Background:

Bialaphos is a natural non-selective phytotoxin produced by certain Streptomyces species. It is a pro-toxin, a tripeptide that is converted in vivo to the active agent phosphinothricin, which is a glutamine analog.[1] L-Phosphinothricin inhibits glutamine synthetase (Ki = 6.1 μM), resulting in accumulation of ammonium and disruption of primary metabolism.[1],[2] The bacterial bar gene encodes a phosphinothricin acetyltransferase, which confers resistance to phosphinothricin.[3] Bialaphos is used in the selection of transgenic plants that express the bar gene, usually under the control of a constitutively active viral promoter.[3]

Reference:
[1]. Duke, S.O., and Dayan, F.E. Modes of action of microbially-produced phytotoxins. Toxins (Basel) 3(8), 1038-1064 (2011).
[2]. Ericson, M.C. Purification and properties of glutamine synthetase from spinach leaves. Plant Physiology 79(4), 923-927 (1985).
[3]. Block, M.D., Botterman, J., Vandewiele, M., et al. Engineering herbicide resistance in plants by expression of a detoxifying enzyme. EMBO Journal 6(9), 2513-2518 (1987).