A method for modifying the nuclear genome of a plant cell at a preselected site, comprising the steps of: a. Contacting a plant cell with a bacterium capable of directing the transfer of T-DNA molecules from said bacterium into the nuclear genome of said plant cell, said bacterium comprising i. a first T-DNA molecule comprising between a first pair of T-DNA borders a chimeric gene encoding a plant-functional double stranded DNA break inducing (DSBI) enzyme, said DSBI enzyme being capable of recognizing and inducing a double stranded DNA break at a recognition site located at or within 2 kb of said preselected site, said chimeric gene comprising the following operably linked elements: 1. a plant expressible promoter; 2. a DNA region encoding a DSBI enzyme; 3. a plant-functional 3' termination and polyadenylation region; and ii. a second T-DNA molecule comprising between a second pair of T-DNA borders a repair DNA molecule for use as a template for repair of said double stranded DNA break b. Selecting a plant cell wherein said repair DNA has been used as a template for repair of the double stranded DNA break, said repair of said double stranded DNA break resulting in a modification of said nuclear genome at said preselected site, wherein said modification is selected from i. a replacement of at least one nucleotide; ii. a deletion of at least one nucleotide; iii. an insertion of at least one nucleotide; or iv. any combination of i. - iii. a. Contacting a plant cell with a bacterium capable of directing the transfer of T-DNA molecules from said bacterium into the nuclear genome of said plant cell, said bacterium comprising i. a first T-DNA molecule comprising between a first pair of T-DNA borders a chimeric gene encoding a plant-functional double stranded DNA break inducing (DSBI) enzyme, said DSBI enzyme being capable of recognizing and inducing a double stranded DNA break at a recognition site located at or within 2 kb of said preselected site, said chimeric gene comprising the following operably linked elements: 1. a plant expressible promoter; 2. a DNA region encoding a DSBI enzyme; 3. a plant-functional 3' termination and polyadenylation region; and ii. a second T-DNA molecule comprising between a second pair of T-DNA borders a repair DNA molecule for use as a template for repair of said double stranded DNA break i. a first T-DNA molecule comprising between a first pair of T-DNA borders a chimeric gene encoding a plant-functional double stranded DNA break inducing (DSBI) enzyme, said DSBI enzyme being capable of recognizing and inducing a double stranded DNA break at a recognition site located at or within 2 kb of said preselected site, said chimeric gene comprising the following operably linked elements: 1. a plant expressible promoter; 2. a DNA region encoding a DSBI enzyme; 3. a plant-functional 3' termination and polyadenylation region; and 1. a plant expressible promoter; 2. a DNA region encoding a DSBI enzyme; 3. a plant-functional 3' termination and polyadenylation region; and ii. a second T-DNA molecule comprising between a second pair of T-DNA borders a repair DNA molecule for use as a template for repair of said double stranded DNA break b. Selecting a plant cell wherein said repair DNA has been used as a template for repair of the double stranded DNA break, said repair of said double stranded DNA break resulting in a modification of said nuclear genome at said preselected site, wherein said modification is selected from i. a replacement of at least one nucleotide; ii. a deletion of at least one nucleotide; iii. an insertion of at least one nucleotide; or iv. any combination of i. - iii. i. a replacement of at least one nucleotide; ii. a deletion of at least one nucleotide; iii. an insertion of at least one nucleotide; or iv. any combination of i. - iii.