An epigenetic control of vernalization has been demonstrated in annual plants such as Arabidopsis and cereals, but the situation remains unclear in biennial plants such as sugar beet that has an absolute requirement for vernalization. The role of DNA methylation in flowering induction and the identification of corresponding target loci also need to be clarified. In this context, sugar beet (Beta vulgaris altissima) genotypes differing in bolting tolerance were submitted to various bolting conditions such as different temperatures and/or methylating drugs. DNA hypomethylating treatment was not sufficient to induce bolting while DNA hypermethylation treatment inhibits and delays bolting. Vernalizing and devernalizing temperatures were shown to affect bolting as well as DNA methylation levels in the shoot apical meristem. In addition, a negative correlation was established between bolting and DNA methylation. Genotypes considered as resistant or sensitive to bolting could also be distinguished by their DNA methylation levels. Finally, sugar beet homologues of the Arabidopsis vernalization genes FLC and VIN3 exhibited distinct DNA methylation marks during vernalization independently to the variations of global DNA methylation. These vernalization genes also displayed differences in mRNA accumulation and methylation profiles between genotypes resistant or sensitive to bolting. Taken together, the data suggest that the time course and amplitude of DNA methylation variations are critical points for the induction of sugar beet bolting and represent an epigenetic component of the genotypic bolting tolerance, opening up new perspectives for sugar beet breeding.

Sugar beet (Beta vulgaris altissima) is a biennial root crop with an absolute requirement for cold exposure to bolt and flower, a process called vernalization. Global DNA methylation variations have been reported during vernalization in several plants. However, few genes targeted by DNA methylation during vernalization have been described. The objectives of this study were to identify differentially methylated regions and to study their involvement in bolting induction and tolerance. Restriction landmark genome scanning was applied to DNA from shoot apical meristems of sugar beet genotypes, providing a direct quantitative epigenetic assessment of several CG methylated genes without prior knowledge of gene sequence. Several differentially methylated regions exhibiting variations of gene-body DNA methylation and expression during cold exposure and/or between genotypes were identified, including an AROGENATE DEHYDRATASE and two RNA METHYLCYTOSINE TRANSFERASE sequences. One RNA METHYLCYTOSINE TRANSFERASE sequence displayed gene-body hypermethylation and activation of expression, while the other was hypomethylated and inhibited by cold exposure. Global RNA methylation and phenolic compound levels changed during cold exposure in a genotype-dependent way. The use of methyl RNA immunoprecipitation of total RNA and reverse transcription-PCR analysis revealed mRNA methylation in a vernalized bolting-resistant genotype for the FLOWERING LOCUS 1 gene, a repressor of flowering. Finally, Arabidopsis mutants for RNA METHYLCYTOSINE TRANSFERASE and AROGENATE DEHYDRATASE were shown to exhibit, under different environmental conditions, early or late bolting phenotypes, respectively. Overall, the data identified functional targets of DNA methylation during vernalization in sugar beet, and it is proposed that RNA methylation and phenolic compounds play a role in the floral transition.

The RNA-3-encoded p25 protein was previously characterized as one of the major symptom determinants of the Beet necrotic yellow vein virus. Previous analyses reported the influence of the p25 protein in root proliferation phenotype observed in rhizomania disease on infected sugar beets (Beta vulgaris). A transgenic approach was developed, in which the p25 protein was constitutively expressed in Arabidopsis thaliana Columbia (Col-0) ecotype in order to provide new clues as to how the p25 protein might promote alone disease development and symptom expression. Transgenic plants were characterized by Southern blot and independent lines carrying single and multiple copies of the transgene were selected. Mapping of the T-DNA insertion was performed on the monocopy homozygote lines. P25 protein was localized both in the nucleus and in the cytoplasm of epidermal and root cells of transgenic plants. Although A. thaliana was not described as a susceptible host for BNYVV infection, abnormal root branching was observed on p25 protein-expressing A. thaliana plants. Moreover, these transgenic plants were more susceptible than wild-type plants to auxin analog treatment (2,4-D) but more resistant to methyl jasmonate (MeJA), abscisic acid (ABA) and to lesser extend to salicylic acid (SA). Hormonal content assays measuring plant levels of auxin (IAA), jasmonate (JA) and ethylene precursor (ACC) revealed major hormonal changes. Global transcript profiling analyses on roots displayed differential gene expressions that could corroborate root branching phenotype and stress signaling modifications.

HighlightSugar beet bolting tolerance is associated, in the shoot apical meristem, with genotype-dependent DNA methylation and expression variations of an integrative gene network involved in environmental perception and flowering.

SummaryThe consumption of fructans as a low caloric food ingredient or dietary fibre is rapidly increasing due to health benefits. Presently, the most important fructan source is chicory, but these fructans have a simple linear structure and are prone to degradation. Additional sources of high-quality tailor-made fructans would provide novel opportunities for their use as food ingredients. Sugar beet is a highly productive crop that does not normally synthesize fructans. We have introduced specific onion fructosyltransferases into sugar beet. This resulted in an efficient conversion of sucrose into complex, onion-type fructans, without the loss of storage carbohydrate content.