Fungal Ally Reshapes Tobacco Root Microbes to Boost Growth in Poor Soil

When soils run short of nutrients, crops face a double burden: they must scavenge scarce minerals while simultaneously defending themselves against the oxidative damage that nutrient stress triggers inside their cells. A new greenhouse study suggests that an ancient partnership may hold the key to easing both burdens at once. Researchers at Anhui Agricultural University in China have shown that inoculating tobacco with the arbuscular mycorrhizal fungus Claroideoglomus etunicatum transforms the bacterial community living around the plant’s roots, building a richer, more cooperative microbial network that helps the plant grow and protects it from stress. The work, published in the journal Advanced Biotechnology, offers one of the most detailed portraits yet of how a single fungal symbiont can reorganize an entire below-ground ecosystem.

Arbuscular mycorrhizal fungi, or AMF, are among the most successful symbionts on Earth, forming associations with roughly 80 percent of plant species, including most agricultural crops. These fungi penetrate or envelop plant roots and extend a web of filaments, called extraradical mycelium, far into the surrounding soil. Through this network they draw in water and hard-to-reach nutrients, particularly phosphorus, whose poor mobility in soil often limits crop growth. In exchange, the plant supplies the fungus with carbon derived from photosynthesis. For decades, agronomists have exploited this partnership to improve crop tolerance to drought, salinity, and nutrient poverty, but the consequences for the wider rhizosphere microbiome, the teaming bacterial ecosystem hugging the root surface, have remained poorly resolved, especially under low-nutrient conditions.

To probe this question, the research team grew tobacco, Nicotiana tabacum, in pots filled with a low-fertility loess-derived soil collected near the university in Anhui Province. The soil was deliberately lean: ammonium nitrogen measured just 3.37 milligrams per kilogram and nitrate nitrogen 2.16 milligrams per kilogram, with modest organic matter of 15.3 grams per kilogram. Before planting, the soil was autoclaved at 121 degrees Celsius for two hours to eliminate native fungi and other microbes, creating a clean slate. Half the pots received 30 grams of an inoculum containing C. etunicatum spores, mycelia, and colonized root fragments at a density of five spores per gram; the control pots received an equal amount of sterilized inoculum to keep background conditions identical. The plants then grew for 60 days in a greenhouse cycling between 28 and 18 degrees Celsius, with three biological replicates per treatment.

The growth results were unambiguous. Compared with uninoculated controls, AMF-colonized tobacco showed significantly greater aboveground and belowground fresh weight and taller plants, with root biomass increasing by 12.90 percent and spore density in the soil rising by 4.51 percent. More striking were the changes inside the plant’s stress-response machinery. Activities of the antioxidant enzymes catalase and superoxide dismutase rose significantly, while levels of malondialdehyde, a molecular marker of lipid peroxidation and membrane damage, fell. Because malondialdehyde accumulates when reactive oxygen species assault cell membranes under stress, its decline indicates that the fungal partnership had measurably blunted the oxidative injury that nutrient scarcity inflicts on tobacco tissue.

To see whether these physiological gains were mirrored in the root-zone microbiome, the team sequenced the V4 region of the bacterial 16S rRNA gene from rhizosphere soil using the Illumina MiSeq platform, processing the reads through the Deblur pipeline to generate amplicon sequence variants. Phylogenetic analysis of the 100 most abundant genera identified twelve core bacterial phyla, dominated by Proteobacteria with 28 genera, Chloroflexi with 16, Actinobacteriota with 14, and Myxococcota with 12. Fungal inoculation significantly reshaped the community structure, enriching beneficial groups across these phyla. Among the genera that shifted most dramatically, the AMF-treated soils favored unclassified members of the Xanthomonadaceae along with Agromyces, Phenylobacterium, and Solimonas, whereas the control soils were dominated by Arenimonas, Sphingoaurantiacus, and Azohydromonas.

