Maize line MS6 was genetically engineered to express male sterility
and 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 PAT
enzyme catalyzes the acetylation of phosphinothricin, detoxifying
it into an inactive compound. The PAT enzyme is not known to have
any toxic properties.
The male-sterile trait was introduced by inserting the barnase
gene, isolated from Bacillus amyloliquefaciens, a common soil
bacterium that is frequently used as a source for industrial
enzymes. The barnase gene encodes for a ribonuclease enzyme (RNAse)
that is expressed only in the tapetum cells of the pollen sac
during anther development. The RNAse affects RNA production,
disrupting normal cell functioning and arresting early anther
development, thus leading to male sterility. The PAT enzyme was
used as a selectable marker enabling identification of 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.