Generating transgenic plants by minimal addition of exogenous DNA- a novel selection marker based on plant tubulins

Most transgenic crops available on the market contain antibiotic resistance genes as selection markers. Although scientifically not justified, this raises public concern about a potential spread of antibiotic resistance genes in soil and intestinal bacteria, possibly accentuated by the agricultural use of transgenic crops. These markers are necessary to generate the transgenic crop, but they are dispensable for the agricultural use of these plants. We propose a selection system for plant transformation that is exclusively based on genetic information already present in the host plant and that does not require antibiotic resistance genes. The approach exploits the natural variability of plant tubulins as essential components of cell growth, cell divisionand stress tolerance in plants. There exist a panel of tubulin promoters that can drive expression either constitutively or, alternatively,in different tissues or in response to different signals. They can be fused with a panel of different tubulin isotypes with distinct physiologycal features to produce a versatile kit of combinations that will confer distinct properties such as stress or herbicide tolerance to specific traget tissues. This is used to construct a selection marker that is based upon truncated tubulins lacking the binding site for aryl-carbamate herbicides. the expression of this truncated tubulin is driven by the homologous tubulin promotor within a DNA fragment free of exogenous sequences (minimal expression unit) using clean DNA technology. The suitability of this selection system is demonstrated for rice as a model system for Graminaceous crops