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Humic Acid Induces an Adaptive Stress Response During Early Signaling in Rice
José Nivaldo de Oliveira Sátiro1, Andrés Calderín García2,3, Andressa Fabiane Faria de Souza1
1Plant Nutrition Laboratory, Department of Soils, Federal Rural University of Rio de Janeiro (UFRRJ), Seropédica, Rio de Janeiro, Brazil.
Physiologia Plantarum
|April 19, 2026
Summary
Humic acid (HA) acts as a plant biostimulant by rapidly reprogramming gene expression in rice roots. This study reveals HA induces a eustress-like state, enhancing antioxidant capacity and activating defense pathways via redox-hormone signaling.
Area of Science:
- Plant Biology
- Molecular Biology
- Biochemistry
Background:
- Humic substances (HS) are known plant biostimulants, but their molecular mechanisms are not fully understood.
- Investigating the early molecular responses to humic acid (HA) can elucidate its biostimulant effects.
Purpose of the Study:
- To investigate the early transcriptional responses of rice roots to vermicompost-derived humic acid (HA).
- To elucidate the molecular mechanisms underlying the biostimulant action of HA.
Main Methods:
- RNA sequencing was used to analyze the transcriptional changes in rice roots after 4 hours of exposure to HA.
- Gene enrichment analysis was performed to identify affected biological pathways and functions.
Main Results:
- HA treatment rapidly induced significant transcriptional reprogramming in rice roots, with 231 genes induced and only 7 repressed.
- Induced genes were enriched in redox homeostasis, glutathione metabolism, and detoxification pathways.
- Activation of phenylpropanoid metabolism, oxylipin biosynthesis, jasmonate signaling, and WRKY/C2H2 transcription factors was observed.
Conclusions:
- HA acts as a chemical eustressor, rapidly modulating reactive oxygen species (ROS)-dependent signaling and integrating it with jasmonate-centered responses.
- This rapid reconfiguration of redox-hormone crosstalk establishes a eustress-like state, mobilizing plant defense and adaptive mechanisms without causing acute stress injury.
- ROS signaling serves as a central hub integrating the biostimulant and adaptive effects of humic substances in plants.
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