What is Tos 17?
Ty-1 copia retrotransposons belong to Long Terminal Repeats retroelements which transpose through a copy and paste mechanism via a RNA intermediate and form the major component of intergenic regions in higher plants (Bennetzen, 2000). Tos17 is a Ty1-copia retrotransposon of rice (Oryza sativa L.) belonging to a 32 member-family, 5 members of which have been found active (Tos10, 17, 19, 25 and 27). Tos17 is the most active among these members, exists in low copy number (1-10) in rice varieties and its transposition is regulated at the transcriptional level during normal culture conditions. During tissue culture, activation is observed and results in an increase of the number of Tos17 copies which is a function of the tissue culture duration (Hirochika et al. 1996, Hirochika 2001). Upon regeneration activation stops and the resulting new inserts are stably inherited in the progeny of regenerated plants.

Due to these favourable features, Tos17 has been used for the insertion mutagenesis of the model cereal plant (Hirochika 2001). Nipponbare ,which has been selected as the standard cultivar by the international rice genome sequencing project (IRGSP) carries only two copies per haploid genome located on chromosomes 7 and 10 and an average of 10 transposed copies of Tos17 scattered over the genome are typically observed in plants regenerated from 5 month-old callus cultures. A collection of 51,625 regenerated rice lines carrying 510,000 insertions has been produced at the National Institute for Agrobiological Sciences (NIAS) in Japan (Miyao et al. 2003). 20,000 of these insertions have already been characterized by sequencing the genomic region flanking new Tos17 integration sites, allowing to readily search for an insertion in a favourite gene for investigating its function.

How far is the international effort of insertion mutagenesis in rice?
Insertion line libraries are one of the most needed resources for discovering and validating gene function in forward and reverse genetics strategies. The Ac/Ds and En/Spm maize transposon systems, the Tos17 endogenous retrotransposon and the T-DNA are the main rice insertion mutagens. Classical point or deletion mutagenesis populations are also developed in association with TILLING for searching lesions in specific genes. A large international effort of coordination of national mutagenesis initiatives (Korea, Australia, Japan, China, Taiwan, Singapore, USA , The Philippines, France) has been launched through an International Rice Functional Genomics Consortium and an international project of screening for drought tolerance-associated traits of these mutant libraries has been commissioned by the Generation Challenge Program. The well characterized Genoplante insertion line library, which is made accessible to the scientific community as seeds become available, is recognized as a major contributor to this international framework. Like in Arabidopsis it is however very likely that most of the exchanges will be done on the basis of FST search. This is reinforced by the quarantine and biosafety regulations restricting international transfer of rice seeds.

Unlike the Arabidopsis community which now benefits from a coverage of the whole genome with ca 300K public FSTs, the rice community which deals with a 4-fold larger genome, (even though all 3 mutagens have insertional preference in gene-rich regions) is still in the need of more characterized insertion sites (estimated to a current 100,000 FST from 200,000 lines). To facilitate the reverse genetics search, Cirad has developed a GGB based rice genome navigator integrating all the available public FST information (Droc et al. 2006).

Bibliography

• Bennetzen JL (2000) Plant Mol Biol 42:251-269
• Hirochika H. (2001) Curr Opin Plant Biol , 4:118-122
• Hirochika H et al. (1996) Proc. Natl. Acad. Sci USA, 93 : 7783- 7788
• Miyao A et al. (2003) Plant Cell, 15, 1771-1780
• Sallaud C et al. (2004) Plant J. 39, 450-464
• IRGSP (2005) Nature, 436 :793-800