Each enriched phylum brings a distinct ecological skill set. Proteobacteria, among the most diverse bacterial phyla known, act as central regulators of global carbon, nitrogen, and sulfur cycling, and their nitrogen-fixing members can enhance soil fertility through nitrification-related processes. Actinobacteriota solubilize phosphorus and mineralize organic nutrients while producing extracellular metabolites that suppress pathogens and sometimes function as growth regulators. Chloroflexi, filament-forming anaerobes that degrade complex carbohydrates and peptides, likely metabolize the organic deposits left by fungal hyphae, explaining their rise in the mycorrhizal rhizosphere. Myxococcota, meanwhile, are predators within the mycelial food web, consuming the carbon-rich compounds that AMF transport from the plant. Together, these groups form a functional support crew whose presence correlates with improved nutrient acquisition and stress tolerance in the host.

Perhaps the most visually compelling evidence came from co-occurrence network analysis, which maps which bacterial taxa tend to appear together and infers cooperative or competitive relationships. The networks differed sharply between treatments. Although the control network contained slightly more nodes and edges, the AMF network showed a higher average clustering coefficient, 0.713 versus 0.705, and, critically, far fewer negative correlations between species. In the control network, 96.13 percent of nodes were peripheral with little topological significance, while the AMF network pushed that proportion to 97.31 percent but concentrated its interactions more centrally, with node-level measures of degree, betweenness, and modularity all significantly higher. The researchers interpret this architecture as evidence that fungal inoculation drives the community toward greater microbial synergy, with more positive, mutualistic links replacing antagonistic ones.

Functional profiling reinforced the picture. Using STAMP statistical comparisons, the team found that seven representative microbial functions differed significantly between treatments, with AMF-treated soils showing enhanced abundance of genes associated with signal transduction and prokaryotic cell communities. Mantel tests revealed that bacterial alpha diversity was tightly linked to cell motility, signal transduction, and prokaryotic community functions, with correlation coefficients exceeding 0.20 at statistical significance. The strongest positive functional correlations appeared among amino acid metabolism, carbohydrate metabolism, and cofactor and vitamin metabolism, suggesting that the enriched community was not merely more diverse but metabolically more capable.

To tie these threads together causally, the researchers built a structural equation model linking the fungus, bacterial network structure, soil and plant enzyme activity, community diversity, community function, and plant antioxidant capacity. The model showed that AMF exerted strong positive effects on bacterial network structure, with a standardized path coefficient of 0.840, and on soil enzyme activity, at 0.573, both highly significant. Plant enzyme activity positively influenced both bacterial community function and the plant’s antioxidant capacity, while bacterial community diversity and function in turn boosted antioxidant defenses. Notably, bacterial community diversity exerted a stronger total effect on antioxidant capacity than the fungus itself, implying that the fungus works largely through its microbial intermediaries rather than on the plant directly.

The implications extend well beyond tobacco, a crop of substantial economic importance in China and one of the most intensively studied mycorrhizal hosts. As agriculture confronts the twin pressures of rising fertilizer costs and the environmental toll of over-application, biological strategies that coax more productivity from lean soils are increasingly attractive. This study demonstrates that a mycorrhizal inoculant does not simply feed the plant; it curates a bacterial entourage, recruiting nutrient cyclers, pathogen suppressors, and carbon scavengers while knitting them into a stabler, more cooperative network. The finding that community diversity, not the fungus alone, is the strongest driver of antioxidant protection suggests that future biofertilizers may need to be designed as ecosystems rather than single strains. For now, the humble threads of a soil fungus have been shown to conduct an entire underground orchestra, and the plant, quite literally, reaps the benefits.

Subject of Research: Effects of arbuscular mycorrhizal fungal inoculation on the rhizosphere bacterial community and antioxidant capacity of tobacco under low-nutrient conditions

Article Title: Effect of Claroideoglomous etunicatums on rhizosphere bacterial community of tobacco under low nutrient conditions

Article References: Chen, J., Geng, X., Zhang, Q., Lin, K., Li, Z., Wang, B., Xiao, Q., & Li, X. (2025). Effect of Claroideoglomous etunicatums on rhizosphere bacterial community of tobacco under low nutrient conditions. Advanced Biotechnology, 3(3), Article 22. https://doi.org/10.1007/s44307-025-00071-x

Image Credits: AI Generated

DOI: 10.1007/s44307-025-00071-x

Keywords: arbuscular mycorrhizal fungi, Claroideoglomus etunicatum, tobacco, rhizosphere, bacterial community, 16S rRNA sequencing, co-occurrence network, antioxidant enzymes, malondialdehyde, soil microbiome, low nutrient stress, structural equation modeling