Long‐culm mutations with dominant genes are induced by mPing transposon in rice

摘要

A recent trend in rice breeding has been the development of semidwarf rice cultivars, which are suitable for high-density cultivation with intensive use of fertilizers (Khush 1999). These cultivars typically feature a short sturdy culm, multiple tillers and upright leaves, and their high productivity has been reported in numerous studies (Tomita 2009). However, over-reliance on semidwarf rice cultivars developed using the single semidwarf gene sd1 (Hedden 2003a, 2003b) is not without its own intrinsic limitations. The attendant risks are especially clear in view of rapidly growing pressure for food supply to support an ever expanding world population in the 21st century. As a solution to our food problem, it is necessary to devise a new strategy that allows dramatically enhanced crop yields. For this, we propose the development of ‘tall dwarf’ rice cultivars that are characterized by increased biomass with long culms and large grains based on the primary semidwarf cultivars. In wheat breeding, genetic information has been elucidated for producing the ideal ‘tall dwarf’ plant type (Gale and Law 1976); that is, information on the functions of individual genes that directly control biomass production and the phenotypic effects of genetic modifications of these genes. However, in the case of rice, genetic studies on the elongation of rice culms have remained scarce (Okuno and Kawai 1978a, 1978b).This study focuses on the above question, and seeks to analyze mutant genes involved in culm elongation in several transposon-induced long-culm mutants, and ultimately provide insights into the development of improved long-culm rice lines. Transposable elements (TEs) are mobile DNA sequences that can change their positions in the genome by inserting themselves into new sites. TEs are classified into two classes according to the mode of propagation: retrotransposons (class I elements) and DNA transposable elements (transposons in a narrow sense, class II elements). While the former transposes through an RNA intermediate by the action of reverse transcriptase, the latter transposes in a DNA form through a cut-and-paste mechanism. For a long time, TEs were dismissed as selfish DNA. Recently, however, TEs have been revealed as major players in genomic evolution, because they cause genome rearrangements and alter the structure and regulation of individual genes (Bennetzen et al. 2000; Biemont and Vieira 2006; Tomita et al. 2008; Tomita 2010).In rice, TEs account for at least 35% of the genome (International Rice Genome Sequencing Project 2005). The numerically predominant type of TE is the miniature inverted-repeat transposable element (MITE) (Bureau et al. 1996; Mao et al. 2000; Turcotte et al. 2001; Feng et al. 2002; Jiang et al. 2004). MITEs are reminiscences of non-autonomous DNA transposons that are distinguished from other TEs because of their small size (<600 bp) and presence of short terminal inverted repeats (TIRs). Miniature Ping (mPing) is the first active MITE discovered in the rice genome (Jiang et al. 2003; Kikuchi et al. 2003; Nakazaki et al. 2003). The MITE mPing was shown to be active in the japonica strain Gimbozu where it had accumulated more than 1000 copies (Monden et al. 2009) and several long-culm mutants have been obtained in the progeny. In this study, three long culm mutants induced by mPing were analyzed for their mutant genes, and dominant long culm genes were identified.