The selectable marker gene routinely used for selection of transgenic plants among these options is (3) Hygromycin (specifically, a hygromycin‑resistance gene such as hpt or hph that allows cells to survive on hygromycin‑containing medium).

Why hygromycin is the correct choice

In plant transformation systems (especially Agrobacterium‑mediated transformation and biolistics), a selectable marker gene is included in the T‑DNA to distinguish transformed plant cells from non‑transformed ones. The hygromycin B phosphotransferase gene (hpt/hph) confers resistance to the potent antibiotic hygromycin B, which efficiently kills non‑transformed plant cells at low concentrations. This makes hygromycin one of the standard, routine selectable markers for plant tissue culture selection, alongside kanamycin‑resistance and some herbicide‑resistance markers.

Explanation of each option

1. Ampicillin

• Ampicillin is a β‑lactam antibiotic used primarily to select transformed bacteria (e.g., E. coli, Agrobacterium) carrying plasmids with an ampR gene.

• Plant cells are relatively insensitive at the concentrations typically used, so ampicillin is not a routine selectable marker for plant transformation.

2. Tetracycline

• Tetracycline also mainly serves as a selection antibiotic for bacterial plasmids.

• While high doses can affect plant organelles, it is not a common, standard selectable marker for transgenic plant recovery in tissue culture.

3. Hygromycin – Correct

• Hygromycin B severely inhibits protein synthesis in plant cells; only cells expressing a hygromycin‑resistance gene survive on hygromycin‑containing medium.

• Because of its strong and reliable selection, hygromycin is widely used as a selectable marker in many dicot and monocot transformation protocols.

4. Carbenicillin

• Carbenicillin (and related β‑lactams) is often added to eliminate Agrobacterium after co‑cultivation, not to select transformed plants.

• It has relatively low toxicity to plant cells at bactericidal doses, so it is not used as the primary selectable marker gene in plant transformation.

SEO‑oriented introduction (for article use)

Selectable marker genes are essential in plant genetic engineering because they let researchers recover only those cells that have integrated the transgene. Among common options, hygromycin-resistance genes are routinely used as selectable markers for transgenic plants, whereas ampicillin, tetracycline and carbenicillin are primarily employed to select or kill bacteria and are not standard plant selection markers in tissue culture.