Introduction: meeting rising food demand with smarter biostimulants
Global food demand is climbing as the world’s population is projected to reach 9.7 billion by 2050. To keep pace, researchers are turning to environmentally friendlier options beyond conventional inorganic fertilizers. Among these, Ascophyllum nodosum-based biostimulants (ANE) are gaining attention for their potential to enhance nutrient uptake, bolster abiotic stress tolerance, and shape the soil’s microbial communities that support plant growth.
Seaweed extracts, especially from brown algae like A. nodosum, are valued for their complex chemistry and observed agronomic benefits. This study investigates how ANE, applied as a soil drench or foliar spray to maize, influences the rhizosphere microbiome and the root exudate profile, and how these changes translate into microbial behavior and plant performance.
ANE alters root exudates and root tissue chemistry
Maize plants were treated with ANE in two growth systems (hydroponic and soil) at multiple concentrations, with treatments including soil drench and foliar spray. A key finding was that ANE triggers changes in root exudates and root tissues that likely serve as signals and nutrients for soil bacteria. Notably, benzoxazinoids—DIMBOA and its breakdown product MBOA—accumulated differently across treatments. The most pronounced root changes occurred with the 0.01% ANE soil drench, where DIMBOA and MBOA showed elevated levels, and DIMBOA/MBOA concentrations were largest in root extracts. These compounds are well known chemo-attractants for rhizosphere microbes and can steer microbial recruitment toward the root surface.
Root extracts, exudates, and rhizosphere soil were subjected to untargeted LC–HRMS analysis. Onward, DIMBOA and MBOA were confirmed as key metabolites linked to ANE treatment. In particular, roots under 0.01% ANE soil drench produced higher DIMBOA and MBOA than controls, aligning with shifts observed in the rhizosphere microbiome.
Microbial responses: chemotaxis, antimicrobial genes, and community shifts
The study used a model plant–microbe interaction to explore how ANE-modulated root signals affect bacteria. In vitro swarming assays showed Pseudomonas protegens CHA0 moved more aggressively toward exudates from ANE-treated maize roots, especially when ANE was applied to the roots (0.01%). This indicates that ANE-induced root signals enhance bacterial chemotaxis toward the rhizosphere.
When P. protegens CHA0 was exposed to root exudates from ANE-treated plants, several chemotaxis and antimicrobial genes were upregulated. The cheW and cheV chemotaxis genes showed notable induction, while pvdS (involved in siderophore production), prnD (pyrrolnitrin biosynthesis), and hcnA (hydrogen cyanide synthesis) were strongly boosted in response to root exudates from treated plants. Notably, the 0.01% ANE root-drench treatment produced the most robust gene induction, suggesting a link between root exudate composition and bacterial antimicrobial potential in the rhizosphere.
Beyond direct root exudate effects, ANE also induced changes in gene expression when bacteria were exposed to ANE in the absence of root exudates, though these responses were generally less pronounced than those driven by root-derived signals.
Rhizosphere outcomes: CFUs and community composition
The rhizosphere soil responded with higher microbial load under ANE treatment. The highest colony-forming unit counts were observed with the 0.01% ANE soil drench, roughly doubling the CFU counts relative to untreated soil. Amplicon sequencing revealed that while the dominant bacterial groups were relatively stable, the secondary community showed pronounced shifts under ANE treatment, with several beneficial genera rising in abundance. In particular, Chryseolinea increased dramatically (about 22-fold) under foliar 0.01% ANE, while Phycisphaera and others showed marked enrichment in soil drench treatments. The top zone of the rhizosphere exhibited the strongest compositional shifts, suggesting that ANE exerts its most evident influence where roots interact most with the soil microbiome.
Weighted UniFrac analyses confirmed distinct clustering for top-zone samples between untreated controls and ANE-treated groups, while bottom-zone samples showed less separation. This supports a scenario in which ANE reshapes the rhizosphere microbiome, with the greatest impact near the root-soil interface.
Implications for agriculture: practical use and future directions
The study demonstrates that ANE, whether applied as a soil drench or foliar spray, can modulate root exudates and drive a cascading effect on rhizosphere bacteria, including key plant-beneficial taxa. The dual mode of action—direct bacterial response to ANE and indirect modulation via root exudates—offers a nuanced mechanism for improving nutrient uptake, disease resistance, and stress tolerance in corn. While field performance can be variable, these findings provide a rationale for optimizing ANE application strategies (e.g., soil drenches targeting root signaling) to cultivate favorable rhizosphere assemblages and enhance crop resilience.
Bottom line
Ascophyllum nodosum–based biostimulants shape the corn rhizosphere by reconfiguring root exudates and recruiting beneficial bacteria through enhanced chemotaxis and antimicrobial activity. This work advances our understanding of how seaweed-derived inputs translate into tangible microbiome changes with potential agronomic benefits.
