5‐ ALA Exacerbates Maize ( Zea mays L. ) Energy Depletion Under Severe Drought‐Flood Abrupt Alternation by Activating Energy‐Costly Phenylpropanoid Metabolism

The plant growth regulator 5‐aminolevulinic acid (5‐ALA) mitigates mild to moderate abiotic stresses. However, its efficacy and mode of action under extreme combined stress conditions remain poorly understood. This study investigated the unconventional effects of 5‐ALA on maize ( Zea mays L.) subjected to severe drought‐flood abrupt alternation (DF) using integrated transcriptomic, proteomic and targeted phytohormone analyses. We discovered that 5‐ALA unexpectedly accelerated plant wilting and mortality. While severe DF stress itself triggered a profound energy crisis by inhibiting photosynthetic carbon fixation and glycolysis, 5‐ALA treatment specifically and strongly activated the jasmonic acid biosynthesis pathway and downstream energy‐intensive defence programs, particularly phenylpropanoid and flavonoid biosynthesis. Multi‐omics integration revealed that this forced activation of defence synthesis occurred under irreversible energy supply collapse, co‐occurring with induced endoplasmic reticulum stress, inhibition of ribosomal protein synthesis and initiation of programmed cell death. Our findings provide the first systemic evidence that 5‐ALA can act as a ‘double‐edged sword’, pushing plants into a critical ‘defense overgrowth‐energy depletion’ dilemma under extremely energy‐deficient conditions. This study redefined the application boundaries of 5‐ALA and highlighted the primacy of plant energy status in determining the outcomes of chemical interventions.