Transformed callus does not necessarily regenerate transformed shoots

Agrobacterium tumefaciens carrying a disarmed Ti plasmid vector was used for the transformation of flax tissue. Transformed callus was obtained from inoculated hypocotyl segments and healthy green shoots were regenerated from this callus. Nopaline assays on shoot tissue were positive for nopaline content if carried out soon after removal of the shoot from the callus but negative if carried out 2-3 weeks after removal. All of these shoots gave rise to kanaymcin sensitive progeny and were most likely escapes arising from non-transformed cells protected from the selective agent by transformed cells in the callus. Careful analysis of regenerated shoots from transformed callus is necessary in order to distinguish escapes from true transgenics.