Effects of various promoter/exon/intron combinations on foreign gene expression and selection efficiency following PEG-mediated transformation of einkorn wheat protoplasts

Various chimeric sequences of different promoter elements (PEs) fused to gus as the reporter or hpt as the selectable marker were constructed, and their effects on gene expression and selection efficiency were tested following PEG-mediated transformation of Triticum monococcum protoplasts. The Ubi1 showed the highest transient expression level, whereas the 35S promoter was the weakest. The PEs showing stronger transient expression also gave more regenerated GUS(+) colonies 40 days after transformation, which were directly caused by higher levels of stable GUS expression. On the other hand, however, when hpt was used as a selectable marker gene, less difference was observed among these PEs in terms of the number of Hyg(R) colonies that survived. We showed that this disagreement between GUS expression and HPT-mediated selection is most likely caused by post-translational mechanisms, thus indicating that the selection efficiency is not a linear but rather a complex sublinear function of the HPT gene expression efficiency.