Selection of drug resistant variants was performed with a diploid cell line growing on agar medium, originally isolated from mature embryos of Orla 260 (Funk Seeds Int.). Several spontaneous and EMS-induced variant lines were isolated with increasing resistance to the proline analogue, azetidine-2-carboxylic acid (AZCA), the methionine analogue, ethionine (ETH), or the lysine analogue, S-2-aminoethyl-L-cysteine (AEC):
 

Drug	ID 50% (M)	Selection level (M)	Number of resistant lines found
AZCA	0.5 x 10-3	3.0 x 10-3	7
ETH	0.02 x 10-3	0.2 x 10-3	35
AEC	0.02 x 10-3	0.3 x 10-3	12

Some of the resistant lines show a stable phenotype after several passages on drug-free medium. For one AEC resistant line, AEC^R-12, on which the work has concentrated, the relative growth in fresh weight is inhibited at ca. 5 times the concentration inhibiting the wild type. On drug-free medium the growth rate of the variant is generally slightly less than the wild type. For AEC^R-12 and AZCA^R-3 the free amino acid content of cells grown in the absence of the analogue was measured. No increase in the corresponding amino acids (lysine and proline) was detected.

A. Strauss and P. J. King

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