Characterization of three VERNALIZATION INSENSITIVE3‐like (VIL) homologs in wild wheat, Aegilops tauschii Coss

摘要

Timing of flowering is a significant trait influencing fitness in higher plants. The transition from vegetative to reproductive growth is a critical developmental switch and a key adaptive trait in both crop and wild cereal species (Cockram et al. 2007). Many temperate plants with a winter growth habit need to be exposed to low temperature for certain periods to transition from vegetative to reproductive growth phases (Amasino 2004). Vernalization is a phenomenon for acceleration of flowering by prolonged exposure to low temperature (Trevaskis et al. 2007). In cultivated grasses, winter-type varieties require vernalization for heading and flowering, whereas spring-type varieties are able to transition from the vegetative to reproductive growth phase without vernalization.In wheat and barley, flowering time is a complex trait controlled by three genetic characteristics, vernalization requirement, photoperiodic sensitivity and narrow-sense earliness (Kato and Yamagata 1988), and the vernalization requirement for flowering is generally controlled by a major locus, Vrn-1 (Flood and Halloran 1986; Worland et al. 1987; Dubcovsky et al. 1998). Two other loci, Vrn-2 and Vrn-3, are also associated with determi nation of the vernalization requirement (Takahashi and Yasuda 1971; Law and Worland 1997; McIntosh et al. 1998). The wheat and barley Vrn-1 loci encode an APETALA1/FRUITFULL-like MADS-domain transcription factor (Murai et al. 2003; Trevaskis et al. 2003; Yan et al. 2003). Vrn-2 encodes ZCCT duplicated proteins with CONSTANS (CO), CO-like and TOC1 (CCT) domains (Yan et al. 2004a), and Vrn-3 is an Arabidopsis FLOWERING LOCUS T (FT) homolog (Yan et al. 2006). Wheat genome contains at least two FT homologs, Vrn-3 and WFT, and transcript accumulation levels of these homologs were upregulated during vernalization treatment (Yan et al. 2006; Shimada et al. 2009). WFT functions as a flowering accelerator (Shimada et al. 2009). Dominant alleles of the Vrn-1 loci contribute to the vernalization-insensitive spring habit, and structural mutations at the Vrn-1 loci, such as insertion/ deletion (indel) events at the promoter region and large deletions in the first intron, generate the dominant alleles in the spring-type cultivars of barley and of einkorn, durum and common wheat (Yan et al. 2004b; Fu et al. 2005; von Zitezewiz et al. 2005; Dubcovsky et al. 2006). Spring growth habit accessions have also been found in wild wheat species including Triticum araraticum Jakubz., Triticum urartu Thum., Triticum boeoticum Boiss. and Aegilops tauschii Coss. (Tsunewaki 1966; Golovnina et al. 2010; Takumi et al. 2011). Of the spring-type accessions in wild wheat species, some of T. boeoticum, T. araraticum and Ae. tauschii contain a dominant allele in Vrn-1, whereas other spring-type accessions from T. urartu and Ae. tauschii carry a winter-type allele of Vrn-1; the causal gene for spring growth habit remains unknown (Golovnina et al. 2010; Takumi et al. 2011). Recent study using many barley varieties showed that three major vernalization genes, Vrn-1, Vrn-2 and Vrn-3, partially explain the variation in vernalization requirement and that other known genes contribute to the remainder of the variation (Saisho et al. 2011).Activation of barley Vrn-1 after vernalization is associated with histone modification of the Vrn-1 locus (Oliver et al. 2009). In Arabidopsis, FLOWERING LOCUS C (FLC), acting as a flowering repressor, plays a central role in the vernalization pathway, and vernali zation promotes flowering through silencing FLC (Michaels and Amasino 1999). The epigenetic silencing of FLC expression is accompanied by chromatin modification (Bastow et al. 2004), which is initiated by a protein complex involving a VERNALIZATION INSENSITIVE3 (VIN3) plant homeodomain (PHD)- containing protein (Sung and Amasino 2004). Another PHD finger protein, VIN3-like 1 (VIL1), is also associated with the epigenetically silenced state of FLC chro matin (Sung et al. 2006). T