Isolation and characterization of aminopterin-resistant cell lines in maize

Aminopterin-resistant cell lines of maize were isolated by two different procedures of callus selection and by plating suspension cultures on drugcontaining medium after mutagen treatment. Efficiencies of different methods of variant selection were compared. Four aminopterin-resistant cell lines were shown to be 10-40 times more resistant than the parental cell line, and they were also resistant to another folate analog, methotrexate. The results suggest that alterations in at least three different cell properties could be responsible for resistance; 1) increased dihydrofolate reductase activity, 2) altered aminopterin sensitivity of dihydrofolate reductase, and 3) reduced drug uptake. One of the resistant cell lines showed more than one alteration, but its resistance proved to be unstable. The results suggest that stable changes which may or may not be of genetic origin and also unstable physiological changes or a combination of both could lead to aminopterin resistance in maize cell cultures.