Biennal bearing in apple – Expression patterns of several floral genes revealed by in-situ hybridization

Biennial bearing corresponds to cyclical changes in cropping characterized by “on” and “off” years with “heavy” and “light” fruit loads, respectively. Within the domesticated apple, there is a large variation among cultivars in the amplitude of cycling and tendency to fruitfulness and regular cropping behavior, suggesting genetic control of this phenomenon. A previous study on a segregating population ‘Starkrimson’ x ‘Granny Smith’, using QTL detection and candidate gene mapping, indicated that four main genomic regions that contain floral integrator genes, meristem identity genes, and gibberellin oxidase genes co-located with QTLs [1]. On the consensus map, SOC1 (MdSOC1-like) and GA 20-oxidase (MdGA20ox1a) were located within the QTL interval of annual yields at the bottom of LG1. The flowering gene TERMINAL FLOWER1 (MdTFL1a) do not colocate with any QTL mapped but it has been described as key flowering genes in Malus [2] [3]. GA20ox1 is involved in the later steps of the gibberellin biosynthetic pathway. MdTFL1 is assumed to repress flowering, thus maintaining the apical meristem vegetative. SOC1 integrates multiple flowering signals derived from photoperiod, temperature, hormone, and age-related signals. The aim of this study was to reveal the temporal and spatial expression patterns of these 3 genes during floral bud induction and initiation by in-situ hybridization