Selectable marker genes

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1. Selectable Marker Genes in Plants Aakifah Amreen H.E

2. Selectable Marker Genes The selectable marker genes are usually an integral part of plant transformation system. They are present in the vector along with the target gene. In a majority of cases, the selection is based on the survival of the transformed cells when grown on a medium containing a toxic substance (antibiotic, herbicide, antimetabolite). This is due to the fact that the selectable marker gene confers resistance to toxicity in the transformed cells, while the non- transformed cells get killed. A large number of selectable marker genes are available and they are grouped into three categories— A ntibiotic resistance genes A ntimetabolite marker genes, and H erbicide resistance genes

3. 1. Antibiotic Resistance Genes: A ntibiotic resistance genes (particularly of E. coli) are used as selectable markers. A ntibiotics can effectively inhibit the protein biosynthesis in the cellular organelles, particularly in chloroplasts. i ) Neomycin phosphotransferase II ( npt II gene): The most widely used selectable marker encoding the enzyme neomycin phospho­transferase II (NPT II). C onfers resistance to the antibiotic kanamycin. The trans-formants and the plants derived from them can be checked by applying kanamycin solution and the resistant progeny can be selected.

4. ii) Hygromycin phosphotransferase (hpt gene): M ore toxic than neomycin and therefore can kill non-transformed plant cells much faster. iii) Aminoglycoside adenyltransferase (aadA gene): C onfers resistance to transform ed plant cells against the antibiotics streptomycin and spectionomycin.

5. 2. Antimetabolite Marker Genes: Dihydrofolate reductase (dhfr gene): The enzyme dihydrofolate reductase, produced by dhfr gene is inhibited by the antimetabolite methotrexate. This dhfr gene fused with CaMV promoter results in a methotrexate resistant marker which can be used for the selection of transformed plants.

6. 3. Herbicide Resistance Markers: Genes that confer resistance to herbicides are in use as markers for the selection of transgenic plants. i) Phosphinothricin acetytransferase (pat/bar gene): Bialophos, phosphinothricin and glufosinate are commonly used herbicides. The pat/bar genes code for phosphinothricin acetyltransferase which converts these herbicides into acetylated forms that are non-herbicidal. Thus, pat/bar genes confer resistance to the transformed plants. ii) Enolpyruvylshikimate phosphate synthase (epsps/ aroA genes): The herbicide glyphosate inhibits photosynthesis and blocks the activity of enolpyruvylshikimate phosphate (EPSP) synthase, a key enzyme involved in the biosynthesis of phenylalanine, tyrosine and tryptophan. The genes epsps/ aroA confer resistance to transgenic plants which can be selected.

7. ii) Bromoxynil nitrilase (bxn gene): The herbicide bromoxynil inhibits photosynthesis (photosystem II). Bromoxynil nitrilase enzyme coded by the gene bxn inactivates this herbicide. The gene bxn can be successfully used as a selectable marker for the selection of transformed plants.