Two CONSTANS-LIKE genes jointly control flowering time in beet.

Breeding vegetative crops (e.g. beets, cabbage, forage grasses) is challenged by two conflicting aims. For field production, flowering must be avoided while flowering and seed set is necessary for breeding and seed production. The biennial species sugar beet makes shoot elongation ('bolting') followed by flowering after a long period of cold temperatures. Field production in northern geographical regions starts in spring. A thickened storage root is formed only during vegetative growth. It is expected that winter beets, which are sown before winter would have a much higher yield potential. However, field production was not possible so far due to bolting after winter. We propose a strategy to breed winter beets exploiting haplotype variation at two major bolting time loci, B and B2. Both genes encode transcription factors controlling the expression of two orthologs of the Arabidopsis gene FLOWERING LOCUS T (FT). We detected an epistatic interaction between both genes because F2 plants homozygous for two B/B2 mutant alleles did not bolt even after vernalization. Fluorescence complementation studies revealed that both proteins form a heterodimer in vivo. In non-bolting plants, the bolting activator BvFT2 was completely downregulated whereas the repressor BvFT1 was upregulated which suggests that both genes acquire a CONSTANS (CO) like function in beet. Like CO, B and B2 proteins house CCT and BBX domains which, in contrast to CO are split between the two beet genes. We propose an alternative regulation of FT orthologs in beet that can be exploited to breed winter beets.

Corrigendum: The genome of Chenopodium quinoa.

This corrects the article DOI: 10.1038/nature21370.

The genome of Chenopodium quinoa.

Chenopodium quinoa (quinoa) is a highly nutritious grain identified as an important crop to improve world food security. Unfortunately, few resources are available to facilitate its genetic improvement. Here we report the assembly of a high-quality, chromosome-scale reference genome sequence for quinoa, which was produced using single-molecule real-time sequencing in combination with optical, chromosome-contact and genetic maps. We also report the sequencing of two diploids from the ancestral gene pools of quinoa, which enables the identification of sub-genomes in quinoa, and reduced-coverage genome sequences for 22 other samples of the allotetraploid goosefoot complex. The genome sequence facilitated the identification of the transcription factor likely to control the production of anti-nutritional triterpenoid saponins found in quinoa seeds, including a mutation that appears to cause alternative splicing and a premature stop codon in sweet quinoa strains. These genomic resources are an important first step towards the genetic improvement of quinoa.

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

The species Beta vulgaris encompasses wild and cultivated members with a broad range of phenological development. The annual life cycle is commonly found in sea beets (ssp. maritima) from Mediterranean environments which germinate, bolt, and flower within one season under long day conditions. Biennials such as the cultivated sugar beet (B. vulgaris ssp. vulgaris) as well as sea beets from northern latitudes require prolonged exposure to cold temperature over winter to acquire floral competence. Sugar beet is mainly cultivated for sugar production in Europe and is likely to have originated from sea beet. Flowering time strongly affects seed yield and yield potential and is thus a trait of high agronomic relevance. Besides environmental cues, there are complex genetic networks known to impact life cycle switch in flowering plants. In sugar beet, BTC1, BvBBX19, BvFT1, and BvFT2 are major flowering time regulators. In this study, we phenotyped plants from a diversity Beta panel encompassing cultivated and wild species from different geographical origin. Plants were grown under different day length regimes with and without vernalization. Haplotype analysis of BTC1, BvBBX19, BvFT1, and BvFT2 was performed to identify natural diversity of these genes and their impact on flowering. We found that accessions from northern latitudes flowered significantly later than those from southern latitudes. Some plants did not flower at all, indicating a strong impact of latitude of origin on life cycle. Haplotype analysis revealed a high conservation of the CCT-, REC-, BBX-, and PEBP-domains with regard to SNP occurrence. We identified sequence variation which may impact life cycle adaptation in beet. Our data endorse the importance of BTC1 in the domestication process of cultivated beets and contribute to the understanding of distribution and adaption of Beta species to different life cycle regimes in response to different environments. Moreover, our data provide a resource for haplotypes identified for the major floral regulators in beet.

Flowering time regulation in crops­what did we learn from Arabidopsis?

The change from vegetative to reproductive growth is a key developmental switch in flowering plants. In agriculture, flowering is a prerequisite for crop production whenever seeds or fruits are harvested. An intricate network with various (epi-) genetic regulators responding to environmental and endogenous triggers controls the timely onset of flowering. Changes in the expression of a single flowering time (FTi) regulator can suffice to drastically alter FTi. FTi regulation is of utmost importance for genetic improvement of crops. We summarize recent discoveries on FTi regulators in crop species emphasizing crop-specific genes lacking homologs in Arabidopsis thaliana. We highlight pleiotropic effects on agronomically important characters, impact on adaptation to new geographical/climate conditions and future perspectives for crop improvement.

The B2 flowering time locus of beet encodes a zinc finger transcription factor.

Sugar beet (Beta vulgaris) is a biennial root crop that grows vegetatively in the first year and starts shoot elongation (bolting) and flowering after exposure to cold temperatures over winter. Early bolting before winter is controlled by the dominant allele of the B locus. Recently, the BOLTING time control 1 (BTC1) gene has been cloned from this locus. BTC1 promotes early bolting through repression of the downstream bolting repressor B. vulgaris flowering locus T1 (BvFT1) and activation of the downstream floral activator BvFT2. We have identified a new bolting locus B2 acting epistatically to B. B2 houses a transcription factor which is diurnally regulated and acts like BTC1 upstream of BvFT1 and BvFT2. It was termed BvBBX19 according to its closest homolog from Arabidopsis thaliana. The encoded protein has two conserved domains with homology to zinc finger B-boxes. Ethyl methanesulfonate-induced mutations within the second B-box caused up-regulation of BvFT1 and complete down-regulation of BvFT2. In Arabidopsis, the expression of FT is promoted by the B-box containing protein CONSTANS (CO). We performed a phylogenetic analysis with B-box genes from beet and A. thaliana but only BvCOL1 clustered with CO. However, BvCOL1 had been excluded as a CO ortholog by previous studies. Therefore, a new model for flowering induction in beet is proposed in which BTC1 and BvBBX19 complement each other and thus acquire a CO function to regulate their downstream targets BvFT1 and BvFT2.

The role of a pseudo-response regulator gene in life cycle adaptation and domestication of beet.

Life cycle adaptation to latitudinal and seasonal variation in photoperiod and temperature is a major determinant of evolutionary success in flowering plants. Whereas the life cycle of the dicotyledonous model species Arabidopsis thaliana is controlled by two epistatic genes, FLOWERING LOCUS C and FRIGIDA, three unrelated loci (VERNALIZATION) determine the spring and winter habits of monocotyledonous plants such as temperate cereals. In the core eudicot species Beta vulgaris, whose lineage diverged from that leading to Arabidopsis shortly after the monocot-dicot split 140 million years ago, the bolting locus B is a master switch distinguishing annuals from biennials. Here, we isolated B and show that the pseudo-response regulator gene BOLTING TIME CONTROL 1 (BvBTC1), through regulation of the FLOWERING LOCUS T genes, is absolutely necessary for flowering and mediates the response to both long days and vernalization. Our results suggest that domestication of beets involved the selection of a rare partial loss-of-function BvBTC1 allele that imparts reduced sensitivity to photoperiod that is restored by vernalization, thus conferring bienniality, and illustrate how evolutionary plasticity at a key regulatory point can enable new life cycle strategies.