<p style="text-align: justify;">Maize lines 676, 678, and 680 were developed to provide a reliable hybrid system based on nuclear male sterility. The transgenic lines were produced by genetically engineering plants to be male sterile and tolerant to the herbicide glufosinate ammonium, where herbicide tolerance was used as a selectable marker to identify the male sterile plants. These transformed maize lines contain an introduced gene (<i>dam</i>) from <i>Escherichia coli</i> that encodes the enzyme DNA adenine methylase (DAM). Expression of this gene in specific plant tissues results in the inability of the transformed plants to produce anthers or pollen, resulting in a male sterile plant.</p>

<p style="text-align: justify;">The maize lines 676, 678, and 680 also contained the <i>pat</i> gene, isolated from the bacterium <i>Streptomyces viridochromogenes</i>, for use as a selectable marker. The <i>pat</i> gene encodes a phosphinothricin acetyl transferase (PAT) enzyme, which, when introduced into a plant cell, confers tolerance to the herbicide glufosinate ammonium. The <i>pat</i> gene was used as a selectable marker to identify transformed plants during tissue culture regeneration, and as a field selection method to identify the male sterile lines prior to flowering. Under field conditions, plants that were not male-sterile could be eliminated by application of the herbicide glufosinate ammonium. The novel hybrid system provided an efficient and effective way to identify male-sterile plants for use in hybrid seed production.</p>

<p style="text-align: justify;">Glufosinate ammonium is the active ingredient in phosphinothricin herbicides (Basta&reg;, Rely&reg;, Finale&reg;, and Liberty&reg;). 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&#39;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. The PAT enzyme detoxifies phosphinothricin by acetylation into an inactive compound.</p>
