Key molecular and metabolic processes used for genetic engineering to improve freezing tolerance in cereals.

Abstract

Genetic Engineering to Improve Freezing Tolerance in Cereals 195stomata closure, which is regulated by abscisic acid (ABA). It is well documented that ABA content increases transiently in the early stage of cold stress response (Galiba et al., 1993). An increased level of ABA was found to coincide with the downregulation of other stress hormones, salicylic acid and jasmonic acid during an early phase of wheat response to cold stress (Kosová et al., 2012). The interaction among plant hormones is reviewed elsewhere . CBF transcription factorsAfter exposure to low temperature, in parallel with the enhanced ABA level, the transcriptome of those plants capable of cold acclimation undergoes a complete reorganization, as revealed by the up-or downregulation of thousands of genes (Greenup et al., 2011;Laudencia-Chingcuanco et al., 2011). As estimated in Arabidopsis, more than 200 transcription factors are involved in the reconfiguration, and may serve as regulators for acclimation (Thomashow, 2010). The best understood cold regulatory pathway is the CBF regulon controlled by the C-repeat binding factors (CBFs), also called dehydration-responsive element binding (DREB1) factors (Thomashow, 2010;Mizoi et al., 2012). The CBFs belong to the AP2/EREBP (APETALA2/ethylene-responsive element binding protein) transcription factor family and possess a plant-specific AP2 DNA binding domain that interacts with the C-repeat elements present in the promoter region of their target genes (Jaglo et al., 2001). CBF expression is induced by different abiotic stresses (cold, drought, salt). The function of CBF genes has been revealed in many plant species. In Arabidopsis, six CBFs have been identified, while in the economically important cereals, the number of CBFs are much higher: 20 in barley (Hordeum vulgare L.) (Skinner et al., 2005), 13 in einkorn (Triticum monococcum) (Miller et al., 2006) and 37 in common wheat (Triticum aestivum L.) (Badawi et al., 2007). CBF genes are positioned in clusters on the homeologous group 5 chromosomes of the Triticeae and coincide with the FR-2 quantitative trait locus (QTL) for freezing tolerance (Vágújfalvi et al., 2003(Vágújfalvi et al., , 2005Miller et al., 2006;Tondelli et al., 2006;Båga et al., 2007;Francia et al., 2007). CBFs in Triticeae are regulated in a complex way, influenced by genotype, induction-temperature and lightregulated factors (Campoli et al., 2009). Analysis of 201 rye (Secale cereale L.) genotypes showed that single nucleotide polymorphisms (SNPs) in ScCBF15 and ScCBF12 genes were significantly associated with frost tolerance (Li et al., 2011). An einkorn mapping population was generated (Miller et al., 2006) and subjected to frost tests (Knox et al., 2008) and it was shown that three CBF genes (TmCBF12, TmCBF14 and TmCBF15) were responsible for the increased frost tolerance, and this improvement was related to higher expression levels of COR14b and DHN5 genes (Knox et al., 2008). In hexaploid wheat, three CBF genes: TaCBF14, TaCBF15 and TaCBF16 were also induced by cold treatm...