evidence for a significant direct water transport by VAM hyphae to plants. Key words: Agropyron repens (couchgrass), Glomus mosseae, nitrogen, phosphorus, vesicular–arbuscular mycorrhiza, water.