With an ever-increasing frequency, it has become desirable to express many
foreign genes in the same transgenic plant. A range of approaches to this p
roblem have been explored in the period since gene transfer was developed a
s a useful tool in plant science. These have ranged from assembly of multip
le transcription units on a single DNA vector, to the execution of several
independent transformations involving single genes followed by successive r
ounds of hybridization to yield plant lines with the desired combination of
genes, to cotransformation with multiple plasmids followed by molecular sc
reening to identify desired transgenic plants. Recent developments have pro
vided new alternatives to the problem of the expression of multiple foreign
genes in plants, approaches that revolve around the expression of polycist
ronic mRNAs. This review summarizes the current status of the area of polyc
istronic gene expression in transgenic plants, within the context of multig
ene expression.