No Flower no Fruit - Genetic Potentials to Trigger Flowering in Fruit Trees

The development of flower buds and a sufficient fruit set are basic requirements for fruit growers to generate a marketable crop. However, fruit trees remain in a juvenile (nonflowering) phase for years, and after a transition period of getting reproductively competent they enter the adult phase of tree life. The reproductive phase is associated with the ability to alternate between the production of vegetative and reproductive buds. Efficient breeding is limited in fruit trees due to the long period of juvenility. Therefore, it is important to accelerate flowering by reducing the juvenile phase of the tree. In fruit production, precocious flowering of the tree is also favoured to reduce the vegetative phase of tree development after planting in order to obtain the earliest fruit crop. Additional critical aspects of flower development in fruit trees, such as alternate bearing, accentuate the necessity to improve our understanding on genetic factors controlling floral initiation as well as flower and fruit development in perennial fruit trees. Most of what we know about regulating floral development is based on research in annual plants, like Arabidopsis thaliana. In this review, we summarize floral transition, meristem development and flower bud formation in Malus domestica, one of the most important representatives of temperate fruit trees. We also focus on current findings of the transition from vegetative to reproductive growth obtained in Arabidopsis and how this knowledge can be applied to fruit trees, particular to apple. We discuss state-of-the-art and future research to manipulate maturation and flower initiation in apple.