Retrotransposon BARE displays Strong Tissue-Specific Differences in Expression

The long terminal repeat (LTR) retrotransposons are ubiquitous in the eukaryotes and account for upwards of 80% of large plant genomes, where they are responsible for much of the changes in genome size and organization over evolutionary time. Retrotransposon copies may be passed to the next generation by one of two ways: replication of existing copies as part of the chromosome; synthesis and propagation of new copies by the combined activities of RNA polymerase II and reverse transcriptase by a “copy and paste” life cycle virtually identical to that of retroviruses, excepting the lack of an extracellular infective phase. The question of where and when in a plant retrotransposons are replicated has been hardly studied. In newly replicated copies of retrotransposons are to be inherited, integration events must occur in cells giving rise clonally to the next generation. The BARE retrotransposon comprises about 10% of the barley (Hordeum vulgare L.) genome. It is actively transcribed, translated, and forms virus-like particles (VLPs). To shed light on the replication strategy of BARE in the plant, we have used immunolocalization and in situ hybridization to examine expression of the BARE on a tissue-specific level. Gag has a highly specific localization in pre-fertilization ovaries. The strong presence of Gag in the floral meristems suggests that newly replicated copies there will be passed to the next generation. To our knowledge, this is the first analysis of the expression of native retrotransposon proteins within a plant to be reported.