5-aminolevulinic acid and copper stress adaptation in plants: direct evidence and mechanistic insights from other stress systems

Copper (Cu) is an essential micronutrient that supports vital plant processes, including photosynthetic electron transport, respiration, and antioxidant metabolism. However, excessive Cu accumulation caused by industrial activities and intensive agriculture disrupts metal homeostasis, promotes severe oxidative stress, impairs chloroplast function, and strictly limits crop productivity. 5-Aminolevulinic acid (5-ALA), a precursor of chlorophyll and heme biosynthesis, has been investigated for mitigating plant stress. Direct experimental evidence under Cu in the studies examined is concentrated in grapevine, where foliar 5-ALA reduced leaf Cu content and lipid peroxidation, increased SOD and POD activities, and altered Cu-related, chlorophyll-metabolism, and photosynthesis-associated transcripts. Evidence for nitrate-reductase-associated nitric oxide signaling, heme involvement, altered metal-transporter expression and thiol-mediated detoxification comes mainly from other metal or metalloid stresses and from salinity or chilling models. These findings identify candidate mechanisms but do not establish the same pathways under Cu. This review therefore distinguishes Cu observations, evidence from other metals, evidence from other abiotic stresses and hypotheses requiring Cu-specific validation. We discuss unresolved molecular targets, dose dependence and the use of functional genetics and multi-omics to test pathway involvement. This perspective defines priorities for Cu-specific mechanistic validation and for assessing agricultural performance under realistic exposure conditions.