Toward understanding transcriptional regulatory networks in abiotic stress responses and tolerance in rice

In most crops, actual yields are only 20% of attainable yields (Boyer 1982). In crop production, a dominant factor limiting yield is abiotic stress such as excess or deficient water, high or low temperature, and high salinity (Boyer 1982). Therefore, improvement of abiotic stress tolerance might increase actual yields extensively in the most crops. As plants are sessile by nature they have evolved adaptive mechanisms against abiotic stress conditions. Recent progress in molecular biology has opened the door to uncovering the adaptive mechanisms at the molecular level in plants (Yamaguchi-Shinozaki & Shinozaki 2005, 2006).A large number of abiotic stress responsive genes have been reported in a variety of plants including rice and Arabidopsis. These genes induced during stress conditions function not only in the protection of cells from stress by production of important metabolic proteins, but also in the regulation of genes, including transcription factors (TFs), for signal transduction in the stress responses. These TFs regulate expression of multiple downstream target genes under stress conditions. These regulatory systems are achieved through specific cis-elements in the promoter regions of target genes, which are termed 'regulons'. Several regulons involved in abiotic stress responses have been identified in Arabidopsis (Nakashima et al. 2009; Qin et al. 2011). The dehydration-responsive element binding protein 1 (DREB1)/C-repeat binding factor (CBF) regulon functions in the cold stress response, whereas the DREB2 regulon acts in heat and osmotic stress responses (Mizoi et al. 2011). The abscisic acid (ABA) responsive element (ABRE) binding protein (AREB)/ABRE binding factor (ABF) regulon functions in ABA-dependent gene expression under osmotic stress conditions (Fujita et al. 2011). In addition, the NAC regulon is shown to be involved in osmotic stress responses (Nakashima et al. 2009).Rice (Oryza sativa L.) is a staple crop for more than half of the world's po