Using firefly luciferase to identify the transition from transient to stable expression in bombarded wheat scutellar tissue

Transformation of wheat by the biolistic procedure is unpredictable and inefficient. To gain insight into the fate of transgenes introduced into wheat scutellar tissue by particle bombardment, the expression of the firefly luciferase gene was followed using low-light video imaging. Luciferase expression can be detected as early as 40 to 50 min after bombardment. After 48 h, the levels of transient expression in individual scutella fall rapidly, becoming undetectable 10 to 20 days later. Luciferase activity was observed to recover in a small percentage of the material and it was iom this material that transformed plants, stably expressing luciferase, were recovered. We concluded that the calluses which recover luciferase expression are stably transformed and we have termed the transition period between transient and stable expression the transformation boundary. The percentage of scutella displaying luciferase activity, as measured at 30 days post-bombardment, was judged a realistic measure of the efficiency of the transformation procedure. The results of our experiments suggest that the selection and regeneration of plants were not major factors contributing to the poor transformation efficiencies associated with biolistic transformation. The results demonstrate that luciferase can be used to assess rapidly and quantify the efficiency of the transformation procedure without the need to produce transformed plants. This will allow different procedures to be rapidly assessed and compared and should provide valuable insight into the conditions required to improve the efficiency of DNA integration and stable expression in species recalcitrant to transformation.