Two Fungal Partners Team Up to Steer Phosphorus Through a Forage Grass

Deep inside the roots and leaves of a native South American forage grass, two very different fungi are quietly deciding where the plant’s phosphorus should go. A new greenhouse study on Bromus auleticus, a prairie brome valued as pasture across the Pampas, shows that the famous grass endophyte Epichloë and a little-known dark septate root fungus called Thelonectria blackeriella act as complementary traffic controllers for one of agriculture’s most precious and most wasted nutrients. When phosphorus is scarce, the Epichloë-infected plants push more of the element into their shoots; when phosphorus is plentiful, plants carrying both fungi hoard it in their roots. The finding, published in Plant and Soil, suggests that the internal choreography of phosphorus inside a plant is not a solo performance by the plant itself but a negotiation with its resident microbes.

Phosphorus is a paradox in modern farming. Cereal crops worldwide use added phosphorus with notoriously low efficiency, and much of what farmers spread on fields ends up locked in soil or washed into waterways, feeding algal blooms downstream. Yet in phosphorus-poor grasslands, the element is often the single biggest limit on regrowth after grazing, because animals strip the leaves that hold most of a pasture’s mobile nutrients. Understanding how a forage grass allocates phosphorus between roots and shoots is therefore not an arcane detail of plant physiology; it is a question about how quickly a grazed pasture bounces back, and how much fertilizer a rancher actually needs to buy.

The research team, led by Natalia L. Terlizzi of the University of Buenos Aires and CONICET, began not in the greenhouse but in a petri dish. From the roots of Bromus auleticus plants they had isolated twenty-nine strains of dark septate endophytes, a loose guild of root-colonizing fungi named for the dark-pigmented walls that separate their cells. Dark septate endophytes have long been something of an enigma: unlike arbuscular mycorrhizal fungi, they do not form the familiar tree-like exchange structures inside root cells, and their effects on hosts range from clear mutualism to apparent neutrality or even parasitism. But many of them share one tantalizing trait, the ability to solubilize insoluble mineral phosphates, unlocking phosphorus that plant roots alone cannot touch.

Screening all twenty-nine strains on NBRIP medium, a standard growth medium built around insoluble tricalcium phosphate, the researchers looked for the classic halo of dissolved phosphate around fungal colonies, first on solid plates and then in liquid culture where solubilization can be quantified. One strain outperformed the rest, and molecular identification of its ribosomal DNA sequences revealed it to be Thelonectria blackeriella. In vitro, the fungus reliably freed inorganic phosphorus from an insoluble mineral source, confirming that it carried the biochemical machinery, typically organic acids and other exuded metabolites, needed to mine phosphorus that would otherwise remain chemically inert in the soil.

With the star solubilizer in hand, the team designed a factorial greenhouse experiment of elegant simplicity. Plants were sorted into four groups depending on their fungal passengers: with or without systemic Epichloë infection, and with or without inoculation by the T. blackeriella strain. Each of these four combinations was then grown under two phosphorus regimes, one without supplemental phosphorus and one with added fertilizer. After harvest, the researchers measured phosphorus concentrations separately in shoots and roots, and distilled the results into a single diagnostic number, the Phosphorus Translocation Factor, calculated as the ratio of shoot phosphorus concentration to root phosphorus concentration. A high factor means the plant is shipping phosphorus upward into the leaves that grazers will eat and that photosynthesis demands; a low factor means phosphorus is being banked below ground.

The results split cleanly along the phosphorus supply gradient, and this is where the story becomes genuinely surprising. Under low phosphorus availability, the decisive player was Epichloë. Plants harboring the shoot-systemic endophyte showed a higher Phosphorus Translocation Factor, meaning they prioritized allocating their scarce phosphorus to aerial tissues. This makes ecological sense: when every atom of phosphorus counts, sending it to the leaves supports photosynthesis and regrowth, and previous work has shown that phosphorus reserves in shoots and roots drive how fast grasses recover after defoliation. Epichloë, which lives its entire life inside the leaf and stem tissue and is transmitted through seed, has an obvious stake in keeping the aerial part of its host vigorous.

