Takada et al. (2019) showed that vernalization requirement in Brassica rapa leafy vegetables is captured by two factors — the steady-state total expression of the BrFLC floral-repressor paralogs before cold, and the rate (sensitivity) with which prolonged cold represses BrFLC expression — and closed by asking whether natural variation in these two factors is regulated by cis-acting sequence variation at BrFLC loci or by trans-acting factors elsewhere in the genome. This problem asks for a genetic and molecular resolution of that question: decompose the natural variation of both determinants into cis and trans components, validate the nine-line correlation between pre-vernalization total BrFLC expression and days to flowering after a fixed cold treatment across a large, genetically diverse B. rapa panel, and identify the cis-regulatory elements and/or trans-acting regulators that set line-specific BrFLC repression kinetics during vernalization. A 2026 field review still frames all three components as open, and accumulating cis-side and trans-side evidence from individual loci and mutants has not yet produced the decomposition.

Background

Vernalization is the acquired competence to flower after a prolonged period of low temperature, an adaptive requirement that prevents flowering before winter in temperate plants. In Arabidopsis thaliana, the central actor is FLOWERING LOCUS C (FLC), a MADS-box transcription factor that represses floral integrators such as FLOWERING LOCUS T (FT); FRIGIDA (FRI), a plant-specific protein, activates FLC expression to create a high-flowering-time (winter-annual) state. During cold, the Polycomb repressive complex 2 (PRC2, a chromatin-modifying complex) deposits tri-methylation of lysine 27 on histone H3 (H3K27me3), a repressive histone mark, at FLC; this mark spreads along the locus and stably silences FLC after cold ends. Brassica rapa — the species of Chinese cabbage, turnip, pak choi and other leafy and root vegetables whose premature bolting after cold damages yield — underwent whole-genome triplication and therefore carries four FLC paralogs (BrFLC1, BrFLC2, BrFLC3, BrFLC5; BrFLC5 is a pseudogene in the reference genome) and two FRI paralogs (BrFRIa, BrFRIb), which makes its vernalization system quantitatively more complex than that of Arabidopsis. Because most cultivars are F1 hybrids and bolting resistance is a central breeding objective, understanding what genetically sets vernalization requirement in this crop matters both mechanistically and practically. Takada et al. (2019) measured flowering time after four weeks of cold across nine diverse B. rapa lines and established that: (i) BrFRIb functions as an activator of BrFLC; (ii) the steady-state sum of BrFLC paralog expression before cold correlates positively with days to flowering after cold, indicating pre-vernalization total BrFLC expression as an indicator of vernalization requirement; (iii) BrFLC loci gain H3K27me3 during cold, which spreads to stabilize repression, but the H3K27me3 accumulation pattern did not differ between the early- and late-flowering lines examined, so it does not explain requirement variation; and (iv) the line with the highest vernalization requirement, BRA2209, showed the lowest rate of BrFLC1 repression during prolonged cold and carries a roughly 410 bp deletion covering part of the 3' end of BrFLC1 (part of the seventh exon including the stop codon plus downstream sequence), hinting that this region contains sequences important for cold response. The paper concludes that 'the steady state expression levels of BrFLCs and the sensitivity of the repression of BrFLCs by cold treatment' are the two determinants of vernalization requirement in B. rapa, and that 'further study will be required to identify whether variations of these two factors are regulated by cis or trans'. Here, cis-acting variation means DNA-sequence differences at the BrFLC loci themselves (promoters, introns, 3' regions) that alter that locus's expression or its responsiveness to cold, while trans-acting variation means allelic differences at other genomic loci (for example transcription factors or chromatin regulators) that affect BrFLC loci in concert.

Problem Statement

Determine the genetic and molecular basis of natural variation in vernalization requirement in Brassica rapa as captured by the two-factor model of Takada et al. (2019), by jointly resolving three linked questions. (1) Cis/trans decomposition: using natural variation among divergent B. rapa lines — for example allele-specific expression of BrFLC paralogs in F1 hybrids, expression-QTL segregation in mapping populations, and/or reciprocal transgenic or allele-swap (genome-edited) tests of BrFLC alleles in common backgrounds — quantify whether variation in (a) the steady-state total pre-vernalization BrFLC expression level and (b) the rate of cold-induced BrFLC repression is controlled predominantly by cis-acting sequence variation at the BrFLC loci (such as the 3' deletion in BrFLC1 of line BRA2209, first-intron insertions, or promoter variants) or by trans-acting factors elsewhere in the genome, including any trans factors that coordinately regulate multiple BrFLC paralogs. (2) Generality of the predictor: test, across a large and genetically diverse panel of B. rapa accessions beyond the original nine lines, whether the pre-vernalization total BrFLC transcript level positively predicts the vernalization requirement (days to flowering after a fixed cold treatment), establishing the strength, generality and limitations of this relationship. (3) Molecular basis of repression kinetics: identify the cis-regulatory elements and/or trans-acting chromatin or transcriptional regulators that set line-specific rates of BrFLC repression during prolonged cold — explaining why high-requirement lines such as BRA2209 repress BrFLC1 slowly, given that H3K27me3 accumulation patterns did not differ between the early- and late-flowering lines examined — and functionally validate the causal variants or regulators. A complete answer resolves all three components under one objective: a mechanistic genetic account of why some B. rapa lineages require prolonged cold while others do not.

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