Genetic control of flower development in apple and the utilisation of transgenic early flowering apple plants in breeding

In recent years much effort has been made to understand genetic regulation of the reproductive process in annual/biennial and perennial plants. A number of genes were identified which trigger the transition from the juvenile to the adult stage, floral induction, flower initiation, flower organ development as well as bud and seed dormancy. Whereas the picture in Arabidopsis thaliana is close to complete, much less is known in trees, like apple Malus × domestica Borkh. Recent results suggest a number of similarities to cues and pathways known in model plants, but individual pathways seem to be different. During the last decade, different flowering gene homologs have been isolated from apple and functionally characterized. Their mRNA expression level was measured to study its correlation to biological processes like floral initiation and flower organ development. A number of selected genes were ectopically expressed in Arabidopsis. Only a few of these genes have been functionally tested in transgenic apple plants. Most studies have focused on genes involved in floral induction. Genes that are able to break the juvenile stage of apple, are of particular interest to shorten the long-lasting generation cycle which is the major drawback in fruit breeding programs. Using the BpMADS4 gene, a FRUITFULL-like gene of silver birch, we produced plants with a shortened juvenile phase. These transgenic plants were used for crosses to introduce genes for resistance to plant pathogens from apple wild relatives to the cultivated apple. Transgenic seedlings were selected which flowered within a few months. These seedlings were evaluated on the presence of the resistance genes using molecular markers and challenging assays. Selected seedlings were then used for pseudo-backcrosses with high quality apple cultivars to reduce significantly the linkage drag coming from the wild apples. Using the system described here one crossbred generation per year is feasible.