Under high phosphorus supply, the pattern inverted, and the dark septate endophyte took center stage. Plants carrying both symbionts, the dual-colonized E+ DSE+ group, accumulated significantly higher phosphorus concentrations in their roots than any other treatment. Rather than shuttling the abundant phosphorus straight to the shoots, these plants appeared to retain it below ground, with the root-dwelling fungus associated with enhanced root phosphorus storage. The authors interpret this as functional complementarity between the two fungi: Epichloë champions aerial allocation when phosphorus is limiting, while the dark septate endophyte facilitates root accumulation when the element is plentiful. In effect, the plant’s internal phosphorus economy shifts its strategy depending on which of its fungal partners has the most relevant expertise for the moment.

Why would a plant want to bank phosphorus in its roots when fertilizer is abundant? The answer may lie in the peculiar life of a forage grass. Grazing or mowing removes the shoots wholesale, and regrowth draws heavily on stored nutrient reserves in the surviving root system. Studies of defoliated grasses have shown that reallocation of nitrogen and phosphorus from roots is a major engine of post-grazing recovery, and that plants with larger phosphorus reserves regrow faster after cutting. A dual-symbiotic plant that stockpiles phosphorus in its roots during times of plenty is, in essence, loading a reserve tank for the next grazing event. For a pasture species like Bromus auleticus, that buffering capacity could translate directly into nutritional resilience, steadier forage supply, and reduced dependence on repeated phosphorus applications.

The study also carries a broader message about how scientists think about plant microbiomes. Most research on phosphorus acquisition has focused on single symbioses, above all the arbuscular mycorrhiza, and on total uptake rather than internal partitioning. This work demonstrates that two coexisting endophytes, one systemic and foliar, one root-localized and dark-pigmented, can exert opposing, context-dependent effects not on how much phosphorus the plant acquires, but on where the plant puts it. The same plant genotype, the same soil, and the same fertilizer level can produce different phosphorus architectures depending on which fungi are present. That kind of hidden regulatory layer is invisible to conventional fertilizer trials, yet it may explain much of the unexplained variation in how pastures and crops respond to phosphorus inputs.

There are, of course, caveats and open questions. The experiment was conducted in a greenhouse under controlled conditions, and field validation across real grazing systems, variable soils, and native microbial communities remains the necessary next step. The precise mechanisms by which Epichloë shifts allocation toward shoots, whether through hormonal signaling, altered root architecture, or changes in the rhizosphere community it is known to influence, are not yet resolved, and the study measured concentration-based partitioning rather than total phosphorus fluxes over time. Still, the core result stands on its own: a phosphate-solubilizing dark septate fungus isolated from the very roots of a native forage grass, working in concert with Epichloë, measurably rewires the plant’s internal phosphorus distribution in a supply-dependent way. As agriculture searches for ways to squeeze more value from finite phosphate rock while cutting runoff, the idea of recruiting a plant’s own fungal residents as allocation managers, rather than merely as uptake boosters, is an appealing and genuinely novel direction. The humble brome grass of the Pampas, it turns out, has been running a sophisticated phosphorus logistics operation all along, with two fungal partners signing off on every shipment.

Subject of Research: How dual fungal endophyte symbiosis modulates internal phosphorus allocation in a forage grass under varying phosphorus supply

Article Title: Epichloë and the dark septate endophyte Thelonectria blackeriella modulate internal phosphorus allocation in a forage grass under varying P supply

Article References: Terlizzi, N. L., Rodríguez, M. A., Mínguez, D. R., Della Mónica, I. F., & Novas, M. V. (2026). Epichloë and the dark septate endophyte Thelonectria blackeriella modulate internal phosphorus allocation in a forage grass under varying P supply. Plant and Soil. https://doi.org/10.1007/s11104-026-09020-6

Image Credits: AI Generated

DOI: 10.1007/s11104-026-09020-6

Keywords: Epichloë, dark septate endophytes, Thelonectria blackeriella, Bromus auleticus, phosphorus allocation, phosphorus solubilization, forage grass, plant symbiosis, phosphorus translocation factor, pasture regrowth, plant microbiome, sustainable agriculture