• crown gall;
  • disease resistance;
  • iaaM;
  • ipt;
  • post-transcriptional gene silencing;
  • tumour symptoms

Transformation with self-complementary oncogene sequences was used to silence the Agrobacterium tumefaciens oncogenes ipt and iaaM. The silencing response was triggered by using a very short chimeric sequence where conserved fragments from both oncogenes were fused in one unique transgene. Most T0 transgenic tobacco lines and T1 seedlings evaluated in vitro had intermediate or very low susceptibility to A. tumefaciens as compared with the wildtype plants. A greenhouse evaluation of whole plants confirmed the lines that were resistant. Low levels of transgene hairpin RNA (hpRNA) coupled with small interfering RNA (siRNA) accumulation correlated with oncogene silencing and, therefore, resistance to crown gall. After infection with the oncogenic strain, much lower levels of the oncogenes’ mRNA were found in resistant lines than in wildtype plants. The frequency of resistant lines, with few or no symptoms, produced with the chimeric construct was similar to the highest reported efficiencies obtained by using sense and antisense whole oncogene sequences.