Deciphering the complex nature of bolting time regulation in Beta vulgaris

Abstract

Only few genetic loci are sufficient to increase the variation of bolting time in Beta vulgaris dramatically, regarding vernalization requirement, seasonal bolting time and reproduction type. Beta species show a wide variation of bolting time regarding the year of first reproduction, seasonal bolting time and the number of reproduction cycles. To elucidate the genetics of bolting time control, we used three F mapping populations that were produced by crossing a semelparous, annual sugar beet with iteroparous, vernalization-requiring wild beet genotypes. The semelparous plants died after reproduction, whereas iteroparous plants reproduced at least twice. All populations segregated for vernalization requirement, seasonal bolting time and the number of reproduction cycles. We found that vernalization requirement co-segregated with the bolting locus B on chromosome 2 and was inherited independently from semel- or iteroparous reproduction. Furthermore, we found that seasonal bolting time is a highly heritable trait (h > 0.84), which is primarily controlled by two major QTL located on chromosome 4 and 9. Late bolting alleles of both loci act in a partially recessive manner and were identified in both iteroparous pollinators. We observed an additive interaction of both loci for bolting delay. The QTL region on chromosome 4 encompasses the floral promoter gene BvFT2, whereas the QTL on chromosome 9 co-localizes with the BR locus, which controls post-winter bolting resistance. Our findings are applicable for marker-assisted sugar beet breeding regarding early bolting to accelerate generation cycles and late bolting to develop bolting-resistant spring and winter beets. Unexpectedly, one population segregated also for dwarf growth that was found to be controlled by a single locus on chromosome 9.

Vernalization Alters Sink and Source Identities and Reverses Phloem Translocation from Taproots to Shoots in Sugar Beet

“…The expression of the flowering-related genes BTC1 and BBX19 in taproot tissue suggested that taproots might also be involved in the perception of vernalization. It is tempting to think into a direction where newly identified (Pfeiffer et al, 2014;Broccanello et al, 2015;Tränkner et al, 2017) or yet undiscovered bolting loci might harbor yet uncharacterized factors which might integrate both, bolting and required sink-source transition, similar to the recently described FT homolog StSPS6A ('tuberigen') in potato (Navarro et al, 2011;Abelenda et al, 2019).…”

Haplotype Variation of Flowering Time Genes of Sugar Beet and Its Wild Relatives and the Impact on Life Cycle Regimes

“…Recently, BvFT2 was proposed as a candidate gene for seasonal bolting time at the SBT-4 locus (Tränkner et al, 2017 ). We identified two and five polymorphisms within BvFT2 and BvFT1 , respectively.…”

“…The SBT-9 / BR1 locus was discovered to control bolting resistance after winter (Pfeiffer et al, 2014 ), where a homolog of the Arabidopsis CLEAVAGE AND POLYADENYLATION SPECIFIC FACTOR 73-I ( CPSF73-I ) was identified as the most promising candidate gene (Tränkner et al, 2016 ). Recently, Tränkner et al ( 2017 ) proposed that two QTL contribute to variation in seasonal bolting. Besides SBT-9 / BR1 , the SBT-4 locus was elucidated to majorly control seasonal bolting and BvFT2 was suggested as a candidate gene.…”

Two CONSTANS-LIKE genes jointly control flowering time in beet

“…Quantitative trait loci (QTL) and major genes controlling bolting time have been mapped to the nine beet chromosomes 2 . The bolting time QTL SEASONAL BOLTING- 4 and -9 ( SBT-4, SBT-9 ) accounts for up to 52% of the phenotypic variation 3 . The phenotypic effect of SBT-4 is likely caused by the major flowering time regulator BvFT2 because they were mapped to the same position on chromosome 4.…”