REGULATION OF FLOWERING TIME IN WHEAT

摘要

Vernalization is the requirement of a long exposure to cold temperatures to induceflowering. In wheat and barley, the VRN-1 and VRN-2 genes are mainly responsiblefor this requirement. The alleles for winter growth habit are the ancestral forms ofthese genes in the Triticeae, and independent mutations in both genes have resultedin the recurrent generation of spring forms in the temperate cereals. The VRN-1 geneis a meristem identity gene (= APETALA1) and is dominant for spring growth habit.Reduction of VRN-1 transcript levels by RNA interference (RNAi) result in a delay inflowering of 2-3 weeks, suggesting that the transcript level of VRN-1 is critical for thedetermination of flowering time in wheat. The spring wheat and barley lines characterized so far show deletions in the promoter or the first intron of VRN-1 suggesting that they are important regulatory regions. Mutations in any of these regions in a single VRN-1 copy in polyploid wheat aresufficient to determine a spring growth habit. We propose that these mutations interferewith the recognition of a represser coded or regulated by VRN-2. Analysis of theallele frequencies of the different combinations of dominant Vrn-Al, -Bl, and -Dl incommon spring wheat varieties from California and Argentina suggests that some ofthese combinations might have an adaptative value . The VRN-2 gene is a Zinc-finger CCT transcription factor and is dominant for the winter growth habit. This gene is down-regulated by vernalization, releasing the transcription of the VRN-1 genes and promoting flowering. Mutations in the CCTdomain or deletions of the VRN-2 gene are associated with a spring growth habit in bothbarley and diploid wheat. Reduction of VRN-2 transcript levels by RNAi in winter wheatvariety Jagger resulted in the increase of VRN-1 transcript levels and in a significantacceleration of flowering time. We have recently proposed the existence a feedback regulatory loop between VRN-1 and VRN-2, which is responsible for the transcription of recessive vrn-1 alleles several days after the initiation of the transcription of the dominant Vrn-1 alleles, in plantsshowing different combinations of dominant and recessive alleles. We found here thatthe induction of the recessive alleles is mediated by the down-regulation of VRN-2 afterthe initiation of transcription of the dominant Vrn-1 allele. Once VRN-2 is repressed, therecessive vrn-1 alleles can be transcribed. Three experiments supporting this hypothesisare discussed.