Melatonin: plant growth regulator and/or biostimulator during stress?

•Melatonin acts as plant growth regulator and modulates growth, rooting, and senescence.•Melatonin also plays a role in the plant anti-stress response.•Melatonin acts as a direct antioxidant and triggers an antioxidative response in plants.•Melatonin can be used as a protective agent for crop improvement and protection. Melatonin regulates the growth of roots, shoots, and explants, to activate seed germination and rhizogenesis and to delay induced leaf senescence. The antioxidant properties of melatonin would seem to explain, at least partially, its ability to fortify plants subjected to abiotic stress. In this Review we examine recent data on the gene-regulation capacity of melatonin that point to many interesting features, such as the upregulation of anti-stress genes and recent aspects of the auxin-independent effects of melatonin as a plant growth regulator. This, together with the recent data on endogenous melatonin biosynthesis induction by environmental factors, makes melatonin an interesting candidate for use as a natural biostimulating treatment for field crops.