Classic breeding of plants allows the creation of new varieties with altered characteristics. Gene technology has the same goal but allows fast insertion and expression of particular genes from any organism in plants. Currently, most transgene plants carry genes for herbicide resistance. In the past few years, CRISPR/Cas9 technology has also been used for development of herbicide-resistant plants. CRISPR/Cas9 originates from the natural immune system of the bacteria Streptococcus pyogenes. This bacteria uses this immune system as a defence against viruses. When a virion infects a bacterium, it releases it's DNA, which inserts into the bacterial CRISPR locus. After transcription, tracrRNA and nuclease Cas9 bind to the repeating segments of the CRISPR RNA. This is a signal for the Cas9 endonuclease to cut the RNA into shorter fragments, which then serve as bacterial memory of virus infection and allow a faster response against a new viral infection. In research, CRISPR/Cas9 is used to edit genomes. This is achieved by artificial synthesis of the guiding RNA that binds to a specific DNA sequence. Cas9 forms a complex with the guiding RNA, which guides the nuclease to the targeted sequence. This technology has already allowed the altering of numerous genes which encode for herbicide resistance. Among these genes are ALS, and genes for ACCase, EPSPS, GS, PSII. Due to the increasing nuber of herbicide-resistant weeds, farmers are in need of new plants with resistance against herbicides. This task would be difficult to achieve quickly with the use of classical breeding techniques due to large genomes and polyploidy of plants. Polyploidy results in each mutation contributing to a relatively low resistance. In my dissertation, I will decribe the molecular basics of herbicide resistance and summarise herbicide resistant plants that have been created to this day. Finally, I will focus on the mechanisms of CRISPR/Cas9.
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