**3. The influence of vernalization genes on the flowering time of bread wheat**

Bread wheat is generally classified as spring or winter types according to its response to low temperatures during the vegetative phase [22–24]. Exposure to low temperatures (0–10°C) for several weeks (usually 6–8 weeks) is necessary for the development of tillers and the induction of flowering in winter wheat, whereas tillering and flowering of spring wheat occur regardless of temperature [22, 25]. The flowering models of the temperate cereals indicate that before vernalization, *Vrn‐3* series is repressed by *Vrn‐2* and long expo‐ sure to low temperatures is necessary for the upregulation of *Vrn‐1* series and downregu‐ lation of *Vrn‐2* in the leaves. Failure of these processes will delay the flowering process [26–28]. As spring approaches, the *Vrn‐3* levels are upregulated (a process mediated by photoperiod genes) and signals are sent from the leaves to shoot apices to increase the *Vrn‐1* transcription above threshold levels for the induction of flowering [10, 28]. Winter wheat types are considered to be ancestral to spring wheat types [29] and the winter alleles of *Vrn‐A1* genes are considered to be ancestral to the spring alleles. The insensitivity of spring wheat to vernalization is due to mutational loss of the repressor binding site in the regulatory region of one or more *Vrn‐1* genes [30] and is responsible for the early flower‐ ing ability of spring wheat [31]. Spring wheat varieties have been bred to adapt to diverse agroclimatic conditions attributing to their much shorter flowering time as compared to winter wheat [4, 32].

Substitution line analyses have identified four major series of genes controlling the length of the vernalization period in bread wheat (**Table 1**). According to Yan et al [17], the *Vrn‐B3* gene (7BS) was identified as a flowering time (FT)‐like gene, as have its homologous *Vrn‐A3* and *Vrn‐D3* on 7AS and 7DS, respectively [19]. Among the identified major genes control‐ ling vernalization response, the *Vrn‐1* series (*Vrn‐A1*, *Vrn‐B1*, *Vrn‐D1*) is the predominant one in reducing vernalization requirement [31, 33]. However, even with fulfilled vernalization requirement, photoperiod sensitive bread wheat cannot flower until a critical day length has been reached [5].


**Table 1.** Vernalization genes/class of genes identified in bread wheat to date.
