Transcriptional regulatory networks in response to abiotic stresses in Arabidopsis and grasses.

Various abiotic stresses such as drought, high salinity, high temperature, and low temperature negatively impact plant growth and productivity of crops. Plants have adapted to respond to these stresses at the molecular, cellular, physiological, and biochemical level, enabling them to survive. Various adverse environmental stresses induce the expression of a variety of genes in many plant species (Xiong et al., 2002; Shinozaki et al., 2003; Bartels and Sunkar, 2005). Numerous stress-induced genes have been identified using microarray experiments (Kreps et al., 2002; Seki et al., 2002). The products of these genes are thought to promote stress tolerance and to regulate gene expression through signal transduction pathways (Xiong et al., 2002; Shinozaki et al., 2003). Abscisic acid (ABA) is produced under water deficit conditions and plays an important role in the stress response and tolerance of plants to drought and high salinity. Exogenous application of ABA induces a number of genes that respond to dehydration and cold stress (Zhu, 2002; Shinozaki et al., 2003). Several reports have characterized genes that are induced by dehydration and cold stress, but do not respond to exogenous application of ABA in Arabidopsis (Arabidopsis thaliana; Zhu, 2002; Yamaguchi-Shinozaki and Shinozaki, 2006). This suggests the existence of ABAindependent and ABA-dependent signal transduction pathways that convert the initial stress signal into cellular responses. To better understand the molecular mechanisms regulating gene expression in response to abiotic stresses, including dehydration and cold stress, studies have initially focused on the analysis of Arabidopsis cisand trans-acting elements and their role in mediating stress responses (Yamaguchi-Shinozaki and Shinozaki, 2006). Recently, abiotic stress-inducible genes and their cisand trans-acting elements were also studied in rice (Oryza sativa), a preferred crop plant to study stress responses because of its commercial value, relatively small genome size (approximately 430 Mb), diploid origin (2x = 24), and close relationship to other important cereal crops. 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).