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Functional genomics of photoperiodic bulb initiation in onion (Allium cepa)

By Andrew Taylor


Bulb initiation is a process which is photoperiodically driven, drawing parallels with flowering. Photoperiodic flowering is well characterised at molecular and genetic levels and occurs when photoreceptors interact with the circadian clock, regulating the expression of CONSTANS (CO), which itself regulates the expression of floral pathway integrating genes such as FLOWERING LOCUS T (FT), leading to floral initiation. Two genes which regulate CO transcription are FLAVIN-BINDING, KELCH REPEAT, F-BOX (FKF1) and GIGANTEA (GI). The onion genome is very large with a high level of duplication, presenting challenges for any molecular-based study. The aim of this study was to test the hypothesis that genes controlling daylength response are conserved between the model plant Arabidopsis and onion and hence between the different end-processes bulbing and flowering.\ud Bulbing ratios were used to measure the response of onion plants to long day (LD) and short day (SD) conditions and the reversibility of the bulbing process. It was shown that bulbing is reversible, with a delay when plants are transferred from SDs to LDs, suggesting the accumulation of an inhibitor.\ud Diurnal expression patterns of onion genes homologous to Arabidopsis flowering time genes were examined using quantitative RT-PCR. Phylogenetic analyses were conducted to validate the identity of the homologues. Molecular and phylogenetic data suggests that an onion GIGANTEA (GI) homologue was isolated. Peaks of expression of ZT10 in LDs and ZT7 in SDs mirror the expression of Arabidopsis GI. Homologues of FKF1 and the circadian clock gene ZEITLUPE (ZTL) were also characterised. The putative FKF1 homologue showed different expression patterns in varieties exhibiting different daylength responses. These differences may contribute to the different daylength responses. A CO-like gene, which is closely related to Arabidopsis COL4, and three FT-like genes were also characterised. It appears that many of the genes controlling daylength response are conserved in onion

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