Identifying genes underlying adaptive traits of bulbing and bolting in onion (Allium cepa L.)

Abstract

Onion (Allium cepa L.) is a biennial plant and completes its life cycle in two growing seasons i.e., forms bulbs in the first year and flowering and seed production occurs in the second year. The poor adaptation of onion cultivars at different latitudes results in substantial losses due to reduced yield and precocious flowering. Onions form bulbs when grown under inductive daylength and an adequate amount of far-red light. Onion cultivars grown at particular latitudes require minimum critical day length to form a bulb.

At the physiological level, bulb initiation in temperate regions is regulated in a similar manner to the photoperiodic flowering of Arabidopsis (Mettananda and Fordham, 1997; Taylor, 2010; Lee et al., 2013). The first aim of this study was to identify photoperiodic pathway genes responsible for the adaptation of onion under different daylength. The expression of photoperiodic pathways gene was measured under the long-day and short-day condition in DH2150 (long-day onion) and Albasile (short-day onion) over a 24 hours period using RT-qPCR. Two FT genes (AcFT1 and AcFT4) regulate bulbing in long-day and short-day onions but respond differently under different daylength. Consistent with their proposed roles, the diurnal experiment suggested that AcFT1 and AcAP1 are only expressed in bulbing, whereas AcFT4 is expressed in non-bulbing population. The photoreceptor, circadian clock, and output pathway genes follow diurnal expression patterns peaking at different times of the day and these genes were not altered in bulbing and non-bulbing populations indicating that they are not involved in the adaptation of two onion cultivars studied here.

To determine the functional role of genes is very challenging in bulb onion due to lack of an efficient genetic transformation method (Eady, et al., 2000; McCallum, 2007). The second aim of this study was to develop an efficient and reliable protocol for generating transformed onion callus cells from shoot tip tissue of germinating seed. Callus was induced efficiently from the two onion cultivars: PLK (Pukekohe long keeper) and DH2107 using Agrobacterium tumefaciens strains GV3101 and EHA105.

Premature flower¬ing before bulb development is an undesirable trait and reduces the quality of the bulbs. Baldwin et al., (2014) found AcBlt1 locus on chromosome 1 which is responsible for bolting. Khosa (2018) found AcVRN1 gene is important for onion flowering. AcVRN1 gene turns on by vernalization but optimum temperature for vernalization varies. The third aim of this study was to develop a molecular marker for the AcVRN1 gene to identify which onion cultivar are susceptible to bolting at early stage of onion development. TAIL-PCR was used to amplify the unknown promoter and intron 1 region. No polymorphism was found in the promoter and intron 1 region of AcVRN1 gene in all the seven varieties studied here.

Overall this study will help to understand molecular and genetic difference between LD and SD onion cultivars, which is important for adapting new onion cultivars at different latitudes.