Suppression of A . tumefaciens tumorigenesis in oncogene-silenced walnut microshoots. Walnut microshoots were inoculated with virulent A. tumefaciens strain 20W-5A. A) Inoculation sites of untransformed microshoots formed large, undifferentiated tumors five weeks post-inoculation. B) Oncogene-silenced line 33-3-1 exhibited no tumor development. C) Small fragments of callus from A. tumefaciens inoculation sites cultured on hormone-free plant growth medium for five weeks: untransformed callus (upper) and callus derived from transgenic line 33-3-1 (lower).