Hormonal Signal Transduction in Rice

Abstract

The compact nature of the rice genome provides a distinct advantage in gene isolation and genomic sequencing in contrast to other cereal crops. Further, the rice genome shows apparent syntenies with many other cereal crops such as wheat, barley, and maize (reviewed by Devos 2005). These syntenies suggest that rice genomics has implications not only for rice genetic studies and breeding, but also for other crops. Thus, rice has been selected as a target species for genome research by a number of research groups, and the International Rice Genome Sequencing Project (IRGSP) was launched in 1998, with the participation of ten countries (Sasaki et al. 2002). In 2004, the IRGSP declared the completion of the whole rice genome sequence, which provides very useful information for studying rice biology. Using this advantage and others in rice biology such as tagging libraries, transformation techniques, and full-length cDNA clones (An et al. 2005; Sasaki et al. 2005), we and other groups have been studying the signal transduction mechanisms of some phytohormones such as gibberellin (GA), brassinosteroid (BR), and auxin. Because dwarf characteristics are favored in plant breeding, many rice dwarf mutants have been identified and some have been used in the analysis of GA and BR (Ashikari and Matsuoka 2002). The sd1 mutant is a good example of how rice GA-related mutants have contributed to progress in basic science and breeding programs. This mutant enabled a dramatic increase in the yield of rice, which contributed significantly to global food security in the 1960s, during the time referred to as the “green revolution” (Khush 1999). We identified the SD1 gene and determined that the gene encodes GA20 oxidase, which catalyzes the final steps in GA biosynthesis (Ashikari et al. 2002; Sasaki et al. 2002). Erect leaves caused by BR deficiency or insensitivity also increase biomass production and grain yield (see Chapter IV.2). Consequently, we performed large-scale screening of GA-, BR-,