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Plant Physiol, April 2001, Vol. 125, pp. 1585-1590
Expression of bar in the Plastid Genome Confers
Herbicide Resistance1
Kerry A.
Lutz,
Jane E.
Knapp,2 and
Pal
Maliga*
Waksman Institute, Rutgers, State University of New Jersey, 190 Frelinghuysen Road, Piscataway, New Jersey 08854-8020
Phosphinothricin (PPT) is the active component of a family of
environmentally safe, nonselective herbicides. Resistance to PPT in
transgenic crops has been reported by nuclear expression of a
bar transgene encoding phosphinothricin
acetyltransferase, a detoxifying enzyme. We report here expression of a
bacterial bar gene (b-bar1) in tobacco
(Nicotiana tabacum cv Petit Havana) plastids that
confers field-level tolerance to Liberty, an herbicide containing PPT.
We also describe a second bacterial bar gene
(b-bar2) and a codon-optimized synthetic
bar (s-bar) gene with significantly elevated levels of expression in plastids (>7% of total soluble cellular protein). Although these genes are expressed at a high level,
direct selection thus far did not yield transplastomic clones,
indicating that subcellular localization rather than the absolute
amount of the enzyme is critical for direct selection of transgenic
clones. The codon-modified s-bar gene is poorly expressed in Escherichia coli, a common enteric
bacterium, due to differences in codon use. We propose to use codon
usage differences as a precautionary measure to prevent expression of
marker genes in the unlikely event of horizontal gene transfer from
plastids to bacteria. Localization of the bar gene in
the plastid genome is an attractive alternative to incorporation in the
nuclear genome since there is no transmission of plastid-encoded genes
via pollen.
1
This work was supported by the Rice
Biotechnology Research Grant from The Rockefeller Foundation, the
National Science Foundation (grant nos. MCB 96-30763 and MCB
99-05043), and the Monsanto Company (to P.M.).
2
Present address: Department of Plant Science, University
of Connecticut, 1376 Storrs Road, Storrs, CT 06269.
*
Corresponding author; e-mail maliga{at}waksman.rutgers.edu; fax
732-445-5735.
© 2001 American Society of Plant Physiologists
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