Transcription factors (TFs) are master regulators that control gene clusters. A single TF can control the expression of many target genes through specific binding of the TF to the cis-acting element in the promoters of respective target genes. This type of transcriptional regulatory system is called regulon. Several major regulons that are active in response to abiotic stress have been identified in Arabidopsis. Dehydration-responsive element binding protein 1 (DREB1)/C-repeat binding factor (CBF) and DREB2 regulons function in ABA-independent gene expression, whereas the ABA-responsive element (ABRE) binding protein (AREB)/ABRE binding factor (ABF) regulon functions in ABA-dependent gene expression (Fig. 1). In addition to these major pathways, other regulons, including the NAC and MYB/MYC regulons, are involved in abiotic stress-responsive gene expression. Recent studies demonstrated that DREB1/ CBF, DREB2, AREB/ABF, and NAC regulons have important roles in response to abiotic stresses in rice (Fig. 1). In this article, we focus on the regulation of gene expression in response to dehydration, high salinity, cold, and heat stresses, with particular emphasis on the role of DREB1/CBF, DREB2, AREB/ABF, and NAC regulons in grasses, including important crops such as rice, wheat (Triticum aestivum), maize (Zea mays), and barley (Hordeum vulgare), in comparison to Arabidopsis. For a more comprehensive overview on the very complex signal transduction pathways controlling abiotic stress responses, we refer the reader to the many excellent review articles that have recently been published (Chinnusamy et al., 2004; Bartels and Sunkar, 2005; Sunkar et al., 2007).
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