Abstract
BackgroundThe gene FLOWERING LOCUS T (FT) and its orthologues play a central role in the integration of flowering signals within Arabidopsis and other diverse species. Multiple copies of FT, with different cis-intronic sequence, exist and appear to operate harmoniously within polyploid crop species such as Brassica napus (AACC), a member of the same plant family as Arabidopsis.ResultsWe have identified six BnFT paralogues from the genome of B. napus and mapped them to six distinct regions, each of which is homologous to a common ancestral block (E) of Arabidopsis chromosome 1. Four of the six regions were present within inverted duplicated regions of chromosomes A7 and C6. The coding sequences of BnFT paralogues showed 92-99% identities to each other and 85-87% identity with that of Arabidopsis. However, two of the paralogues on chromosomes A2 and C2, BnA2.FT and BnC2.FT, were found to lack the distinctive CArG box that is located within intron 1 that has been shown in Arabidopsis to be the binding site for theFLC protein. Three BnFT paralogues (BnA2.FT, BnC6.FT.a and BnC6.FT.b) were associated with two major QTL clusters for flowering time. One of the QTLs encompassing two BnFT paralogues (BnC6.FT.a and BnC6.FT.b) on chromosome C6 was resolved further using near isogenic lines, specific alleles of which were both shown to promote flowering. Association analysis of the three BnFT paralogues across 55 cultivars of B. napus showed that the alleles detected in the original parents of the mapping population used to detect QTL (NY7 and Tapidor) were ubiquitous amongst spring and winter type cultivars of rapeseed. It was inferred that the ancestral FT homologues in Brassica evolved from two distinct copies, one of which was duplicated along with inversion of the associated chromosomal segment prior to the divergence of B. rapa (AA) and B. oleracea (CC). At least ten such inverted duplicated blocks (IDBs) were identified covering a quarter of the whole B. napus genome.ConclusionSix orthologues of Arabidopsis FT were identified and mapped in the genome of B. napus which sheds new light on the evolution of paralogues in polyploidy species. The allelic variation of BnFT paralogues results in functional differences affecting flowering time between winter and spring type cultivars of oilseed Brassica. The prevalent inverted duplicated blocks, two of which were located by four of the six BnFT paralogues, contributed to gene duplications and might represent predominant pathway of evolution in Brassica.
Highlights
The gene FLOWERING LOCUS T (FT) and its orthologues play a central role in the integration of flowering signals within Arabidopsis and other diverse species
FT is expressed in the phloem of leaves, with the small protein moving as a long-distance signal to the shoot apical meristem (SAM) where it interacts with FD, a bZIP transcription factor, to form a complex of FT/FD heterodimer
Isolation and genetic mapping of BnFT paralogues Thirty-five potential FT orthologue-containing BAC clones were identified by Southern blot screening of the JBnB BAC library, which was developed from the doubled haploid B. napus cv
Summary
The gene FLOWERING LOCUS T (FT) and its orthologues play a central role in the integration of flowering signals within Arabidopsis and other diverse species. Forward and reverse genetics has allowed identification of genes in the context of environmental and developmental cues that mediate the onset of flowering, and allowed detailed characterization of the photoperiod, vernalization, gibberellin and autonomous pathways Major integrators of these pathways include FLOWERING LOCUS T (FT), along with SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1) and LEAFY (LFY) [1,2,3,4,5]. FT is expressed in the phloem of leaves, with the small protein moving as a long-distance signal to the shoot apical meristem (SAM) where it interacts with FD, a bZIP transcription factor, to form a complex of FT/FD heterodimer This activates the floral meristem identity genes APETALA 1 (AP1) and FRUITFUL (FUL) to promote flowering [7,8,11,12]
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