Regulation of VRN-1 Vernalization Genes in Normal and Transgenic Polyploid Wheat

Abstract

Vernalization, the requirement of a long exposure to low temperatures to accelerate flowering, is an essential adaptation of plants to cold winters. The vernalization gene VRN-1 plays an important role in this process in diploid (Triticum monococcum) and polyploid wheat (Triticum aestivum). We have recently shown that the diploid wheat VRN-A m 1 gene was similar to the Arabidopsis (Arabidopsis thaliana L. Heynh.) APETALA1 meristem identity gene. We also showed that dominant Vrn-A m 1 alleles were the result of loss-of-function mutations in regulatory regions recognized by a VRN-1 repressor, likely VRN-2. This model predicts that only the dominant Vrn-1 allele will be transcribed in lines carrying both recessive and dominant alleles. Here, we confirm this prediction in young isogenic lines of hexaploid wheat carrying different dominant Vrn-A1, Vrn-B1, and Vrn-D1 alleles, and also in heterozygous VRN-1 diploid wheat plants. However, a few weeks later, transcripts from the recessive alleles were also detected in both the polyploid and heterozygous diploid spring plants. Transcription of the recessive alleles was preceded by a reduction of the transcript levels of VRN-2. These results suggest that the dominant Vrn-1 allele or a gene regulated by VRN-1 down-regulates the VRN-2 repressor facilitating the transcription of the recessive alleles in unvernalized plants. We also show here that the level of VRN-1 transcripts in early developmental stages is critical for flowering initiation. A reduction of VRN-1 transcript levels by RNA interference delayed apex transition to the reproductive stage, increased the number of leaves, and delayed heading time by 2 to 3 weeks. We hypothesize that the coordinated transcription of dominant and recessive alleles may contribute to an earlier attainment of the VRN-1 transcript level threshold required to trigger flowering initiation in polyploid wheat.Common wheat (Triticum aestivum) was one of the first crops domesticated by man and, since then, has traveled with human populations to a wide range of environments. Wheat adaptability to this wide range of environments is partially due to the exploitation of genetic variation in daylength sensitivity and vernalization requirement, which provide a flexible regulation of flowering time. Vernalization, the requirement of a prolonged exposure to low temperatures to accelerate flowering, is particularly important for fall-planted wheat varieties to prevent flower development during winter and protect the environmentally sensitive floral organs. Wheat varieties that require vernalization to flower are referred to as winter wheats, whereas those without a vernalization requirement are identified as spring wheats.Allelic variation at the VRN-1 locus is one of the main sources of genetic differences in vernalization requirement in both diploid (Triticum monococcum) and polyploid wheats (for review, see McIntosh et al., 2003). Common wheat is a hexaploid species (2n 5 42, genomes AABBDD) and carries three homoeologous copies of the VRN-1 gene, o...

Identification of the VERNALIZATION 4 gene reveals the origin of spring growth habit in ancient wheats from South Asia

“…VRN1, VRN2, FT and ACTIN primers for SYBR GREEN quantitative PCR were developed in previous studies (10,70,71). VRN-A1, VRN-B1 and VRN-D1 expression was tested using genome specific qRT-PCR primers described before (69). Quantitative RT-PCR was performed using SYBR Green and a 7500 Fast Real-Time PCR system (Applied Biosystems).…”

“…4). It is tempting to speculate that the T2783C SNP in RIP-3 VRN-A1 can contribute to the higher transcript levels in VRN-A1 relative to VRN-B1 and VRN-D1 observed in winter wheat plants after 6 wk of vernalization (69). We are currently generating the genetic stocks required to compare the expression levels of the different RIP-3 alleles at the VRN-A1 locus.…”

Discrete Developmental Roles for Temperate Cereal GrassVERNALIZATION1/FRUITFULL-Like Genes in Flowering Competency and the Transition to Flowering

“…In different accessions of winter wheat and barley, expression levels of ZCCT1 (VRN2) are generally highest under warm long-day conditions (Yan et al, 2004b;Trevaskis et al, 2006), whereas levels of VRN1 are highest under cold conditions (Murai et al, 2003;Yan et al, 2003Yan et al, , 2004bLoukoianov et al, 2005;Trevaskis et al, 2006). In contrast, transcription of VRN1 in spring and facultative growth habits are independent of vernalization, either being constitutively expressed at high levels (Danyluk et al, 2003;Yan et al, 2003) or being upregulated in an age-dependent manner (Trevaskis et al, 2003(Trevaskis et al, , 2006von Zitzewitz et al, 2005).…”

RNA interference for wheat functional gene analysis

“…Using a similar strategy, Loukoianov et al (2005) targeted VRN1, a MADS-box transcription factor that promotes flowering and is up-regulated by vernalization (Yan et al 2003). They designed a 294-bp trigger sequence from T. monococcum VRN1 that excluded the conserved MADS-and K-box domains.…”

“…However, this has not always been the case as the VRN1 RNAi transgene (Loukoianov et al 2005) showed poor transmission through the pollen (M. Bonafede and J. Dubcovsky, unpublished data). Progeny tests additionally demonstrated that homozygous transgenic plants appear to have a stronger reduction of the target transcripts, accumulate more small interfering RNA (siRNA), and have more severe phenotypic changes than the heterozygous plants, suggesting a dosage effect of RNAi in hexaploid wheat (Travella et al 2006).…”