RETROTRANSPOSONS IN THE GENOMES OF HIGHER PLANTS

Retrotransposons with high copy numbers and extensive sequence heterogeneity are widely distributed in higher plants. Retrotransposons can be divided into three major classes according to their structure organization and the amino acid sequences of the encoded reverse transcriptase. They are Ty1 copia like retrotransposons, Ty3 gypsy like retrotransposons, and LINE (long interspersed nuclear elements) like retrotransposons. Retrotransposons have been found in the flanking regions of normal plant genes. These elements may provide regulatory sequences for gene expression and may be involved in gene duplication during evolution. So far, transcriptionally active retrotransposons are only identified in the root of tobacco under normal developmental and growth conditions. Most of the retroelements in plants appear to be “genomic parasites. But they contribute greatly to the genetic diversity and genome size variability of plants. Some silent retrotransposons in plant genomes only possess the capability to transpose under certain circumstances, such as tissue culture or stimulation by microbial elicitors. The expression of these retrotransposons produces mutant progeny which may be useful for crop improvement and natural selection. Recently, active retrotransposons of tobacco were introduced into a few heterogeneous plant species. These elements transposed in the genome of new hosts which reveals the possibility of using retroelements to study plant gene functions.