Activation of retrotransposons by stresses and external changes is common in all eukaryotic systems, including plants. The transcription of the tobacco Tnt1 retrotransposon was studied in its natural host as well as in Arabidopsis and tomato. It is activated by factors of microbial origin, by external stresses, and by viral, bacterial, and fungal attacks. Tnt1 expression is linked with the biological responses of the plant to the elicitor or to the pathogen attack and in particular with the early steps of the metabolic pathways leading to the activation of plant defense genes. In most cases, the basic features of Tnt1 regulation in tobacco are maintained in tomato and Arabidopsis, but some host-specific regulations were shown. The U3 region of the Tnt1 LTR contains the major cis-acting components of Tnt1 transcriptional activation in association with the plant defense responses. Furthermore, the Tnt1 U3 region, and especially the tandemly repeated BII boxes, contains several sequences similar to well-characterized motifs involved in the activation of several plant defense genes. The possible origin of Tnt1 regulatory sequences as well as the biological implications of Tnt1 activation by pathogen attacks are discussed.