Specific expression of the tobacco Tnt1 retrotransposon in protoplasts.

Abstract

The Tntl transposable element of tobacco belongs to the retrotransposon family and shares the structural features of viral retroelements including two long terminal repeats (LTRs) which are known to contain promoter regions. We show that two Tntl RNAs of 5.2 and 6.5 kb can be found. The 5.2 kb RNA matches with the size of the Tntl elements so far isolated (5.3 kb), whilst the evidence suggests that the 6.5 kb RNA could be a chimaeric RNA initiated in a gene in which Tntl has inserted. The Tntl 5.2 kb RNA starts in the LTR, and the LTR can promote the expression of a translational LTR-fl-glucuronidase (GUS) fusion at a high level in transient expression assays. The Tntl 5.2 kb RNA and the LTR-GUS fusion of transgenic tobacco plants are specifically expressed in leaf-derived protoplasts whereas they are not expressed in leaf tissue. The 5.2 kb RNA is also transcribed at low levels in roots. This RNA is induced after 2 h of maceration in the protoplast isolation medium, and its level declines rapidly after protoplast isolation. The induction requires only the presence of cell wall hydrolases, and is independent of wounding and plasmolysis. The induction of Tntl expression is not mediated by typical oligosaccharide elicitors released from the cell wall known to mediate defense gene responses. Tntl transcription features provide a first example of tissue culture-induced mutagenesis in plants and a molecular basis for some of the somaclonal variation events.