Hidden Fungal Allies Help Peas Survive Zinc-Starved Soils Through Whole-Plant Signaling

Zinc may be a micronutrient, but for crops it is anything but minor. When soils run short of this essential element, plants pay a heavy price: chlorophyll synthesis falters, photosynthesis slows, membranes leak, and growth stalls. The problem is global in scale, because zinc availability is notoriously poor in the calcareous and alkaline soils that dominate many of the world’s agricultural regions. Legumes such as garden pea are especially vulnerable, since zinc shortages undermine not only vegetative growth but also nodulation and nitrogen fixation, the very traits that make peas valuable in sustainable rotations. A new study published in Discover Plants now shows that an ancient underground partnership may offer a way out, and that the benefits travel far beyond the roots where the partnership begins.

The research, led by Shifat Ara Saiful and Ahmad H. Kabir of the University of Rajshahi and Lamar University together with colleagues in Bangladesh and the United States, focused on Rhizophagus irregularis, one of the most widespread arbuscular mycorrhizal fungi, or AMF. These fungi colonize the roots of most terrestrial plants and push out vast networks of hyphae into the surrounding soil, effectively extending the root system’s reach. Because hyphae can explore soil pores and access nutrient pools beyond the depletion zone that forms around roots, mycorrhizal plants can tap zinc reserves that non-colonized roots simply cannot reach. The fungi also release organic acids and chelating compounds that dissolve poorly available zinc, and they can regulate the expression of plant zinc transporters that move the metal from fungal structures into root cortical cells.

To test how much this symbiosis matters under zinc starvation, the team grew pea seedlings in carefully controlled hydroponic systems with either sufficient zinc, at 2.0 micromolar zinc sulfate, or deficient zinc, at 0.1 micromolar. Some plants received an inoculum of R. irregularis spores while others did not, producing four treatment groups. After five weeks, microscopic examination of trypan blue-stained roots confirmed that colonization occurred only in inoculated plants, which displayed the telltale fungal structures of mycelia, arbuscules, and vesicles. Molecular verification followed: PCR amplification of the fungus’s 18S rRNA gene produced clear bands in inoculated samples, and quantitative PCR showed significantly higher fungal abundance in colonized roots, regardless of whether zinc was plentiful or scarce.

The damage inflicted by zinc deficiency was unmistakable. Zinc-starved peas showed stunted shoots and roots, reduced dry weight of both organs, and lower turgid weight, a sign of impaired water status. Chlorophyll scores measured with a SPAD meter dropped, as did the photosynthetic performance index and the maximum quantum efficiency of photosystem II, both captured with chlorophyll fluorescence analysis. At the cellular level, the stress was equally evident: roots accumulated hydrogen peroxide, a reactive oxygen species, and leaked electrolytes, indicating membrane damage and disrupted redox balance. In short, zinc deficiency pushed the seedlings toward oxidative injury and photosynthetic collapse, a pattern consistent with zinc’s established roles in enzyme activation, protein synthesis, membrane integrity, and protection against reactive oxygen species.

Inoculation with R. irregularis dramatically reversed this decline. Colonized plants grown under zinc deficiency regained shoot height, root length, and biomass, and their chlorophyll scores and photosynthetic parameters rose to levels statistically comparable to healthier treatments. Hydrogen peroxide accumulation and electrolyte leakage fell significantly in the inoculated, zinc-starved plants, pointing to a protective effect on membranes and antioxidant systems. The best overall performers were plants that enjoyed both ample zinc and the fungal partner, but the crucial finding was that the fungus could largely compensate when zinc was limiting, restoring growth and physiology toward control levels.

Elemental analysis by inductively coupled plasma mass spectrometry revealed how the fungus reshaped mineral nutrition. Zinc concentrations in both roots and leaves fell sharply under deficiency, yet AMF inoculation restored tissue zinc levels significantly. The deficiency also disturbed the balance of other metals: iron and copper accumulated to excess in zinc-starved roots and leaves, likely through non-specific metal transporters and disrupted ion homeostasis, while manganese rose in roots. AMF colonization partially normalized iron and copper levels and further increased root manganese, suggesting the fungus fine-tunes the whole micronutrient network rather than simply delivering more zinc. Notably, other measured elements did not change across treatments, indicating that the mycorrhizal effect was element-specific.

