Melatonin - a new plant biostimulator?

Melatonin (MEL) is a highly conserved molecule occurring in evolutionarily distant organisms such as bacteria, mono- and multicellular algae, higher plants, invertebrates and vertebrates. Although until recently, this molecule was mainly known as an animal hormone and neurotransmitter, its role in plants is currently being extensively investigated. MEL, N-acetyl-5-methoxytryptamine was found in various agronomically important vegetables, fruits, grains and herbs. Its concentration varies from ng to pg per gram fresh weight. MEL is present in all plant organs with its highest level found in seeds. Since the germ tissues are highly vulnerable to oxidative damage, and MEL is well known as an effective amphiphilic free radical scavenger, MEL may play an important role in plant antioxidant defence system. Especially in desiccated seed tissues, where the activity of detoxifying enzymes is limited, MEL may be essential for protecting plant germ and reproductive tissues from oxidative injuries. Recent studies provide solid evidence for MEL acting as plant growth and development regulator as well as a biostimulator, especially under unfavourable environmental conditions. Various plant species rich in MEL exhibit higher capacity for stress tolerance. MEL is also involved in stress-affected developmental transitions including flowering, fruiting and senescence. Plants are equipped with an enzymatic system for MEL biosynthesis; they are also able to synthesize a MEL precursor, tryptophan. In addition to in vivo synthesis, plants can also absorb exogenously provided MEL from the environment. These, and particularly the evidence that in plants MEL induces resistance against stresses, suggest that our concept of seed enrichment with exogenous MEL is justified. Our experiments proved that exogenous MEL applied into seeds by pre-sowing treatment (priming) improved their vigour and germination efficiency as well as seedling growth.