Melatonin was first discovered in bovine or cow pineal gland. It was then found in many species of animals including humans. Melatonin is widespread and most extensively studied in vertebrates, mammals, reptiles, amphibians, bony and cartilagenous fishes and birds.

It was then noted that some organisms outside animal kingdom also possessed melatonin. This included dinoflagellate alga Lingulodinium polyedrum. Further studies showed that melatonin was found in many algae and in several species of fungi, protists, and prokaryotes. Studies aimed to look for melatonin in higher plants as well.

Melatonin in plants

Melatonin has been identified since then in many plants including Feverfew (Tanacetum parthenium), and St John's wort (Hypericum perforatum). 

Some fruits and grains like bananas and grapes, rice, wheat, barley, and oats are rich in melatonin. Other foods include herbs, olive oil, coffee, tea, wine and beer. Excess consumption of these does not raise the blood levels of melatonin in humans significantly.

Role of melatonin in plants

• Like animals, melatonin helps plants respond to light and dark cycles or the photoperiod.
• Melatonin helps them respond to harsh environments.
• Melatonin also plays the role of antioxidant in many plants. This may be the original function of melatonin in organisms with the other functions were added with evolution. In plants it acts as a direct free radical scavenger and an indirect antioxidant. Melatonin directly detoxifies the hydroxyl radical (OH), hydrogen peroxide, nitric oxide, peroxynitrite anion, peroxynitrous acid, and hypochlorous acid. Additionally, melatonin increases the activity of several antioxidative enzymes.
• Plants tend to have a much higher level of melatonin since they lack mobility to cope with harsh environments.
• Evidence also indicates that melatonin has an ability to increase the production of crops. This may be possible by melatonin’s role in preservation of chlorophyll, promotion of photosynthesis, and stimulation of root development. Plants genetically engineered to make enhanced melatonin content could lead to increase in crop production in agriculture and improved general health of humans.

Differences between plant melatonin and animal melatonin

The enzyme that synthesized melatonin - arylalkylamine N-acetyltransferase (AANAT) – has not been identified in plants. The serotonin N-acetylating enzyme in plants may differ greatly from the animal AANAT with regard to sequence and structure. This means there are multiple evolutionary origins of enzymes with these catalytic properties.