The present invention discloses a system for site-specific modification of ALS gene by a CRISPR-Cas9 system to produce herbicide-resistant rice, and uses thereof. The system for site-specific modification in a plant genome of the present invention comprises a vector for site-specific modification in a plant genome and a donor DNA; wherein the vector for site-specific modification in a plant genome comprises a Cas9 protein expression cassette, gRNA expression cassettes and a donor DNA; the gRNA expression cassettes encode two gRNAs targeting two target sites of a target DNA of a plant of interest, respectively; the target DNA has a fragment to be site-specifically modified which is positioned between the two target site.
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