An analysis on the accumulation of serotonin indicates the positively correlation between MdT5H5 and serotonin content in apple fruits.

• The 5-HT level was higher in the pulp than in the peel. • The distribution of 5-HT in apple fruits was skewed. • MdT5H5 may play an important role in the synthesis of 5-HT in apple. • 5-HT accumulated gradually during apple fruit development. • The bagging reduced the 5-HT content in apple fruits. Serotonin (5-hydroxytryptamine, 5-HT), a tryptophan derivative, is an important functional component in fruits. Although 5-HT was found in most fruits, the accumulation characteristics of its biosynthesis in apple fruits remain unclear. Here, we conducted a large-scale survey of 148 apple fruits in 5-HT content, and investigated the biosynthesis differences of 5-HT. Our results displayed a broad skewed normal distribution of 5-HT in apple fruits. 5-HT accumulated in all tissues of apple plants, with the highest content in the seeds and the higher content in the pulp than that of the peel. A significant increase of 5-HT at ripening stage was observed in the fruits with high 5-HT content, however, the 5-HT content did not change significantly during fruit development in the fruits with low 5-HT content. The bagging reduced the 5-HT biosynthesis compared to the fruits exposed to light conditions. In addition, the expression level of MdT5H5 was positively correlated with the 5-HT content, suggesting that MdT5H5 may play a key regulatory role in the 5-HT biosynthesis in apple fruits. [ABSTRACT FROM AUTHOR]