Isolation and expression pattern of genes related to flower initiation in the evergreen azalea, Rhododendron × pulchrum ‘Oomurasaki’

The flowering of the evergreen azalea, ‘Oomurasaki’ (Rhododendron × pulchrum), extended over two seasons was studied. During the first season, most of the shoot apical meristems transformed into floral meristems and formed flower buds in August. After the dormancy of the flower buds from November to February, flower bud development continued during the second season and flowering occurred in May. To ascertain the approximate period of flower initiation, we monitored the spring shoot and apical bud growth. The putative period for flower initiation was indicated to occur between late June and August. To identify the genes involved in flower initiation, we isolated homologs of the putative flowering-related genes, LEAFY (LFY), TERMINAL FLOWER 1 (TFL1), and APETALA 3 (AP3), and investigated their expression patterns in the shoot apex and apical buds during the first season using Reverse Transcription-quantitative PCR (RT-qPCR). We founding that RpLFY expression increased continually from 14 July to 4 August but that RpTFL1 expression showed trends toward a decrease during the increasing RpLFY expression in apical parts. Subsequently, RpAP3 expression began to increase on 4 August. Collectively, the results estimated that RpLFY and RpTFL1 are involved in flower initiation in ‘Oomurasaki’. We propose that the increased expression of RpLFY and the decreased expression of RpTFL1 at late July mark the beginning of flower initiation.

► Shoot phenology and expression pattern of flowering-related gene were investigated to understand the timing of flower initiation in evergreen azalea ‘Oomurasaki’ (Rhododendron × pulchrum).
► Shoot phenology may indicated that flower initiation occur between late June and August.
► In the putative period of flower initiation, LFY expression increased and TFL1 expression decreased.
► Expression pattern of these genes may determine the timing of flower initiation.