Goldacre Paper The central role of the VERNALIZATION1 gene in the vernalization response of cereals

Many varieties of wheat (Triticum spp.) and barley (Hordeum vulgare L.) require prolonged exposure to cold during winter in order to flower (vernalization). In these cereals, vernalization-induced flowering is controlled by the VERNALIZATION1 (VRN1) gene. VRN1 is a promoter of flowering that is activated by low temperatures. VRN1 transcript levels increase graduallyduringvernalization,with longer cold treatments inducinghigher expression levels.ElevatedVRN1 expression is maintained in the shoot apex and leaves after vernalization, and the level of VRN1 expression in these organs determines how rapidly vernalized plants flower. Some alleles ofVRN1 are expressedwithout vernalization due to deletions or insertions within the promoter or first intron of the VRN1 gene. Varieties of wheat and barley with these alleles flower without vernalization and are grown where vernalization does not occur. The first intron of the VRN1 locus has histone modifications typically associated with the maintenance of an inactive chromatin state, suggesting this region is targeted by epigenetic mechanisms that contribute to repression ofVRN1 before winter. Other mechanisms are likely to act elsewhere in the VRN1 gene to mediate low-temperature induction. This review examines how understanding the mechanisms that regulateVRN1provides insights into the biologyof vernalization-inducedflowering in cereals andhow thiswill contribute to future cereal breeding strategies. Additional keywords: barley, flowering, wheat. Vernalization-induced flowering Plants growing in temperate regions time flowering to coincide with favourable seasonal conditions. Winter frost can damage cold-sensitive floral organs, whereas heat and water stress during summer can reduce fertility, so flowering often occurs in spring when conditions are optimal (see King and Heide 2009). One cue that promotes spring flowering is prolonged exposure to cold during winter, or vernalization. Vernalization occurs in many plants (see Chouard 1960; Amasino 2004; King and Heide 2009) but the focus of this review is the molecular mechanisms that control vernalization-induced flowering in economically important cereal crops such as wheat (Triticum aestivum L.) and barley (Hordeum vulgare L.). The vernalization response in cereals In temperate regions, wheat and barley are sown in autumn then over-winter before flowering in spring. When sown in spring, these same cereal varieties typically show delayed flowering or fail to flower altogether. Several researchers recognised that the cold of winter is a critical factor required to trigger flowering of these plants and that this is lacking when plants are sown in spring (see McKinney 1940; Chouard 1960). For example, Gassner showed that germinating wheat or rye (Secale cereale M. Bieb.) seeds at normal growth temperatures can cause a strong delay of flowering, whereas germination at low temperatures can stimulate flowering (Gassner 1918). He concluded that many cereals have a requirement for cold, or “Kaltbedürfnis”, which must be satisfied to allow flowering (Gassner 1918). This phenomenon later came to be referred to as vernalization (vernalis meaning “pertaining to spring”; see Chouard 1960). Vernalization promotes the transition to reproductive development at the shoot apex (Flood and Halloran 1984). Thus, after vernalization, the production of leaf primordia ceases and floral primordial appear at the shoot apex (Bonnet 1935, 1936;Zadoks et al. 1974). These develop into inflorescence branches that bear the florets (flowers) (Bonnet 1935, 1936; Zadoks et al. 1974). Vernalization is sufficient to trigger the transition to reproductive development, but long days are then required for rapid inflorescencedevelopment and stemelongation (Purvis 1934;Gott et al. 1955) (Fig. 1). Critically, vernalization is a prerequisite for the acceleration of flowering by long days, so long days cause rapid flowering only after plants have been vernalized (Purvis 1934). This combination of vernalization requirement and daylength sensitivity ensures that flowering is CSIRO PUBLISHING Review www.publish.csiro.au/journals/fpb Functional Plant Biology, 2010, 37, 479–487 CSIRO 2010 10.1071/FP10056 1445-4408/10/060479 delayed until after winter to avoid frost damage but then occurs rapidly as daylength increases during