Serotonin is an important chemical in plants as well as in animals. The serotonin found in plants is termed phytoserotonin and this form of serotonin is involved in several plant functions.

In plants, serotonin was first found and reported in a legume called Mucuna pruriens. Later, nearly 42 plant species from 20 families were reported to contain significant amounts of serotonin.

Since serotonin regulates intestinal activity, its presence in the fruits of plants may serve as a way of ensuring seeds are passed through and expelled by the digestive tract quickly, in much the same way as some fruit-based laxatives do. Serotonin is also found in the spines of stinging nettles, triggering pain if the spines are touched, just as its presence in insect venom does.

In drying seeds, serotonin helps to dispose of accumulating ammonia. The ammonia is incorporated into L-tryptophan. From there, it is decarboxylated to give tryptamine. Tryptamine then undergoes hydroxylation by cytochrome P450 monooxygenase to give serotonin.

The greatest concentration of serotonin in plants has been found in walnuts and hickory. In pineapples, banana, kiwi fruit, plums and tomatoes the concentration of serotonin is around 3 to 30 mg/kg. In general, vegetables contain moderate levels of serotonin, at around 0.1 to 3 mg/kg.

Unlike the precursors to serotonin such as 5HTP and tryptophan, serotonin itself does not cross the blood–brain barrier and consuming foods that contain serotonin has no effect on the serotonin level in the brain.

Plant function

Phytoserotonin also plays a role in the following aspects of plant function:

• Growth regulation
• Xylem sap exudation
• Flowering
• Ion permeability
• Plant morphogenesis
• Regulation of ripening