Russian dandelion [Taraxacum kok-saghyz (TKS)] is an herbaceous perennial that can also be grown as an annual. Latex can be found in the root, in specialized cells called laticifers (Whaley and Bowen, 1947) and this species could be a source of natural rubber, essential for the fabrication of over 40,000 products vital to industries including transportation, health care, and construction (Mooibroek and Cornish, 2000). TKS grows well in southern Ontario and the northern United States, and it is currently under development as a new crop to introduce natural rubber production to these regions (van Beilen and Poirier, 2007).
The transition from vegetative to reproductive development in plants is cued by endogenous, as well as environmental signals, such as photoperiod and temperature (Thomas et al., 2006). TKS can require a period of cool temperatures, known as vernalization, to induce flowering (Borthwick et al., 1943). This is common in temperate perennials because it encourages flowering after winter, during the favorable conditions of spring (Andres and Coupland, 2012). Although the natural distribution of TKS is restricted to a relatively small area along the Alatau mountain range in Kazakhstan, the species can grow in a number of regions with favorable climates (Whaley and Bowen, 1947).
Variation for flowering habit is observed in natural populations. Early flowering, spring-type, plants do not require vernalization and flower ≈50 d after planting in a greenhouse with a 16-h photoperiod (K.J.M. Hodgson-Kratky and D.J. Wolyn, unpublished data). Winter-type plants, in contrast, grow vegetatively and generally will not flower without a cold period (Hodgson-Kratky et al., 2015).
The floral induction pathway has been studied extensively in the model plant, Arabidopsis thaliana. In this species, two major loci determine flowering habit: flowering locus C (FLC) and frigida (FRI) (Koornneef et al., 1994). Winter-type plants carry dominant functional alleles at each of these two loci, and homozygous recessive genotypes with inactive fri and/or flc alleles result in early flowering (Gendall and Simpson, 2006; Johanson et al., 2000). Control of flowering through FLC/FRI is conserved in many plant species (Irwin et al., 2012; Kuittinen et al., 2008; Reeves et al., 2007; Risk et al., 2010; Schranz et al., 2002; Zhang et al., 2009).
The genetic pathway controlling the vernalization requirement for flowering in temperate cereals, such as wheat (Triticum aestivum), barley (Hordeum vulgare), and rye (Secale cereale), evolved independently of that in A. thaliana, and is regulated primarily by three genes: Vernalization 1 (VRN1) (Yan et al., 2003), VRN2 (Yan et al., 2004), and VRN3 (Yan et al., 2006). Spring-type plants possess recessive null mutations at the VRN2 locus (Dubcovsky et al., 2005; Yan et al., 2004) or dominant mutations in the promoter region of VRN1 (Dubcovsky et al., 2005; Fu et al., 2005; Tranquilli and Dubcovsky, 2000; Yan et al., 2004) or VRN3, which cause high expression in these two genes regardless of environmental conditions (Yan et al., 2006).
Flowering habit is an important trait in TKS breeding because winter-type plants have higher rubber yields than spring-type plants (Whaley and Bowen, 1947). Therefore, populations under development for high rubber are also selected for vernalization requirement and thus it would be useful to understand the inheritance of flowering habit. Based on the genetic pathways identified in A. thaliana and cereals, multiple interacting loci influencing the trait are predicted in TKS. The objectives of this research were to determine the number of major genes controlling the vernalization requirement in TKS and interlocus interactions.
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