Gene Expression Profiles Involved in Development of Freezing Tolerance in Common Wheat
Wheat Cold Acclimation and Freezing Tolerance
Exposure of plants to low, nonfreezing temperatures leads to an increase in freezing tolerance, and this adaptive process, called cold acclimation, involves drastic physiological, biochemical and metabolic changes. Most of these alterations are regulated through changes in gene expression. One of the mechanisms behind development of freezing tolerance is induction of the Cor (cold-responsive)/Lea (late-embryogenesis-abundant) gene family (Thomashow 1999). In common wheat, major loci controlling freezing tolerance (Fr-1 and Fr-2) have been assigned to the long arm of group five chromosomes (Galiba et al. 1995; Snape et al. 1997). Fr-2 is coincident with a cluster of genes encoding C-repeat binding factors (CBFs) in wheat and barley (Miller et al. 2006; Francia et al. 2007), which directly induce the downstream Cor/Lea gene expression during cold acclimation (Takumi et al. 2008). In expression quantitative trait locus (eQTL) analysis of Cor/Lea and CBF genes, four eQTLs controlling cold-responsive genes were found, and the major eQTL with the greatest effect was located on the long arm of chromosome 5A (Motomura et al. 2013). The 5AL eQTL region, which plays important roles in development of freezing tolerance in common wheat (Motomura et al. 2013), coincides with a region homoeologous to a frost-tolerance locus (Fr-Am2) reported as a CBF cluster region in einkorn wheat (Vágújfalvi et al. 2003; Miller et al. 2006). Allelic differences at Fr-A2 might be a major cause of cultivar differences in extent of freezing tolerance in common wheat (Motomura et al. 2013). It was recently reported that large deletions in the CBF cluster at Fr-B2 significantly reduced frost tolerance in tetraploid and hexaploid wheat (Pearce et al. 2013). In barley, two QTLs for lowtemperature (LT) tolerance, Fr-H1 and Fr-H2, are found on the long arm of chromosome 5H (Francia et al. 2004), and the Vrn-H1/Fr-H1 genotype affects both the expression of CBF genes at Fr-H2 and LT tolerance (Stockinger et al. 2007; Chen et al. 2009). Thus, the barley Vrn-H1/Fr-H1 and Fr-H2 regions function to develop freezing tolerance through Cor/Lea gene expression during cold acclimation. In contrast to barley, the functions of Vrn-A1/Fr-A1 and Vrn-D1/Fr-D1 in regulation of cold-responsive gene expression in common wheat remain unclear.
A lot of other genes, including Wlip19 and Wabi5 bZIP transcription factor genes (Kobayashi et al. 2008a, b), contribute to cold acclimation and freezing tolerance in common wheat. These transcription factors, which act in abscisic acid (ABA) signaling, bind to ABA-responsive elements in the promoters of Cor/Lea genes. Thus, ABA induces expression of a variety of genes that function in the regulation of gene expression, signal transduction and abiotic stress tolerance in common wheat. In fact, ABA sensitivity strongly affects the basal levels of freezing tolerance (Kobayashi et al. 2006, 2008c), and some QTLs on wheat chromosomes controlling ABA sensitivity at the seedling stage are also related to Cor/Lea gene expression and putatively associated with freezing tolerance (Kobayashi et al. 2010). Recent reports showed that QTLs for ABA sensitivity at the seedling stage could be also associated with dehydration tolerance, seed dormancy and preharvest sprouting tolerance (Iehisa et al. 2014a, b). The QTLs for ABA sensitivity do not correspond to Fr-1 and Fr-2, and the two Fr loci act independently of ABA signal transduction pathways (Fig. 27.1).