5-Aminolevulinic Acid: from pyrrole biosynthetic precursor to multifunctional plant growth regulator

5-Aminolevulinic acid (ALA) is a non-protein δ-amino acid and an essential precursor of tetrapyrrole compound biosynthesis. Nowadays, it is a well-known natural plant growth regulator with multiple biological regulatory functions. In this review, we summarize the regulatory effects of ALA in promoting plant growth and the development of organs such as roots, stems, leaves, flowers, and fruits under normal conditions as well as stressful conditions. We emphasize the newly revealed signaling transduction and transcriptional regulatory mechanisms of ALA in maintaining root functions against abiotic stresses, improving leaf photosynthetic performance, and enhancing fruit appearance and flavor qualities as well as storage. Although most of the current reports on ALA are still apparent effect descriptions rather than mechanism explorations, studies suggest that ALA can facilitate agricultural development toward higher yield, quality, efficiency, and safety. The regulatory mechanisms of ALA at different levels need further study in the future.