Regulation of Flowering Time in Wheat

Abstract

Vernalization is the requirement of a long exposure to cold temperatures to induce flowering. In wheat and barley, the VRN-1 and VRN-2 genes are mainly responsible for this requirement. The alleles for winter growth habit are the ancestral forms of these genes in the Triticeae, and independent mutations in both genes have resulted in the recurrent generation of spring forms in the temperate cereals. The VRN-1 gene is 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 in flowering of 2–3 weeks, suggesting that the transcript level of VRN-1 is critical for the determination 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 are sufficient to determine a spring growth habit. We propose that these mutations interfere with the recognition of a repressor coded or regulated by VRN-2. Analysis of the allele frequencies of the different combinations of dominant Vrn-A1, -B1, and -D1 in common spring wheat varieties from California and Argentina suggests that some of these 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 CCT domain or deletions of the VRN-2 gene are associated with a spring growth habit in both barley and diploid wheat. Reduction of VRN-2 transcript levels by RNAi in winter wheat variety Jagger resulted in the increase of VRN-1 transcript levels and in a significant acceleration 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 plants showing different combinations of dominant and recessive alleles. We found here that the induction of the recessive alleles is mediated by the down-regulation of VRN-2 after the initiation of transcription of the dominant Vrn-1 allele. Once VRN-2 is repressed, the recessive vrn-1 alleles can be transcribed. Three experiments supporting this hypothesis are discussed