Rice Genetics IV - IRRI books - International Rice Research Institute

Gene silencing and itsreactivation in transgenic riceT.C. Hall, S.P. Kumpatla, P. Kharb, L. Iyer, M. Cervera, Y. Jiang, T. Wang,G. Yang, P. Teerawanichpan, J. Narangajavana, and J. DongGenome surveillance systems of higher organisms protect not only againstintragenomic parasites such as retroposons and transposons but also againstinvasive DNA introduced via genome transformation, resulting in (trans)genesilencing at both the transcriptional and posttranscriptional levels. Transcriptionalgene silencing of both bialaphos resistance and Btt CryIIIA crystal toxingenes was encountered in experiments targeted to provide resistance to therice water weevil. Extensive methylation of the transgenes, especially of thepromoter elements, was evident. Germination of seedlings from silenced linesin the presence of 5-azacytidine (which prevents cytosine methylation) led tothe reactivation of bialaphos resistance in progeny of silenced plants. TheCaMV 35S promoter has been implicated in many instances of silencing andthe possibility that some promoters are especially prone to silencing is supportedby the finding that RCg2, a rice root-specific promoter, is silenced inmore than 80% of transformants. Genes flanking RCg2/uidA are rarely silenced,showing that silencing can be highly targeted. Multicopy insertionsare very susceptible to silencing and epistatic interactions between multicopyand single-copy inserts are documented. Several strategies to alleviate silencinghave been considered, such as sequence diversification and flankingtransgene inserts with matrix attachment regions.The ability to isolate genes and transfer them among organisms so that they are expressedin an alien genomic background is surely one of the most exciting discoveriesof the 20th century. Such expression has provided great insight into the fundamentalprinciples of biology as well as opening new practical opportunities for the improvementof crops and the development of novel pharmaceutical products. The realizationthat some transgenes do not express as anticipated is a significant challenge for geneticengineering of crops, but is also leading to an unanticipated understanding ofepigenetic phenomena that were previously perplexing (Wolffe and Matzke 1999).Indeed, over the past decade it has become abundantly clear that some genome surveillancesystems complement and may even evolutionarily precede better knownGene silencing and its reactivation in transgenic rice 465