Root exudate flux as a quantitative language of phytobiome assembly and stress resilience.

Abstract

Climate change threatens crop productivity and soil function, increasing reliance on rhizosphere microbiomes that support plant nutrition, stress tolerance and disease suppression. Plants shape these communities through root exudation, which transfers an estimated 13.4 Pg C yr-1, about 9% of global gross primary productivity, belowground. Yet most studies report exudate composition rather than the concentrations and rates microorganisms perceive. This review synthesises the literature, with particular emphasis on studies published between 2015 and 2026, especially those from 2020 onwards, while incorporating foundational earlier studies. It proposes a quantitative signalling framework in which absolute concentration, release flux and spatial gradient influence microbial responses at defined activation thresholds. Exudate chemistry, biosynthesis, transport and uptake are first examined, distinguishing passive diffusion sustained by the microbial sink from transporter-mediated release, root re-uptake and phytosiderophore-mediated iron acquisition. Sampling and analytical approaches, including HRMS-based metabolomics, stable isotope tracing, imaging and kinetic modelling, are then evaluated, separating relative from absolute quantification. Because most high-precision flux data derive from hydroponic or sterile systems, where wheat phytosiderophore release exceeds that in soil by about 50-fold, in situ microdialysis, corrections for sorption and degradation, and minimum reporting standards are proposed. Signal-response modules and allelochemical signalling at the root-microbe-soil interface are discussed, followed by multi-omics integration, where static metabolite pools and transcripts cannot resolve flux unless anchored by pulse labelling with LC-MS, stable isotope probing and NanoSIMS. Stress-induced quantitative reprogramming of exudation is interpreted within this framework. Exudation measured in soil is concluded to be a net flux, and unresolved compound-specific thresholds remain the principal barrier to predictive rhizosphere manipulation.

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