Herbicide-resistant alfalfa cells: an example of gene amplification in plants.

We have selected alfalfa suspension cell lines that are 20- to 100-fold more resistant than wild-type cells to the nonselective herbicide L-phosphinothricin, a mixed competitive inhibitor of glutamine synthetase (GS). GS enzyme levels are three- to sevenfold elevated in the variant cell line. Partial amino acid sequences of two cyanogen bromide cleavage peptides have been determined on GS purified from the variant cell line and used to confirm the identity of a 276-base pair GS cDNA clone. Southern blot analysis of wild-type and variant cell DNAs using this cDNA as a probe indicate that resistance to L-phosphinothricin is a consequence of a four- to 11-fold amplification of one GS gene resulting in about an eightfold increase in mRNA levels, and an increased enzyme synthesis sufficient to overcome the toxic effects of the inhibitor.