Induction of a hypersensitive response by chimeric helicase sequences of tobamoviruses U1 and Ob in N-carrying tobacco

Recently, the helicase domain of the Tobacco mosaic virus (TMV)-U1 replicas e proteins (designated MOREHEL:U1) was identified as the elicitor of the N gene-mediated hypersensitive response (HR) in tobacco. In this study, we us ed agroinfiltration to express the equivalent MOREHEL domain of the non-HR- inducing tobamovirus strain TMV-Ob. It appeared that this MOREHEL:Ob sequen ce did not elicit a HR in N gene-carrying tobacco. Both MOREHEL sequences w ere divided into eight subdomains, and chimeras of MOREHEL sequences from U 1 and Ob were constructed. Expression of these chimeric MOREHEL sequences r evealed that, in the TMV-U1 MOREHEL sequence, at least four domains involve d in full HR induction were present. The presence of at least three of thes e four domains seems a minimal requirement for HR induction. Two additional domains may play a minor role in HR induction. To study the elicitor funct ion of the chimeras during the TMV life cycle, chimeric MOREHEL domains wer e introduced into full-length TMV cDNA clones. These constructs, however, w ere unable to establish an infection in Nicotiana benthamiana or Nicotiana tabacum plants.