MICROBIAL ELICITORS OF PLANT DEFENSE RESPONSES ACTIVATE TRANSCRIPTION OF A RETROTRANSPOSON

The Tnt1 retrotransposon was isolated from tobacco after transposition. Tnt1 expression is very tow at the plant level and under most stress conditions but is strongly induced during protoplast isolation. It is shown here that transcription of the Tnt1 retrotransposon can be activated by several microbial factors having the common ability to elicit a hypersensitive response in tobacco. These elicitors include Onozuka crude extracts from the fungus Trichoderma viride, elicitins purified from Phytophthora fungal species and culture supernatants of the bacterium Erwinia chrysanthemi. Activation of Tnt1 expression correlates with the biological activity of purified elicitins. On detached leaves, dose response to two different elicitins, the highly necrotic cryptogein from P. cryptogea and the less toxic capsicein from P. capsici, is parallel to the necrotic dose response. A dose of 1 mu g cryptogein induces Tnt1 transcription to a high level similar to that induced by Onozuka 1 mg ml(-1) whilst 10-100-fold more capsicein is required to induce this level. In whole plants treated with cryptogein, activation of Tnt1 transcription is restricted to necrotic organs. In tobacco cell suspension cultures treated with E. chrysanthemi culture supernatants, the accumulation of Tnt1 RNA correlates with the accumulation of the phytoalexin capsidiol a few hours later. Two hours after contact with the elicitor, cells accumulate a high level of Tnt1 RNA, this level being almost 10-fold higher than that observed in protoplasts in response to Onozuka 1 mg ml(-1). Our results suggest that Tnt1 activation might reflect a local and early plant response to microbial stress.