Extensive Rewiring of Transcriptional Regulatory Networks and Pleiotropic Phenotypes in Tobacco Expressing a Hairpin of the Tnt1 Retrotransposon

Tnt1 is one of the few transpositionally active LTR-retrotransposons, present in over 600 copies in the Nicotiana tabacum genome. Under normal growth conditions, Tnt1 expression is limited to basal levels. Whenever tobacco plants are under biotic or abiotic stress, Tnt1 expression is induced along with other plant defense response genes. Because retrotransposons are able to shape their host genomes and are active in critical moments such as stress responses, we hypothesized that Tnt1 may have a biological role more complex than presently assumed. To address this hypothesis, we generated transgenic tobacco expressing a hairpin of a fragment of the Tnt1 coding region. Transgenic plants showed pleiotropic phenotypes such as cell death spots on the leaves or extreme phenotypes like abnormal development in aerial and root portions, early flowering and callose deposition (which is known to be related to plant pathogen defense). RNA sequencing was performed to examine changes in the transcriptional profiles, revealing a rewiring of transcriptional regulatory networks related to stress responses in the transgenic plants. Ethylene is known to be a signalling compound in plant defense responses. We found genes of the ethylene cascade up-regulated, and ethylene overproduction was demonstrated both in 08 and 95 days-old in vitro plants. Our results suggest that copies of Tnt1 could be part of gene regulatory networks that control stress and defense responses acting as modulator to some networks and as an intermediate part of others.