Codon-optimized Cas9 endonuclease encoding polynucleotide

It was now found that the expression of a nucleotide sequence as described in the method of the invention in a plant cell results in much higher rates of indels compared to those seen in cells transformed with a control nucleic acid molecule. Thus, the invention is directed to codon-optimized Cas9 endonuclease-encoding polynucleotide. Accordingly, the present invention provides a method for modifying a target site in the genome of a plant cell, the method comprising providing one or more guide RNA and a Cas endonuclease to said plant cell, wherein said guide RNA and Cas endonuclease are capable of forming a complex that enables the Cas endonuclease to introduce a double strand break at said target site, and wherein the Cas9 endonuclease is expressed in the plant cell from a polynucleotide comprising an codon-optimized Cas9 endonuclease encoding nucleic acid molecule with a nucleotide sequence selected from the disclosed nucleotide sequences.