THE EXPRESSION OF THE TOBACCO TNT1 RETROTRANSPOSON IS LINKED TO PLANTDEFENSE RESPONSES

Activation of retrotransposons by stresses and external changes is com mon in all eukaryotic systems, including plants. The transcription of the tobacco Tnt1 retrotransposon was studied in its natural host as we ll as in Arabidopsis and tomato. It is activated by factors of microbi al origin, by external stresses, and by viral, bacterial, and fungal a ttacks. Tnt1 expression is linked with the biological responses of the plant to the elicitor or to the pathogen attack and in particular wit h the early steps of the metabolic pathways leading to the activation of plant defense genes. In most cases, the basic features of Tnt1 regu lation in tobacco are maintained in tomato and Arabidopsis, but some h ost-specific regulations were shown. The U3 region of the Tnt1 LTR con tains the major cis-acting components of Tnt1 transcriptional activati on in association with the plant defense responses. Furthermore, the T nt1 U3 region, and especially the tandemly repeated BII boxes, contain s several sequences similar to well-characterized motifs involved in t he activation of several plant defense genes. The possible origin of T nt1 regulatory sequences as well as the biological implications of Tnt 1 activation by pathogen attacks are discussed.