spring, thereby avoiding heat and water stress during summer (Fig. 2). A similar combination of vernalization requirement and daylength sensitivity is found in many plants from temperate zones (see Thomas and Vince-Prue 1997; King and Heide 2009). The effect of vernalization is cumulative, so increasing durations of cold accelerate flowering to greater extents until a point when the vernalization response is saturated (Gott et al. 1955). Acceleration of flowering by vernalization is also temperature dependent and, typically, there is an optimal temperature for vernalization between 0 and 10 C that will saturate the vernalization response more rapidly than warmer or colder temperatures (Gassner 1918; Chouard 1960). Thus, vernalization triggers a quantitative flowering response in cereals, and the effectiveness of vernalization is both time and temperature dependent. Vernalization is remembered When cells from vernalized wheat are used to regenerate plants through tissue culture, the resulting plants do not require vernalization to flower (Marci nska et al. 1995). Similarly, wheat cells can be vernalized during tissue culture to give rise to plants that flower rapidly with no further requirement for vernalization (Whelan and Schaalje 1992). Furthermore, maturing seeds on the spike (inflorescence) can be exposed to prolonged cold and then allowed toundergo the normal process of seed drying and germination to give rise to seedlings that flower without vernalization (Gregory and Purvis 1936). These observations suggest that cereals retain a cellular memory of vernalization. The idea that plants retain a memory of vernalization is consistent with the way vernalization promotes flowering: when germinating seeds are exposed to prolonged cold, there are no visible signs of floral development at the end of vernalization, but plants undergo rapid floral development when placed at normal growth temperatures after vernalization (Purvis 1934; Flood and Halloran 1984; Sasani et al. 2009). So exposure to cold is remembered and this exerts an after-effect during subsequent development (Chouard 1960). After plants flower, thememory of vernalization is presumably reset in seeds to allow the vernalization response to reoccur in the next generation. The molecular basis of vernalization-induced flowering in cereals The VERNALIZATION1 gene (VRN1) controls vernalizationinduced flowering in cereals (see Trevaskis et al. 2007a; Distelfeld et al. 2009). VRN1 encodes a MADS box transcription factor (a class of transcription factor named after the archetypal genes MCM1, AGAMOUS, DEFICIENS, SRF1) related to genes that promote flowering in other plant species (Danyluk et al. 2003; Trevaskis et al. 2003; Yan et al. 2003). VRN1 transcripts are present at low basal levels but increase during prolonged cold treatment (Danyluk et al. 2003; Trevaskis et al. 2003; Yan et al. 2003). This response is quantitative, with longer cold treatments inducing higher transcript levels (Danyluk et al. 2003; Yan et al. 2003; von Zitzewitz et al. 2005; Sasani et al. 2009). This parallels the degree to which Fig. 2. Seasonal flowering responses of temperate cereals. Flowering of autumn-sown (vernalization-responsive) cereals is delayed before winter because neither daylength nor vernalization response pathways are active. During winter, plants are vernalized, promoting the transition to reproductive development and making plants competent to respond to long days. Subsequently, plants flower rapidly as days lengthen during spring. Fig. 1. The influence of vernalization and daylength on development of the shoot apex of wheat. Vernalization-responsive varieties of wheat or barley are sown in autumn and require vernalization (blue arrow) during winter to promote the transition to reproductive development at the shoot apex. The transition to reproductive development ismarkedby the appearance of lateral inflorescence primordia at the shoot apex, in addition to the leaf primordia, which gives rise to distinctive double ridges (DR). After plants have been vernalized, increasing daylength (yellow arrow) can accelerate inflorescence development and stem elongation, so development proceeds rapidly to the terminal spikelet stage (TS) and eventual head emergence (H) in spring. Wheats or barleys with reduced photoperiod sensitivity or vernalization requirement will show other flowering behaviours. 480 Functional Plant Biology B. Trevaskis