Targeted molecular trait stacking in cotton through targeted double-strand break induction.

Recent developments of tools for targeted genome modification have led to new concepts in how multiple traits can be combined. Targeted genome modification is based on the use of nucleases with tailor-made specificities to introduce a DNA double-strand break (DSB) at specific target loci. A re-engineered meganuclease was designed for specific cleavage of an endogenous target sequence adjacent to a transgenic insect control locus in cotton. The combination of targeted DNA cleavage and homologous recombination-mediated repair made precise targeted insertion of additional trait genes (hppd, epsps) feasible in cotton. Targeted insertion events were recovered at a frequency of about 2% of the independently transformed embryogenic callus lines. We further demonstrated that all trait genes were inherited as a single genetic unit, which will simplify future multiple-trait introgression.

Spontaneous mutation frequency in plants obscures the effect of chimeraplasty.

In this study we tested the performance of chimeraplasts, chimeric RNA/DNA oligonucleotides, for the creation of directed changes in chromosomal sequences in tobacco and oilseed rape. As target genes for chimeraplasty, the endogenous als gene and two transgenes, bar and a fusion between egfp and bar, were used. In experiments in which similar numbers of cells were treated with and without chimeraplasts, delivery of chimeraplasts did not lead to increased numbers of herbicide-resistant or egfp fluorescent calli. Sequence analysis of the target genes revealed a range of sequence changes rather than the sequence modification intended through chimeraplast design. Importantly, in both the presence and absence of chimeraplasts, similar types of changes were obtained at similar frequencies. These data suggest that the observed changes at the target codon result primarily from spontaneous mutation.