Herbicide-tolerant maize

Glufosinate ammonium herbicide tolerant maize produced by inserting the phosphinothricin acetyltransferase (bar) gene from Streptomyces hygroscopicus to confer tolerance to the herbicide phosphinothricin (Glufosinate ammonium).

Zea mays hybrid A188 x B73

pDPG165

The native bar gene initiation codon was modified from GTG to ATG to conform with plant codon usage.

Southern blot analysis indicated that a single copy of the T-DNA was inserted into the host genome with several deletions and rearrangements of some genetic elements. The insertion contains a single intact copy of the bar gene and single incomplete copies of the 35s promoter and beta-lactamase gene. Furthermore, bacterial promoter region for ß-lactamase gene is present in DLL 25, but the gene is truncated in the 3' region. It was determined (1) that the truncated ß-lactamase gene present in DLL 25  line did not, when introduced into the E. coli, produce active ß-lactamase, and (2) that the truncated ß-lactamase gene present in the DLL 25 line does not produce any protein detectable on a Western blot. The insert also contains two partial copies of the T7 terminator element.

The maize line DLL25 (synonym B16) was genetically engineered to express tolerance to glufosinate ammonium, the active ingredient in phosphinothricin herbicides (Basta®, Rely®, Finale®, and Liberty®). Glufosinate chemically resembles the amino acid glutamate and acts to inhibit an enzyme, called glutamine synthetase, which is involved in the synthesis of glutamine. Essentially, glufosinate acts enough like glutamate, the molecule used by glutamine synthetase to make glutamine, that it blocks the enzyme's usual activity. Glutamine synthetase is also involved in ammonia detoxification. The action of glufosinate results in reduced glutamine levels and a corresponding increase in concentrations of ammonia in plant tissues, leading to cell membrane disruption and cessation of photosynthesis resulting in plant withering and death.

Glufosinate tolerance in this maize line is the result of introducing a gene encoding the enzyme phosphinothricin-N-acetyltransferase (PAT) isolated from the common aerobic soil actinomycete, Streptomyces hygroscopicus, the same organism from which glufosinate was originally isolated. The PAT enzyme catalyzes the acetylation of phosphinothricin, detoxifying it into an inactive compound.