Vernalization

Abstract: Winter wheats require several weeks at low temperature to flower. This process, vernalization, is controlled mainly by the VRN1 gene. Using 6,190 gametes, we found VRN1 to be completely linked to MADS-box genes AP1 and AGLG1 in a 0.03-centimorgan interval flanked by genes Cysteine and Cytochrome B5. No additional genes were found between the last two genes in the 324-kb Triticum monococcum sequence or in the colinear regions in rice and sorghum. Wheat AP1 and AGLG1 genes were similar to Arabidopsis meristem identity genes AP1 and AGL2, respectively. AP1 transcription was regulated by vernalization in both apices and leaves, and the progressive increase of AP1 transcription was consistent with the progressive effect of vernalization on flowering time. Vernalization was required for AP1 transcription in apices and leaves in winter wheat but not in spring wheat. AGLG1 transcripts were detected during spike differentiation but not in vernalized apices or leaves, suggesting that AP1 acts upstream of AGLG1. No differences were detected between genotypes with different VRN1 alleles in the AP1 and AGLG1 coding regions, but three independent deletions were found in the promoter region of AP1. These results suggest that AP1 is a better candidate for VRN1 than AGLG1. The epistatic interactions between vernalization genes VRN1 and VRN2 suggested a model in which VRN2 would repress directly or indirectly the expression of AP1. A mutation in the promoter region of AP1 would result in the lack of recognition of the repressor and in a dominant spring growth habit.

Abstract: Plants respond to the changing seasons to initiate developmental programmes precisely at particular times of year. Flowering is the best characterized of these seasonal responses, and in temperate climates it often occurs in spring. Genetic approaches in Arabidopsis thaliana have shown how the underlying responses to changes in day length (photoperiod) or winter temperature (vernalization) are conferred and how these converge to create a robust seasonal response. Recent advances in plant genome analysis have demonstrated the diversity in these regulatory systems in many plant species, including several crops and perennials, such as poplar trees. Here, we report progress in defining the diverse genetic mechanisms that enable plants to recognize winter, spring and autumn to initiate flower development.

Abstract: Winter wheat and barley varieties require an extended exposure to low temperatures to accelerate flowering (vernalization), whereas spring varieties do not have this requirement. In this study, we show that in these species, the vernalization gene VRN3 is linked completely to a gene similar to Arabidopsis FLOWERING LOCUS T (FT). FT induction in the leaves results in a transmissible signal that promotes flowering. Transcript levels of the barley and wheat orthologues, designated as HvFT and TaFT, respectively, are significantly higher in plants homozygous for the dominant Vrn3 alleles (early flowering) than in plants homozygous for the recessive vrn3 alleles (late flowering). In wheat, the dominant Vrn3 allele is associated with the insertion of a retroelement in the TaFT promoter, whereas in barley, mutations in the HvFT first intron differentiate plants with dominant and recessive VRN3 alleles. Winter wheat plants transformed with the TaFT allele carrying the promoter retroelement insertion flowered significantly earlier than nontransgenic plants, supporting the identity between TaFT and VRN-B3. Statistical analyses of flowering times confirmed the presence of significant interactions between vernalization and FT allelic classes in both wheat and barley (P < 0.0001). These interactions were supported further by the observed up-regulation of HvFT transcript levels by vernalization in barley winter plants (P = 0.002). These results confirmed that the wheat and barley FT genes are responsible for natural allelic variation in vernalization requirement, providing additional sources of adaptive diversity to these economically important crops.

Abstract: Vernalization, the acceleration of flowering by a long period of cold temperature, ensures that many plants overwinter vegetatively and flower in spring. In Arabidopsis, allelic variation at the FRIGIDA (FRI) locus is a major determinant of natural variation in flowering time. Dominant alleles of FRI confer late flowering, which is reversed to earliness by vernalization. We cloned FRI and analyzed the molecular basis of the allelic variation. Most of the early-flowering ecotypes analyzed carry FRI alleles containing one of two different deletions that disrupt the open reading frame. Loss-of-function mutations at FRI have thus provided the basis for the evolution of many early-flowering ecotypes.