Plant retrotransposons

Retrotransposons are mobile genetic elements that transpose through reverse transcription of an RNA intermediate. Retrotransposons are ubiquitous in p lants and play a major role in plant gene and genome evolution. In many cas es, rebrotransposons comprise over 50% of nuclear DNA content, a situation that can arise in just a few million years. Plant retrotransposons an struc turally and functionally similar to the retrotransposons and retroviruses t hat are found in other eukaryotic organisms. However, there are important d ifferences in the genomic organization of retrotransposons in plants compar ed to some other eukaryotes, including their often-high copy numbers, their extensively heterogeneous populations, and their chromosomal dispersion pa tterns. Recent studies are providing valuable insights into the mechanisms involved in regulating the expression and transposition of retrotransposons . This review describes the structure, genomic organization, expression, re gulation, and evolution of retrotransposons, and discusses both their contr ibutions to plant genome evolution and their use as genetic tools in plant biology.