**Background:** L-aminoguanidine (AG) has been widely used as a pharmacological inhibitor in plant research, primarily characterized as a diamine oxidase (DAO) or copper amine oxidase (CuAO) inhibitor. However, accumulating evidence suggests AG has multiple targets in plants, leading to controversial and sometimes contradictory results. This review aims to synthesize current knowledge on AG's effects on polyamine (PA) homeostasis, nitric oxide (NO) biosynthesis, and protein glycation in plants, to provide a more complete understanding of its mechanisms of action.
**Methods:** This is a narrative review that compiles and critically discusses published experimental evidence on AG's effects in plants. The authors organized the review around three main proposed mechanisms: (1) DAO inhibition affecting PA catabolism, (2) hypothetical NO synthase inhibition reducing NO content, and (3) antiglycation activity. They summarized data from multiple studies across various plant species, including Vicia faba, Hordeum vulgare, Glycine max, Lycopersicum esculentum, Lupinus luteus, Nicotiana benthamiana, Triticum aestivum, Cicer arietinum, Mesembryanthemum crystallinum, Nicotiana tabacum, Solanum lycopersicum, Medicago truncatula, Arabidopsis thaliana, Capsicum annuum, Helianthus annuus, Zea mays, Olea europaea, and Pisum sativum. AG concentrations used in these studies ranged from 10 µM to 5 mM, depending on the plant species, organ, and experimental conditions.
**Key Results:** As a DAO inhibitor, AG was shown to reduce GABA content by 22% in Lupinus luteus seedlings (Legocka et al.), and 5 mM AG inhibited DAO in fava beans with simultaneous reduction in GABA levels. In Medicago truncatula, 1 mM AG completely inhibited nodulation. AG concentrations as low as 10 µM were effective in dark-induced leaf senescence studies in barley, while stress conditions often required higher concentrations (0.5–5 mM). As a putative NOS inhibitor, AG at 1 mM was used in pepper leaves to demonstrate that Arg-dependent NO synthase activities were major contributors to NO production. In Arabidopsis thaliana, cuao8 mutants showed impaired NO biosynthesis related to inhibited DAO activity, and AG treatment confirmed this connection. The antiglycation effects of AG are the least studied in plants; AG is known to scavenge dicarbonyl compounds and prevent AGE formation due to its nucleophilic hydrazine and guanidine moieties, and it can inhibit ascorbate oxidation at 1–5 mM concentration. The authors note that only a few studies used the same AG concentrations under the same conditions, making cross-study comparisons difficult.
**Clinical Implications:** While this review focuses on plant biology, the findings have indirect clinical relevance. Understanding AG's mechanisms in plants can inform agricultural practices for enhancing crop stress tolerance to salinity, drought, and heavy metal stress, which has implications for food security and nutrition. The review also notes that pulses (legumes) have high GABA content and are considered functional foods. Additionally, AG's antiglycation properties are relevant to aging research, as glycation processes occur in both plants and animals. The authors emphasize that AG should be regarded as a modulator of PA metabolism rather than a specific DAO inhibitor, and future research should use modern biotechnological approaches (RNASeq, metabolomics, chemical modification) to decipher its complex mode of action across different plant species, developmental stages, and stress conditions.