Characterization of oncogene‐silenced transgenic plants: implications for Agrobacterium biology and post‐transcriptional gene silencing

SUMMARY Agrobacterium tumefacienstumorigenesis is initiated by the horizontal transfer of a suite of oncogenes that alter hormone synthesis and sensitivity in infected plant cells. Transgenic plants silenced for theiaaMandiptoncogenes are highly recalcitrant to tumorigenesis, and present a unique resource to elucidate fundamental questions related toAgrobacteriumbiology and post‐transcriptional gene silencing (PTGS). The oncogene‐silenced transgenic tomato line 01/6 was used to characterizeA. tumefaciensgrowthin plantaand to screen foriaaMandiptsequence variants. Even in the absence of macroscopic and microscopic indications of tumorigenesis,A. tumefaciensis capable of long‐term survival in the hypocotyl tissues of the 01/6 line.A. tumefaciensgrowth, however, is significantly reduced in the 01/6 line, with populations decreased by 96% relative to wild‐type at 52 days post‐inoculation. In addition, the 01/6 line displayed suppression of tumorigenesis against all 35 tested strains ofA. tumefaciens. High target homology is an absolute requirement of PTGS, therefore this result suggests that regions of theiaaMandiptoncogenes are very highly conserved across mostA. tumefaciensstrains. Finally, graft transmissibility of oncogene silencing was assessed by grafting various non‐silenced tomato genotypes on to the 01/6 line. Phenotypic and molecular evidence (tumorigenesis and absence of small interfering RNAs, respectively) suggest that oncogene silencing is not graft‐transmissible, at least to wild‐type and antisenseiaaM ‐over‐expressing genotypes.