Genetic regulation of day length adaptation and bulb formation in onion (Allium cepaL.)

Abstract

Genetic studies aimed at onion improvement have been limited because of outcrossing, high heterozygosity and a very large genome size with a high level of repetitive DNA. Onion bulb initiation is photoperiod-dependent, which places a significant barrier to adapting new varieties for growth at different latitudes. In comparison to photoperiodic regulation of flowering, relatively little is known about genetic regulation of the bulbing process. This project aims to test the hypothesis that the genetic regulation of bulb formation in response to daylength is analogous to the daylength regulation of flowering and to identify genes involved in daylength adaptation in onion. A comprehensive set of developmental, diurnal and spatial mRNA and quantitative expression experiments were carried out to investigate the bulbing response. Bulbing ratios were used to measure the bulbing response of onion plants and the reversibility of the bulbing process under long day (LD) and short day (SD) conditions. RNA-Seq analysis provided a large number of differentially expressed transcripts in onion in response to daylength. Five FT and three COL genes were identified in onion including two novel COL sequences. AcFT1 was expressed in LD, which might induce bulb formation, while AcFT4 was expressed in SD, which might inhibit bulb formation. AcFT5 and AcFT6 were expressed in LD and might also be involved in bulb formation itself. FKF1, GI and COL2 showed good diurnal expression patterns consistent with photoperiod sensing and regulation of FT1. All FT genes exhibited different diurnal expression patterns peaking at different times of the day. FKF1, COL2, COL3, FT1, FT4, LFY and GA3ox1 genes showed distinctive patterns of tissue specific expression in onion. FT genes did not show any variation in expression that would account for the difference in critical daylength between the LD and SD varieties of onion