Laminarin and rhizobacteria (Paenibacillus alvei T22) elicit metabolic reprogramming of wheat seedlings: A metabolomics-guided mode-of-action discovery of biostimulants for plant growth and defence priming

Plant biostimulants, including laminarin (seaweed extract-SWE) and plant growth-promoting rhizobacteria (PGPR), are known to enhance crop performance. Multi-component biostimulants, which combine microbial and non-microbial agents, show promise for enhanced plant physiological responses and defence activation. While individual mechanisms of SWEs and PGPR have been extensively documented, their combined metabolic effects and integration mechanisms remain poorly characterised. We used untargeted metabolomics to profile metabolic responses in plants treated with laminarin, Paenibacillus alvei T22 (PGPR), and their combination. Three distinct metabolic traits emerged: laminarin treatment elevated citrate levels and depleted malate, indicating maximum TCA cycle flux, while dramatically increasing tyrosine, phenylalanine, and tryptophan, which drove comprehensive phenylpropanoid activation with peak accumulation of coumaric acid, caffeic acid, and ferulic acid. PGPR treatment resulted in moderate citrate elevation, stable malate levels, selective increases in aromatic amino acids, and targeted accumulation of phenolic acids, primarily focused on defensive compounds. Combination treatment produced intermediate metabolite levels rather than additive effects, with controlled citrate elevation, prevented malate depletion, and optimised phenolic acid production. This first comparative mechanistic analysis of microbial versus non-microbial biostimulants reveals distinct metabolic strategies: energy-driven growth acceleration versus defence-focused priming. It demonstrates that their combination produces balanced metabolic optimisation, achieving high functional capacity while maintaining metabolic stability and preventing extremes, rather than synergistic amplification, challenging conventional assumptions about biostimulant interactions. These biostimulants reprogram plant metabolism through distinct pathway-level mechanisms revealed by metabolic network analysis, unlocking the molecular basis of superior plant performance. These discoveries provide the mechanistic framework for designing next-generation biostimulant formulations tailored to specific crop requirements, environmental challenges, and performance targets in precision agriculture, for sustainable agricultural intensification through targeted metabolic reprogramming.