Apple tree flowering is mediated by low level of melatonin under the regulation of seasonal light signal

Melatonin regulates the seasonal reproduction in photoperiodic sensitive animals. Its function in plants reproduction has not been extensively studied. In the current study, the effects of melatonin on the apple tree flowering have been systematically investigated. For consecutive 2-year monitoring, it was found that the flowering was always associated with the drop of melatonin level in apple tree. Melatonin application before flowering postponed apple tree flowering with a dose-dependent manner. The increased melatonin levels at a suitable range also resulted in more flowering. The data indicated that similar to the animals, the melatonin also serves as the signal of the environmental light to regulate the plant reproduction. It was mainly the blue and far-red light to regulate the gene expression of melatonin synthetic enzymes and melatonin production in plants. The seasonal alterations of the blue and far-red lights coordinated well with the changes of the melatonin levels and led to decreased melatonin level before flowering. The mechanism studies showed that melatonin per se inhibits all the four flowering pathways in apple. The results not only provide the basic knowledge for melatonin research, but also uncover melatonin as a chemical message of light signal to mediate plant reproduction. This information can be potentially used to control flowering period and prolong the harvest time, helpfully to open a new avenue for increasing crop yield by melatonin application.