At the molecular level, the fungus appeared to switch on the plant’s own zinc-import machinery. Quantitative real-time PCR showed that expression of the zinc transporter genes ZIP1 and ZIP2 increased in the roots of zinc-deficient plants, and that AMF inoculation pushed their expression even higher. ZIP-family transporters are central to zinc uptake and redistribution during micronutrient shortage, so their induction implies that the symbiosis works on two fronts: the fungal hyphae mobilize and deliver zinc from the soil, while the host simultaneously upgrades its internal transport pathways to receive and move the metal. This coordinated regulation helps explain how colonized plants achieved higher tissue zinc concentrations despite the severely limited supply in the growth medium.

The study also uncovered a striking metabolic dimension. Amino acid profiling of roots and leaves showed that zinc deficiency reprogrammed nitrogen metabolism extensively. Stress-associated amino acids such as histidine, lysine, threonine, valine, aspartic acid, glutamic acid, and proline accumulated, while growth-related amino acids including leucine, isoleucine, glycine, and tyrosine declined. This pattern reflects a classic stress response, shifting resources toward osmoprotection, nitrogen remobilization, and signaling at the expense of growth metabolism. AMF inoculation restored several of the depleted amino acids, particularly leucine, isoleucine, glycine, and tyrosine, in both roots and leaves. Because branched-chain amino acids support energy metabolism and stress recovery, and glycine and tyrosine feed antioxidative metabolism and secondary metabolite biosynthesis, their restoration suggests the fungus helps rebalance metabolism toward cellular repair and growth.

Perhaps the most compelling evidence came from a split-root experiment, in which the researchers divided each plant’s root system between two sealed compartments and applied different treatments to each side. When only one compartment was colonized by AMF while the other suffered zinc deficiency, the entire plant still benefited: shoot height, shoot dry weight, and chlorophyll scores improved even though half the root system never touched the fungus. In the treatment where both compartments received the fungus under sufficient zinc, plants matched the fully healthy controls. Compartment-specific effects did appear locally, with colonized roots showing greater elongation and lower hydrogen peroxide, but these localized changes were modest compared with the strong shoot-level recovery. The conclusion is that AMF trigger long-distance systemic signaling, likely involving phytohormones, reactive oxygen species, calcium, and nutrient-responsive pathways, that coordinates photosynthesis, biomass accumulation, and antioxidant defense across the whole plant.

The authors are careful to note the limits of their work. All experiments used pea seedlings in hydroponics, which allows precise control of zinc and fungal inoculation but cannot fully reproduce the complexity of soil-grown plants in the field. As a legume, pea also undergoes developmental shifts once rhizobial nodulation and nitrogen fixation begin, which could alter nutrient demand and root physiology in ways that seedling-stage data cannot capture. Future studies validating these responses across growth stages, in real soils, and in interaction with rhizobia are clearly needed. Even so, the findings carry real weight for sustainable agriculture. They show that a single mycorrhizal fungus can simultaneously improve zinc acquisition, restore metabolic balance, protect the photosynthetic apparatus, and broadcast resilience through systemic signals. As zinc deficiency constrains yields across vast tracts of micronutrient-poor farmland, AMF-based bioinoculants emerge as a promising, environmentally sound complement to fertilizers, one that works with the plant’s own biology rather than simply pouring more metal into the ground.

Subject of Research: Arbuscular mycorrhizal fungus-mediated systemic tolerance to zinc deficiency in pea plants

Article Title: AMF-mediated systemic regulation induces Zn-deficiency tolerance through nutrient and metabolic adjustment in pea plants

Article References: Saiful, S. A., Akter, M. N., Hasan, M. R., Reza, M. S., Mondal, T., Mim, F., Mostofa, M. G., Rahman, M. M., Karim, M. R., & Kabir, A. H. (2026). AMF-mediated systemic regulation induces Zn-deficiency tolerance through nutrient and metabolic adjustment in pea plants. Discover Plants, 3(1), Article 409. https://doi.org/10.1007/s44372-026-00886-5

Image Credits: AI Generated

DOI: 10.1007/s44372-026-00886-5

Keywords: arbuscular mycorrhizal fungi, Rhizophagus irregularis, zinc deficiency, pea, ZIP transporters, amino acid metabolism, systemic signaling, photosynthesis, oxidative stress, mineral homeostasis, biofertilizer, plant physiology