Genetic Mechanisms of Vernalization Requirement Duration in Winter Wheat Cultivars
Abstract Wheat is cultivated across more land area than any other grain crops. Wheat cultivars are classified as two general types: winter wheat with variable low temperature requirement for a proper flowering time (vernalization) and spring wheat without the requirement, based on their qualitative vernalization requirement. Winter wheat cultivars are classified as three types, weak winter, semi-winter and strong winter, according to their quantitative vernalization requirement to reach a vernalization saturation point or achieve the maximum vernalization effect. Three vernalization genes, VRN1, VRN2, and VRN3, were cloned using a positional cloning approach and a one-gene model of qualitative variation in vernalization requirement between spring and winter wheat. A major gene for the vernalization requirement duration in winter wheat was mapped using a population of recombinant inbred lines (RILs) that were generated from two winter wheat cultivars, 'Jagger' and '2174'. Furthermore, the cloning population was developed using a RIL to backcross with 2174, which was segregated in a 3:1 ratio of the early flowered plants and the late flowered plants after the population was vernalized for 3 weeks. The wild type Jagger vrn-A1a allele for less vernalization was dominant over the 2174 vrn-A1b allele for more vernalization, and the two alleles encoded the vrn-A1 proteins with two point mutations. A third haplotype with one of the point mutations was found in common wheat. Gene markers were developed to direct breeding of semi-winter and strong winter wheat cultivars to adapt to different geographical areas and changing climates.
Keywords Flowering time • Gene cloning • Semi-winter wheat • Vernalization requirement duration • Winter wheat
Wheat (Triticum aestivum, 2n = 6x = 42, AABBDD) is one of the mostly cultivated crops in different geographical areas in the world. Wheat is traditionally divided into two types, winter wheat with vernalization requirement and spring wheat without this requirement. Vernalization is an exposure of the plant to a few weeks of low temperature in order to accelerate its ability to transition from vegetative to reproductive development, providing an adaptive mechanism for winter wheat to synchronize its developmental transition with seasonal changes in temperature (Rawson et al. 1998). Winter wheat is sown in autumn, whereas spring wheat is sown in autumn or spring season. Advances in understanding the genetic basis and molecular mechanisms of winter wheat development are critically important to ensure that winter wheat flowers at a proper time.
Vernalization has significant effects at typically 2–10 °C, with dramatic decline at temperatures above 11 °C and an apparent loss of effects above 18 °C (Brooking 1996). Based on their various vernalization requirement durations with a combination of the amplitude and duration of low temperatures in different geographic areas, winter wheat cultivars are typically categorized into three types: a weak winter type that is stimulated to flower by brief exposure to low temperature, a semiwinter type that requires 2–4 weeks of cold exposure for flowering, and a strong winter type that needs 4–6 weeks of cold exposure (Crofts 1989). The winter wheat cultivar Yeoman reportedly requires up to 12 weeks at low temperature to attain a vernalization saturation point (Berry et al. 1980).
Recent studies suggest that global climate will inevitably produce significant changes in air temperatures (Körner and Basler 2010). Average global surface air temperature rose 0.5 °C in the twentieth century or 4–7 °C in the past million years between the ice ages and the warm interglacial periods (Solomon et al. 2007). The global mean temperature is projected to continue its increase by roughly 3 °C (Kerr 2007) or 5 °C by the end of the twenty-first century (Semenov and Halford 2009). Various crops, requiring specific environmental cues for growth and development, will inevitably respond differently to changing climate (Craufurd and Wheeler 2009; Lanning et al. 2010). As various simulation models have shown, winter wheat is more vulnerable to changing climate due to its higher sensitivity to temperatures for proper flowering time and successful grain reproduction (Morison and Long 1995). Higher temperatures in a winter season will lead to insufficient or failed vernalization hence delayed reproductive development for successful grain production of winter wheat.
Three genes controlling vernalization requirement in wheat were cloned, including VRN1 (Yan et al. 2003), VRN2 (Yan et al. 2004), and VRN3 (Yan et al. 2006). VRN1 is an orthologue of AP1 in Arabidopsis, and a dominant Vrn1 allele originated from mutations in the promoter or first intron of a recessive wild type vrn1 gene. VRN2 encodes a novel transcription factor containing a Zinc finger and a CCT domain, and a recessive vrn2 allele had a point mutation in coding region resulting in an alteration of an amino acid at the CCT domain or complete deletion, compared to the dominant allele as a wild type. VRN3 is an orthologue of Arabidopsis FT, and allelic variation at VRN3 is related with mutations in its promoter or intron one. VRN1 and VRN3 were cloned in populations that were segregated as a 3:1 ratio of early and late flowering plants, indicating that both VRN1 and VRN3 are promoters of flowering. VRN2 was cloned in a population that was segregated as a 1:3 ratio of early and late flowering plants, indicating that this gene is a repressor of flowering. Heading date in each of these populations was controlled by one gene, whereas the other two were fixed at the same allele.
In our previous studies on winter wheat, we found that the vrn-A1 locus was associated with variation in the stem elongation in the winter wheat Jagger × 2174 RIL population (Chen et al. 2009), and this locus influenced subsequent timing of heading and physiological maturity when characterized in the field for 3 years (Chen et al. 2010). This locus is very sensitive to temperature change and bears close association with variation in development in the winter wheat population that may be caused by vernalization requirement duration across years. The present study aimed to genotype wheat germplasm and to establish genetic models accounting for strong winter, semi-winter, and weak winter wheat cultivars.