Bolting and flowering control in sugar beet: relationships and effects of gibberellin, the bolting gene B and vernalization

Abstract

Background and aims: Bolting, the first visible sign of reproductive transition in beets (Beta vulgaris), is controlled by the dominant bolting gene B (B allele), which allows for flowering under long days (LDs, >14 h light) without prior vernalization. The B-locus carries recessive alleles (bb) in sugar beet (Beta vulgaris L. spp. vulgaris), so that vernalization and LDs are required for bolting and flowering. Gibberellin growth hormones (GAs) control stem elongation and reproductive development, but their role during these processes in sugar beet is not defined. We aimed to investigate the involvement of GAs in bolting and flowering in sugar beet, and also its relationship with the vernalization requirement as defined by the B-gene.

Methodology: Plants segregating for the B allele were treated with exogenous GA(4) under inductive (16 h light) and non-inductive (8 h light) photoperiods, with and without prior vernalization treatment. A co-dominant polymerase chain reaction (PCR) marker was used to genotype the B-gene locus. Bolting and flowering dates were scored, and bolt heights were measured as appropriate. Analysis of variance was used to determine the effects and interactions of GAs, the B allele and vernalization on bolting and flowering. The effects of the B allele on bolting were also verified in the field.

Principal results: Application of GAs or the B allele could initiate bolting independently. When the B allele was absent, the applied GAs promoted stem growth, but did so only in vernalized plants, irrespective of photoperiod. Under LDs, bolt height before flowering in plants carrying the B allele (BB; Bb) was not significantly influenced by GAs. The timing and frequency of flowering were influenced by the B allele without interactive effects from GAs.

Conclusions: In sugar beet, GA acts independently of the B allele and photoperiod to induce bolting. Vernalization enables GA action independently of the B allele; hence, the dominant B allele may not directly participate in vernalization-induced bolting.

Fig. 1

The effects of applied GA and vernalization on stem growth. The numbers of extended internodes were counted and the final stem heights used to calculate the average internode length in F2Bb plants treated with exogenous GA with and without prior vernalization. Plants were grown under SDs (8 h light) in the CE chamber at 22 °C. Significant GA effects were observed only in vernalized plants.

Fig. 2

Typical phenotypes observed among F2Bb plants grown in the LD (16 h light) CE room. The plants are from left to right: non-bolted, representative of the biennial bb genotype; bolted with reversion to rosette growth, representative of the annual Bb genotype; and bolted with flowers, representative of the annual BB genotype. Such phenotypes were observed in LD conditions irrespective of GA treatment.

Fig. 3

The mean number of days to bolt for annual genotypes in populations 950619 and 960701. The lower boundary of the box indicates the 25th percentile; the upper boundary line of the box indicates the 75th percentile; the broken line in the box indicates the median; and the whiskers below and above the box indicate, respectively, the 5th and 95th percentiles. F2 plant populations segregating at the B-gene locus were grown in the field and scored for bolting when bolts were at least 5 cm tall as defined in Smit (1983). All bolting plants were genotyped and segregated into homozygous and heterozygous pools for analysis to determine the effects of the dominant B allele.

Fig. 4

A simple model of events expected to result in phasic transitions in the apical shoot meristem of sugar beet during reproductive growth. Differences between annual types carrying the dominant B allele (BB; Bb) and biennial types (bb) are represented. Arrows point to the downstream pathway or process most significantly affected by the relevant upstream component. Essentially, bolting in annual types is dependent on the LD photoperiod pathway acting through the B allele, whereas in biennial types bolting requires vernalization to activate the GA signalling pathway. The B allele and GA pathways, therefore, converge on bolting, after which the plants proceed to flowering; a process that is favoured under LD conditions in both annual and biennial types. In SDs (8 h light) only, interactive effects exist between GA and the B-allele such that significantly more plants bolt when GA is applied (P < 0.01). This is indicated by the arrow with the broken line and is not the normal condition in sugar beet. N.B. The recessive bolting alleles (bb) at the B-gene locus are generally used by breeders as markers for vernalization requirement, in order to delay bolting in cultivated biennial sugar beet.