Temporal and Spatial Expression of Arabidopsis Gene Homologs Control Daylength Adaptation and Bulb Formation in Onion (Allium cepa L.)

Genetic studies aimed at onion improvement have been limited because of high heterozygosity, a very large genome size with a high level of repetitive DNA and a biennial life cycle. Onion bulb initiation is daylength-dependent, which places a significant barrier to adapting new varieties for growth at different latitudes. Compared to the photoperiodic regulation of flowering, relatively little is known about genetic regulation of the bulbing process. This study aims to identify the role of gene sequences involved in daylength-regulated bulb formation and tissue specific expression of onion. A comprehensive set of developmental and spatial quantitative mRNA expression experiments were carried out to investigate expression of onion FLOWERING LOCUS T (AcFT), LEAFY (AcLFY) and GIBBERELLIN-3 OXIDASE (GA3ox1) during the bulbing response. Bulbing ratios were used to measure the response of onion plants under long day (LD) and short day (SD) conditions. AcFT1 was expressed in LD, which induces bulb formation, while AcFT4 was expressed in SD, which inhibits bulb formation. AcFT5 and AcFT6 were expressed in LD and might also be involved in bulb formation itself. All AcFT, AcLFY and GA3ox1 genes showed distinctive patterns of tissue specific expression in onion, with AcFT genes found primarily in the sites of perception in the leaf and LFY in the basal tissues, the site of response. The results are consistent with AcFT1 expression being the signal for LD-induced bulb initiation and AcFT4, being involved in suppressing bulbing in SD.